The laws of inheritance established by Mendel. Basic genetic concepts

Patterns of heredity. G. Mendel's laws, their statistical nature and cytological foundations

The main laws of heredity were established by the outstanding Czech scientist Gregor Mendel. G. Mendel began his research with monohybrid crossing, in which parental individuals differ in the state of one trait. The seed pea he chose is a self-cutting plant, so the descendants of each individual are pure lines. Together, peas can be artificially cross-pollinated, allowing hybridization and the production of heterozygous (hybrid) forms. As maternal (P), plants of a pure line with yellow seeds were taken, and as parent (P) - with green. As a result of such crossing, the seeds of plants (hybrids of the first generation - F1) turned out to be uniform - yellow. That is, only dominant traits appeared in the phenotype of F1 hybrids.

The monotony of the first hybrid generation and the identification of only a dominant trait in hybrids is called the law of dominance or Mendel's law.

Splitting - the phenomenon of the manifestation of both states of the traits in the second generation of hybrids (F2), is due to the difference in the allelic genes that determine them.

There are self-pollinating F1 plants with yellow seeds that produce offspring with yellow and green seeds; the recessive trait does not disappear, but is only temporarily suppressed, reappears in F2 in the ratio of 1/4 of the green seeds and 3/4 of the yellow ones. That is exactly - 3:1.

The manifestation in the phenotype of a quarter of hybrids of the second generation of a recessive trait, and three-fourths of a dominant one, is called the splitting law, Mendel's II law.

Later, G. Mendel complicated the conditions in the experiments - he used plants that differed in different states of two (dihybrid crossing) or more traits (polyhybrid crossing). When crossing pea plants with smooth yellow seeds and wrinkled green seeds, all hybrids of the first generation had smooth yellow seeds - a manifestation of Mendel's law - the uniformity of hybrids of the first generation. But among the F2 hybrids, there were four phenotypes.

Based on the results obtained, G. Mendel formulated the law of independent combination of trait states (the law of independent inheritance of traits). This is Mendel's third law. In di- or polyhybrid crossing, the splitting of the states of each trait in the offspring occurs independently of the others. Dihybrid crosses are characterized by splitting according to the 9:3:3:1 phenotype, and groups with new combinations of characters appear.

Incomplete dominance is an intermediate nature of inheritance. There are alleles that are only partially dominant over recessive ones. Then the hybrid individual has a degree of trait in the phenotype, which distinguishes it from the parent. This phenomenon is called incomplete dominance.

Methods for checking the genotype of hybrid individuals

As is known, with complete dominance, individuals with a dominant and heterozygous set of chromosomes are phenotypically the same. It is possible to determine their genotype using analyzing crosses. It is based on the fact that individuals homozygous for a recessive trait are always phenotypically similar. This is the crossing of a recessive homozygous individual with an individual with a dominant trait but an unknown genotype.

Upon receipt of a uniform F1, each parent forms only one type of gamete. So, the dominant individual is homozygous for the genotype (AA).

If, when an individual with a dominant trait is crossed with an individual with a recessive homozygous trait, the resulting offspring has a splitting of 1: 1, then the studied individual with a dominant trait is heterozygous (Aa).

1. Features of the method of hybridological analysis. Laws of Mendel.
2. Types of gene interaction.
3. Linked inheritance of traits.
4. cytoplasmic inheritance.

Method hybridological analysis , consisting in crossing and subsequent accounting for splits (ratios of phenotypic and genotypic varieties of descendants), was developed by the Czech naturalist G. Mendel (1865). The features of this method include: 1) taking into account, when crossing, not the entire diverse complex of traits in parents and descendants, but analysis of the inheritance of individual alternative traits identified by the researcher; 2) quantitative accounting in a series of successive generations of hybrid plants that differ in individual characteristics; 3) individual analysis of progeny from each plant.

Working with self-pollinating garden pea plants, G. Mendel chose for the experiment varieties (pure lines) that differ from each other in alternative manifestations of traits. Mendel processed the data obtained mathematically, as a result of which a clear pattern of inheritance of individual traits of parental forms by their descendants in a number of subsequent generations was revealed. Mendel formulated this regularity in the form of the rules of heredity, later called Mendel's laws.

The crossing of two organisms is called hybridization. monohybrid (monogenic)) is called the crossing of two organisms, in which the inheritance of one pair of alternative manifestations of a trait is traced (the development of this trait is due to a pair of alleles of one gene). Hybrids of the first generation are uniform in terms of the trait under study. In F1, only one of a pair of alternative variants of the seed color trait appears, called dominant. These results illustrate Mendel's first law - the law of uniformity of hybrids of the first generation, as well as the rule of dominance.

Mendel's first law can be formulated as follows: when crossing homozygous individuals that differ in one or more pairs of alternative traits, all hybrids of the first generation will be uniform in these traits. Hybrids will show dominant traits of their parents.

In the second generation, splitting according to the studied trait was found

The ratio of descendants with dominant and recessive manifestation of the trait was close to ¾ to ¼. Thus, Mendel's second law can be formulated as follows: in the case of monohybrid crossing of heterozygous individuals (F1 hybrids), in the second generation, splitting is observed according to the variants of the analyzed trait in the ratio of 3:1 by phenotype and 1:2:1 by genotype. To explain the distribution of traits in hybrids of successive generations, G. Mendel suggested that each hereditary trait depends on the presence in somatic cells of two hereditary factors received from the father and mother. To date, it has been established that Mendel's hereditary factors correspond to genes - loci of chromosomes.

Homozygous plants with yellow seeds (AA) form gametes of the same variety with the A allele; plants with green seeds (aa) form gametes with a. Thus, using modern terminology, the hypothesis " purity of gametes” can be formulated as follows: “In the process of formation of germ cells, only one gene from an allelic pair enters each gamete, because, in the process of meiosis, one chromosome from a pair of homologous chromosomes enters the gamete.

Crossing, in which inheritance is traced in two pairs of alternative traits, is called dihybrid, for several pairs of signs - polyhybrid. In Mendel's experiments, when crossing a pea variety with yellow (A) and smooth (B) seeds, with a pea variety with green (a) and wrinkled (b) seeds, F1 hybrids had yellow and smooth seeds, i.e. dominant traits appeared (hybrids are uniform).

Hybrid seeds of the second generation (F2) were divided into four phenotypic groups in the ratio: 315 - with smooth yellow seeds, 101 - with wrinkled yellow, 108 - with smooth green, 32 - with green wrinkled seeds. If the number of offspring in each group is divided by the number of offspring in the smallest group, then in F2 the ratio of phenotypic classes will be approximately 9:3:3:1. So, according to Mendel's third law, genes of different allelic pairs and their corresponding traits are transmitted to offspring regardless from each other combining in all sorts of combinations.

With the complete dominance of one allele over the other, heterozygous individuals are phenotypically indistinguishable from those homozygous for the dominant allele and they can only be distinguished using hybridological analysis, i.e. by offspring, which is obtained from a certain type of crossing, called analyzing. Analyzing is a type of crossing in which the test individual with a dominant trait is crossed with an individual homozygous for the recessive apple.

If the dominant individual is homozygous, the offspring from such a cross will be uniform and splitting will not occur. In the event that an individual with a dominant trait is heterozygous, splitting will occur in a ratio of 1: 1 in terms of phenotype and genotype.

Gene Interaction

In some cases, the action of different genes is relatively independent, but, as a rule, the manifestation of signs is the result of the interaction of products of different genes. These interactions may be related to allelic, so with non-allelic genes.

Interaction between alleles Genes are carried out in the form of three forms: complete dominance, incomplete dominance and independent manifestation (codominance).

Previously, Mendel's experiments were considered, which revealed the complete dominance of one allele and the recessiveness of the other. Incomplete dominance is observed when one gene from a pair of alleles does not provide the formation of a protein product sufficient for the normal manifestation of a trait. With this form of gene interaction, all heterozygotes and homozygotes differ significantly in phenotype from each other. At codedominance in heterozygous organisms, each of the allelic genes causes the formation of a trait controlled by it in the phenotype. An example of this form of interaction of alleles is the inheritance of human blood groups according to the ABO system, determined by gene I. There are three alleles of this gene Io, Ia, Ib, which determine blood group antigens. The inheritance of blood groups also illustrates the phenomenon plural allelism: in the gene pools of human populations, gene I exists in the form of three different alleles, which are combined in individual individuals only in pairs.

Interaction of non-allelic genes. In some cases, two (or more) pairs of non-allelic genes can influence one trait of an organism. This leads to significant numerical deviations of phenotypic (but not genotypic) classes from those established by Mendel in dihybrid crosses. The interaction of non-allelic genes is divided into main forms: complementarity, epistasis, polymerization.

At complementary interaction, the trait manifests itself only in the case of the simultaneous presence of two dominant non-allelic genes in the genotype of the organism. An example of a complementary interaction is the crossing of two different varieties of sweet peas with white flower petals.

The next type of interaction of non-allelic genes is epistasis, in which the gene of one allelic pair suppresses the action of the gene of another pair. A gene that suppresses the action of another is called epistatic genome(or suppressor). The suppressed gene is called hypostatic. Epistasis can be dominant or recessive. An example of dominant epistasis is the inheritance of the color of the plumage of chickens. Gene C in its dominant form determines the normal production of the pigment, but the dominant allele of another gene I is its suppressor. As a result, chickens that have a dominant allele of the color gene in the genotype turn out to be white in the presence of the suppressor. The epistatic action of the recessive gene illustrates the inheritance of coat color in house mice. Agouti color (reddish-gray coat color) is determined by the dominant gene A. Its recessive allele a in the homozygous state causes black color. The dominant gene of the other pair C determines the development of the pigment, homozygotes for the recessive allele c are albinos with white hair and red eyes (lack of pigment in the coat and iris).

The inheritance of a trait, the transmission and development of which, as a rule, is due to two alleles of one gene, is called monogenic. In addition, genes from different allelic pairs are known (they are called polymeric or polygenes), approximately the same effect on the trait.

The phenomenon of simultaneous action on a trait of several non-allelic genes of the same type is called polymerism. Although polymeric genes are not allelic, but since they determine the development of one trait, they are usually denoted by a single letter A (a), indicating the number of allelic pairs with numbers. The action of polygenes is most often summative.

Linked inheritance

An analysis of the inheritance of several traits simultaneously in Drosophila, carried out by T. Morgan, showed that the results of analyzing crosses of F1 hybrids sometimes differ from those expected in the case of their independent inheritance. In the offspring of such a crossing, instead of a free combination of traits from different pairs, a tendency was observed to inherit predominantly parental combinations of traits. This type of inheritance has been called linked. Linked inheritance is explained by the location of the corresponding genes on the same chromosome. As part of the latter, they are transmitted from generation to generation of cells and organisms, preserving the combination of parental alleles.

The dependence of the linked inheritance of traits on the localization of genes in one chromosome gives reason to consider chromosomes as separate clutch groups. An analysis of the inheritance of the trait of eye color in Drosophila in the laboratory of T. Morgan revealed some features that made it necessary to single out as a separate type of inheritance of traits sex-linked inheritance.

The dependence of the results of the experiment on which of the parents was the carrier of the dominant variant of the trait made it possible to suggest that the gene that determines the color of the eyes in Drosophila is located on the X chromosome and has no homologue on the Y chromosome. All the features of sex-linked inheritance are explained by the unequal dose of the corresponding genes in representatives of different - homo- and heterogametic sex. The X chromosome is present in the karyotype of each individual, therefore, the traits determined by the genes of this chromosome are formed in both female and male representatives. Individuals of the homogametic sex receive these genes from both parents and pass them on to all offspring through their gametes. Members of the heterogametic sex receive a single X chromosome from a homogametic parent and pass it on to their homogametic offspring. In mammals (including humans), males receive X-linked genes from their mothers and pass them on to their daughters. At the same time, the male sex never inherits the paternal X-linked trait and does not pass it on to his sons.

Actively functioning genes of the Y chromosome that do not have alleles on the X chromosome are present in the genotype of only the heterogametic sex, and in the hemizygous state. Therefore, they manifest themselves phenotypically and are transmitted from generation to generation only among representatives of the heterogametic sex. So, in humans, a sign of hypertrichosis of the auricle (“hairy ears”) is observed exclusively in men and is inherited from father to son.

We will start by presenting Mendel's laws, then we will talk about Morgan, and in the end we will say why genetics is needed today, how it helps and what are its methods.

In the 1860s, the monk Mendel began to study the inheritance of traits. This was done before him, and for the first time it is mentioned in the Bible. The Old Testament says that if the owner of livestock wanted to get a certain breed, then he fed some sheep with peeled branches if he wanted to get offspring with white wool, and uncleaned if he wanted to get the skin of black cattle. That is, how traits are inherited worried people even before the Bible was written. Why, before Mendel, could they not find the laws of transmission of traits in generations?

The fact is that before him, researchers chose a set of features of one individual, which were more difficult to deal with than with one feature. Before him, the transmission of signs was often considered as a single complex (like - she has a grandmother's face, although there are a lot of individual signs here). And Mendel registered the transmission of each trait separately, regardless of how other traits were transmitted to descendants.

It is important that Mendel chose signs for the study, the registration of which was extremely simple. These are discrete and alternative signs:

1. discrete (discontinuous) features: a given feature is either present or absent. For example, a sign of color: a pea is either green or not green.
2. alternative features: one state of a feature excludes the presence of another state. For example, the state of such a trait as color: a pea is either green or yellow. Both states of a trait cannot appear in one organism.

Mendel's approach to the analysis of descendants was one that had not been used before. This is a quantitative, statistical method of analysis: all descendants with a given trait state (for example, green peas) were combined into one group and their number was counted, which was compared with the number of descendants with a different trait state (yellow peas).

As a sign, Mendel chose the color of seeds of sowing peas, the state of which was mutually exclusive: the color is either yellow or green. Another sign is the shape of the seeds. Alternate states of the trait are shape or wrinkled or smooth. It turned out that these signs are stably reproduced in generations, and appear either in one state or in another. In total, Mendel studied 7 pairs of signs, tracking each one separately.

When crossing, Mendel investigated the transmission of traits from parents to their offspring. And that's what he got. One of the parents gave in a series of generations during self-pollination only wrinkled seeds, the other parent - only smooth seeds.

Peas are self-pollinators. In order to get offspring from two different parents (hybrids), he had to make sure that the plants did not self-pollinate. To do this, he removed the stamens from one parent plant, and transferred pollen from another plant to it. In this case, the resulting seeds were hybrid. All hybrid seeds in the first generation were the same. All of them were smooth. We call the manifested state of the trait dominant (the meaning of the root of this word is dominant). Another state of the trait (wrinkled seeds) was not found in hybrids. We call this state of the trait recessive (inferior).

Mendel crossed the plants of the first generation inside himself and looked at the shape of the resulting peas (this was the second generation of the offspring of the cross). Most of the seeds were smooth. But the part was wrinkled, exactly the same for the original parent (if we were talking about our own family, we would say that the grandson was exactly like his grandfather, although mom and dad did not have this condition at all). He conducted a quantitative study of what proportion of offspring belong to one class (smooth - dominant), and which to another class (wrinkled - recessive). It turned out that about a quarter of the seeds were wrinkled, and three quarters were smooth.

Mendel carried out the same crosses of the first generation hybrids for all other traits: seed color, flower color, etc. He saw that the 3:1 ratio was maintained.

Mendel crossed in one direction (dad with a dominant trait, mom with a recessive trait) and in the other (dad with a recessive trait, mom with a dominant trait). At the same time, the qualitative and quantitative results of the transfer of traits in generations were the same. From this we can conclude that both female and paternal inclinations of the trait make the same contribution to the inheritance of the trait in offspring.

The fact that in the first generation the trait of only one parent is manifested, we call the law of uniformity of hybrids of the first generation or the law of dominance.

The fact that in the second generation the signs of both one parent (dominant) and the other (recessive) reappear allowed Mendel to suggest that it is not the trait as such that is inherited, but the deposit of its development (what we now call the gene). He also suggested that each organism contains a pair of such inclinations for each trait. Only one of the two inclinations passes from parent to child. The deposit of each type (dominant or recessive) passes to the descendant with equal probability. When two different inclinations (dominant and recessive) are combined in a descendant, only one of them appears (dominant, it is indicated by a capital letter A). The recessive deposit (it is denoted by a small letter a) does not disappear in the hybrid, since it appears as a trait in the next generation.

Since in the second generation exactly the same organism appeared as the parent, Mendel decided that the deposit of one trait “does not get smeared”, when combined with another, it remains just as pure. Subsequently, it was found that only half of its inclinations are transmitted from this organism - germ cells, they are called gametes, carry only one of two alternative signs.

In humans, there are about 5 thousand morphological and biochemical traits that are inherited quite clearly according to Mendel. Judging by the splitting in the second generation, alternative inclinations of one trait were combined with each other independently. That is, the dominant trait could appear in combinations like Ah, aa and AA, and recessive only in combination aa.

Let us repeat that Mendel suggested that it is not the trait that is inherited, but the inclinations of the trait (genes) and that these inclinations do not mix, therefore this law is called the law of gamete purity. Through the study of the inheritance process, it was possible to draw conclusions about some characteristics of the inherited material, that is, that the makings are stable in generations, retain their properties, that the makings are discrete, that is, only one state of the trait is determined, that there are two of them, they are combined randomly, etc. d.

At the time of Mendel, nothing was known about meiosis, although they already knew about the nuclear structure of the cell. The fact that the nucleus contains a substance called nuclein became known only a couple of years after the discovery of Mendel's laws, and this discovery was in no way connected with him.

All conclusions of the above material can be formulated as follows:

1) Each hereditary trait is determined by a separate hereditary factor, deposit; in the modern view, these inclinations correspond to genes;

2) Genes are preserved in their pure form in a number of generations, without losing their individuality: this was proof of the basic position of genetics: the gene is relatively constant;

3) Both sexes equally participate in the transfer of their hereditary properties to offspring;

4) Reduplication of an equal number of genes and their reduction in male and female germ cells; this position was a genetic prediction of the existence of meiosis;

5) Hereditary inclinations are paired, one is maternal, the other is paternal; one of them may be dominant, the other recessive; this provision corresponds to the discovery of the principle of allelism: a gene is represented by at least two alleles.

The laws of inheritance include the law of splitting of hereditary traits in the offspring of a hybrid and the law of independent combination of hereditary traits. These two laws reflect the process of transmission of hereditary information in cell generations during sexual reproduction. Their discovery was the first actual proof of the existence of heredity as a phenomenon.

The laws of heredity have a different content, and they are formulated in the following form:

• The first law is the law of discrete (genetic) hereditary determination of traits; it underlies the theory of the gene.
• The second law is the law of the relative constancy of the hereditary unit - the gene.
• The third law is the law of the allelic state of the gene (dominance and recessiveness).

The fact that Mendel's laws are associated with the behavior of chromosomes during meiosis was discovered at the beginning of the 20th century during the rediscovery of Mendel's laws at once by three groups of scientists independently of each other. As you already know, a feature of meiosis is that the number of chromosomes in a cell is halved, chromosomes can change their parts during meiosis. This feature characterizes the situation with the life cycle in all eukaryotes.

In order to test the assumption of the inheritance of inclinations in this form, as we have already said, Mendel also crossed the descendants of the first generation, which have yellow seeds with parental green (recessive) ones. Crossing into a recessive organism he called analyzing. As a result, he got a one-to-one split :( Ah X aa = Ah + Ah + aa + aa). Thus, Mendel confirmed the assumption that in the organism of the first generation there are inclinations of the traits of each of the parents in a ratio of 1 to 1. Mendel called the state when both inclinations of the trait are the same homozygous, and when they are different - heterozygous.

Mendel took into account the results obtained on thousands of seeds, that is, he conducted statistical studies that reflect a biological pattern. The very laws he discovered will also apply to other eukaryotes, such as fungi. Shown here are fungi in which the four spores resulting from one meiosis remain in a common shell. Analyzing crossing in such fungi leads to the fact that in one shell there are 2 spores with the trait of one parent and two with the trait of the other. Thus, the splitting 1:1 in the analyzing crossing reflects the biological regularity of the splitting of the makings of one trait in each meiosis, which will look like a statistical regularity if all the spores are mixed.

The fact that the parents had different states of one trait indicates that the inclinations for the development of the trait can somehow change. These changes are called mutations. Mutations are neutral: hair shape, eye color, etc. Some mutations lead to changes that disrupt the normal functioning of the body. These are short-leggedness in animals (cattle, sheep, etc.), eyelessness and winglessness in insects, hairlessness in mammals, gigantism and dwarfism.

Some mutations may be harmless, such as hairlessness in humans, although all primates have hair. But sometimes there are changes in the intensity of hair on the body and in people. N.I. Vavilov called this phenomenon the law of homological series of hereditary variability: that is, a trait typical of only one of two related species can be found with some frequency in individuals of a related species.

This slide shows that mutations can be quite noticeable, we see a Negro family in which a white albino was born. His children are more likely to be pigmented because the mutation is recessive and its frequency is low.

We have talked before about signs that are fully manifest. But this is not the case for all signs. For example, the phenotype of heterozygotes may be intermediate between the dominant and recessive trait of the parents. So, the color of the fruit in eggplant in the first generation changes from dark blue to a less intense purple. At the same time, in the second generation, the splitting by the presence of color remained 3: 1, but if we take into account the color intensity, the split became 1: 2: 1 (the color is dark blue - AA, purple - 2 Ah and white - aa, respectively) In this case, it is clear that the manifestation of the trait depends on the dose of the dominant allele. Segregation by phenotype corresponds to cleavage by genotype: classes AA, Ah and aa, in a ratio of 1:2:1.

Once again, we highlight the role of Mendel in the development of science. No one before him thought that there could be inclinations of signs at all. It was believed that in each of us there is a little man, inside him there is another little man, etc. Conception has something to do with its appearance, but according to the mechanism, a ready-made little man is already present from the very beginning of his growth. Such were the dominant ideas, which, of course, had a drawback - according to this theory, with a large number of generations, the homunculus should have turned out to be smaller in size than an elementary particle, but then they did not know about particles J.

How did Mendel know which trait is dominant and which is recessive? He did not know anything of the kind, he simply took some principle of organizing experience. Conveniently, the traits he observed were varied: height, size, flower color, bean color, and so on. He did not have an a priori model of the mechanism of inheritance, he derived it from the observation of the transmission of a trait in generations. Another feature of his method. He found that the proportion of individuals with a recessive trait in the second generation is a quarter of all offspring. That is, the probability that this pea is green is 1/4. Let's say it turned out an average of 4 peas in one pod. Will there be in each pod (this is the offspring of two and only two parents) 1 green and 3 yellow peas? No. For example, the probability that there will be 2 green peas is 1/4 x 1/4 = 1/16, and that all four are green is 1/256. That is, if you take a bunch of beans, with four peas in each, then every 256th will have all the peas with recessive traits, that is, green. Mendel analyzed the offspring of many identical pairs of parents. Crossbreeding was talked about because they show that Mendel's laws appear as statistical, and are based on a biological pattern - 1: 1. That is, gametes of different types in EVERY meiosis in a heterozygote are formed in an equal ratio - 1: 1, and the patterns appear statistically, since the descendants of hundreds of meioses are analyzed - Mendel analyzed more than 1000 descendants in crossing each type.

First, Mendel studied the inheritance of one pair of traits. He then wondered what would happen if two pairs of signs were observed at the same time. Above in the figure, on the right side, such a study is illustrated by the thought of pairs of signs - the color of peas and the shape of peas.

Parents of one type gave yellow and round peas during self-pollination. Parents of another type produced green and wrinkled peas during self-pollination. In the first generation, he received all the peas yellow, and round in shape. The resulting splitting in the second generation is conveniently considered using the Penet lattice. A 9:3:3:1 split was obtained (yellow and round: yellow and wrinkled: green and round: green and wrinkled). Splitting for each pair of features occurs independently of each other. The ratio 9zhk + 3zhm + 3bk + 1sm corresponds to an independent combination of the results of two crosses (3g + 1h) x (3k + 1m). That is, the makings of signs of these pairs (color and shape) are combined independently.

Let's calculate how many different phenotypic classes we got. We had 2 phenotypic classes: yellow and green; and on another basis 2 phenotypic classes: round and wrinkled. And in total there will be 2*2=4 phenotypic classes, which we got above. If we consider three traits, then there will be 2 3 = 8 classes of phenotypic classes. Mendel went as far as dihybrid crosses. The makings of all traits, fortunately Mendel, were in peas on different chromosomes, and there were 7 pairs of chromosomes in peas. Therefore, it turned out that he took traits that combined independently in the offspring.

Humans have 23 pairs of chromosomes. If we consider any one heterozygous trait for each chromosome, a person may have 2 23 ~ 8 * 10 6 phenotypic classes in the offspring of one married couple. As mentioned in the first lecture, each of us contains about 1 difference per 1000 positions between father's and mother's chromosomes, that is, about a million differences between father's and mother's chromosomes in total. That is, each of us is a descendant of a million-hybrid cross, in which the number of phenotypic classes is 2,100,000. In practice, this number of phenotypic classes in the offspring of one pair is not realized, because we have only 23 chromosomes, not a million. It turns out that 8*10 6 is the lower limit of the possible diversity in the offspring of a given couple. Based on this, one can understand that there can not be two absolutely identical people. The probability of a mutation of a given nucleotide in DNA in one generation is about 10 -7 - 10 -8, that is, for the entire genome (3 * 10 9) there will be about 100 de novo changes between parent and child. And the total differences in the father's half of your genome from the mother's half are about 1,000,000. This means that old mutations in your genome are much more frequent than newly emerged ones (10,000 times).

Mendel also carried out analyzing crosses - crossing with a recessive homozygote. In a descendant of the first generation, the combination of genes has the form AaB b. If you cross it with a representative with a completely recessive set of genes ( aabb), then there will be four possible classes that will be in the ratio 1:1:1:1, in contrast to the crossing discussed above, when we got a splitting of 9:3:3:1.

Shown below are some statistical criteria - which ratios of numbers should be considered as expected, say 3:1. For example, for 3:1 - out of four hundred peas it is unlikely that exactly 300 to 100 will turn out. If it turns out, for example, 301 to 99, then this ratio can probably be considered equal to 3 to 1. And 350 to 50 is probably not equal to 3 to 1.

The chi-square (χ 2) statistical test is used to test the hypothesis that the observed distribution matches the expected one. This Greek letter is pronounced in Russian as “chi”, and in English as “chi” (chi).

The value of χ 2 is calculated as the sum of the squared deviations of the observed values ​​from the expected value, divided by the expected value. Then, according to a special table for a given value of χ 2, the probability value is found that such a difference between the observed and expected value is random. If the probability is less than 5%, then the deviation is not considered random (the figure of five percent is chosen by agreement).

Will there always be some hereditarily predetermined trait? After all, this default assumption underlies the interpretation of the data obtained by Mendel.

It turns out that this can depend on many reasons. There is such an inherited trait in humans - six-fingeredness. Although we, like all vertebrates, normally have five fingers.

The probability of manifestation of the deposit of a trait in the form of an observed trait (here, six-fingeredness) may be less than 100%. In the photo, a person has 6 fingers on both legs. And his twin does not necessarily show this sign. The proportion of individuals with a given genotype who exhibit the corresponding phenotype was called penetrance (this term was introduced by the Russian geneticist Timofeev-Resovsky).

In some cases, the sixth toe may simply be indicated by some skin growth. Timofeev-Resovsky proposed to call the degree of manifestation of a trait in an individual expressiveness.

Especially clearly not 100% connection of the genotype with the phenotype can be traced in the study of identical twins. Their genetic constitution is one to one, and their signs coincide to varying degrees. Below is a table that shows the coincidence of signs for identical and non-identical twins. Various diseases are taken as signs in this table.

A trait that is present in most individuals in their natural habitat is called the wild type. The most common trait is often dominant. Such a relationship may have an adaptive value useful to the species. In humans, dominant traits are, for example, black hair, dark eyes, curly hair. By the way, since the corresponding genes are on different chromosomes, you can get a curly black man who will be blond - nothing forbids this.

Why does it happen that in a monohybrid crossing, the three genotypic classes in the offspring of the second generation correspond in some cases to three phenotypic classes (eggplant blue violet and white), and in another case to two classes (yellow or green peas)? Why is the manifestation of the dominant trait incomplete in one case, and complete in the other? An analogy can be drawn with photographic film. Depending on the amount of light, the frame can turn out completely transparent, gray and completely black. The same is with genes. For example, maize has the Y gene, which determines the formation of vitamin A. When the dose of the Y gene per cell increases from one to three, the activity of the enzyme it encodes changes linearly and, in this case, the formation of vitamin A and the color of the grain increase. (In corn, the main part of the grain is the endosperm. Each endosperm cell has three genomes - two from mom and one from dad). That is, many traits depend on the dose of the allele quantitatively. The more copies of the allele of the desired type, the greater will be the value of the trait controlled by it. Such a relationship is constantly used in biotechnology.

Mendel could safely have not discovered his laws. Research on peas allowed Mendel to discover his laws, because peas are a self-pollinating plant, and therefore homozygous without coercion. During self-pollination, the proportion of heterozygotes decreases in proportion to two to the power of the generation number. This was Mendel's luck - if the proportion of heterozygotes was large, then no patterns would be observed. When he then took cross-pollinators, the patterns broke down, which greatly upset Mendel, because he thought he had discovered something private. It turned out not.

Above, we talked about the inheritance of qualitative traits, and usually most of the traits are quantitative. Their genetic control is rather complicated. Quantitative signs are described through the average value of the sign value and the range of variation, which is called the reaction norm. Both the average value and the reaction rate are species-specific indicators that depend both on the genotype and environmental conditions. For example, the life expectancy of a person. Although the Bible says that the prophets lived for 800 years, but now it is clear that no one lives more than 120-150 years. A mouse, for example, lives for two years, although it is also a mammal. Our height, our weight - these are all quantitative signs. There are no people 3-4 meters tall, although there are elephants, for example. Each species has its own average for each quantitative trait and its own range of variation.

Patterns of inheritance are open in the study of qualitative traits.

Most of our features are quantitative.

The values ​​of trait values ​​in a representative sample of individuals of a given species are characterized by a certain average and breadth of its variation, which is called the reaction norm and depends both on the genotype and on the conditions for the formation of the trait.

Topic 4.2 Basic patterns

heredity

Terminology 1.Alternative- Contrasting features. 2. clean lines- plants in a row in which splitting is not observed during self-pollination. 3. hybrid method- Obtaining hybrid offspring and its analysis. 4. parent individuals- R. 5. Males – ♂. 6. females – ♀. 7. Crossbreeding– X. 8. hybrids F 1 , F 2 , F n . nine. Monohybrid- crossing individuals with one contrasting trait. Patterns of inheritance of traits The quantitative patterns of inheritance of traits were discovered by the Czech amateur botanist G. Mendel. Having set a goal to find out the patterns of inheritance of traits, he, first of all, drew attention to the choice of the object of study. For his experiments, G. Mendel chose peas - those of his varieties that clearly differed from each other in a number of ways. One of the most significant points in the whole work was the determination of the number of characters by which the plants to be crossed should differ. G. Mendel realized for the first time that starting with the simplest case - the differences between parents in one single trait and gradually complicating the task, one can hope to unravel the whole tangle of patterns of transmission of traits from generation to generation, i.e. their inheritance. Here the strict mathematics of his thinking was revealed. It was this approach that allowed G. Mendel to clearly plan the further complication of experiments. In this respect, Mendel stood above all contemporary biologists. Another important feature of his research was that he chose organisms belonging to pure lines for experiments, i.e. such plants, in a number of generations of which, during self-pollination, splitting according to the studied trait was not observed. No less important is the fact that he observed the inheritance of alternatives, i.e. contrasting features. For example, the flowers of one plant were purple and the other white, the plant was tall or short, the beans were smooth or wrinkled, and so on. Comparing the results of experiments and theoretical calculations, G. Mendel especially emphasized the average statistical nature of the laws he discovered. Thus, the method of crossing individuals that differ in alternative traits, i.e. hybridization, followed by strict consideration of the distribution of parental traits in offspring, is called hybridiological. The patterns of inheritance of traits, identified by G. Mendel and confirmed by many biologists on a variety of objects, are currently being formulated in the form of laws that are universal in nature. The law of uniformity of the first generation of hybrids Monohybrid cross. To illustrate the law of uniformity of the first generation - Mendel's first law, let's reproduce his experiments on monohybrid crossing of pea plants. Monohybrid is the crossing of two organisms that differ from each other in one pair of alternative traits. Consequently, with such crossing, the patterns of inheritance of only two variants of the trait are traced, the development of which is due to a pair of allelic genes. For example, a sign is the color of seeds, options are yellow or green. All other features characteristic of these organisms are not taken into account. If you cross pea plants with yellow and green seeds, then all the hybrid descendants obtained as a result of this crossing will have yellow seeds. The same picture is observed when crossing plants that have a smooth and wrinkled seed shape - all seeds in hybrids will be smooth. Consequently, only one of each pair of alternative traits appears in the hybrid of the first generation. The second sign, as it were, disappears, does not appear. The predominance of the trait of one of the parents in the hybrid Mendel called dominance. A trait that manifests itself in a hybrid of the first generation and suppresses the development of another trait was called dominant, opposite, i.e. the suppressed trait is recessive. It is customary to designate a dominant trait with a capital letter (A), a recessive trait with a lowercase letter (a). Mendel used in experiments plants belonging to different pure lines, or varieties, the descendants of which in a long series of generations were similar to their parents. Therefore, in these plants, both allelic genes are the same. Thus, if there are two identical allelic genes in the genotype of an organism, i.e. two genes that are absolutely identical in nucleotide sequence, such an organism is called homozygous. An organism can be homozygous for dominant (AA) or recessive (aa) genes. If the allelic genes differ from each other in the nucleotide sequence, for example, one is dominant and the other is recessive (Aa), such an organism is called heterozygous. Mendel's first law is also called the law of dominance or uniformity, since all individuals of the first generation have the same manifestation of a trait inherent in one of the parents. It is formulated like this: When crossing two organisms belonging to different pure lines (two homozygotes), differing from each other in a pair of alternative traits, the entire first generation of hybrids (F 1) will be uniform and will carry the trait of one parent. With regard to color, Mendel established that red or black would dominate over white, with pink and grey, of varying saturation, being intermediate colors. Mendel proposed graphic symbols for signs: P - parents, ♂ - male, ♀ - female,
, - gametes, X - crossing, F 1, F 2, F n - offspring. The first Mendel's law is shown in Figure 1.

Figure 1. Mendel's first law

All offspring have the same intermediate color, which does not contradict Mendel's first law.

test questions

1. Mendel's biological material. 2. Alternative features in Mendel's experiments. 3. Pure lines and their definition. 4. Essence of the hybridiological method. 5. Monohybrid crossing. 6. Dominant and recessive traits. 7. Allelic genes. 8. Mendel's first law. The law of uniformity.

Topic 4.2.1 Incomplete gene dominance

Terminology 1. allelic genes- genes located in the same loci of homologous chromosomes. 2. dominant trait- suppressing the development of another. 3. recessive trait- repressed. 4. Homozygote A zygote that has the same genes. 5. heterozygote A zygote that has different genes. 6. Split- divergence of traits in the offspring. 7. Crossing over- crossover of the chromosome. In the heterozygous state, the dominant gene does not always completely suppress the manifestation of the recessive gene. In some cases, the F 1 hybrid does not fully reproduce any of the parental traits, and the expression of the trait is of an intermediate nature with a greater or lesser deviation towards a dominant or recessive state. But all individuals of this generation show uniformity in this trait. The intermediate nature of inheritance in the previous scheme does not contradict Mendel's first law, since all descendants of F 1 are uniform. incomplete dominance is a widespread phenomenon. It was discovered when studying the inheritance of flower color in snapdragons, the structure of bird feathers, the color of wool in cattle and sheep, biochemical characteristics in humans, etc. Multiple allelism. So far, examples have been examined in which the same gene was represented by two alleles - dominant (A) and recessive (a). These two states of the gene are due to mutation. A gene can mutate multiple times. As a result, several variants of allelic genes arise. The totality of these allelic genes, which determine the variety of trait options, is called a series of allelic genes. The occurrence of such a series due to repeated mutation of one gene is called multiple allelism or multiple allelomorphism. Gene A can mutate to state a 1, a 2, a 3, and n. Gene B located in another locus is in the state b 1 , b 2 , b 3 , b n . For example, in the Drosophila fly, a series of alleles for the eye color gene is known, consisting of 12 members: red, coral, cherry, apricot, etc. to white, determined by the recessive gene. Rabbits have a series of multiple alleles for coat color. This causes the development of a solid color or lack of pigmentation (albinism). Members of the same series of alleles may be in different dominant-recessive relationships with each other. It should be remembered that only two genes from a series of alleles can be in the genotype of diploid organisms. The remaining alleles of this gene in different combinations are included in pairs in the genotypes of other individuals of this species. Thus, multiple allelism characterizes the diversity of the gene pool, i.e. the totality of all genes that make up the genotypes of a certain group of individuals or an entire species. In other words, multiple allelism is a species trait, not an individual trait. Mendel's Second Law - The Law of Splitting If the descendants of the first generation, identical in terms of the trait being studied, are crossed among themselves, then in the second generation the traits of both parents appear in a certain numerical ratio: 3/4 individuals will have a dominant trait, 1/4 - recessive. By genotype, F 2 will contain 25% of individuals homozygous for dominant alleles, 50% of organisms will be heterozygous, and 25% of the offspring will be homozygous for recessive alleles. The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some of which are recessive, is called splitting. Therefore, splitting is the distribution of dominant and recessive traits among offspring in a certain numerical ratio. The recessive trait in hybrids of the first generation does not disappear, but is only suppressed and manifests itself in the second hybrid generation. Thus, Mendel's second law (see Fig. 2) can be formulated as follows: when two descendants of the first generation are crossed with each other (two heterozygotes), in the second generation, splitting is observed in a certain numerical ratio: according to the phenotype 3: 1, according to the genotype 1: 2:1.

Figure 2. Mendel's second law

With incomplete dominance in the offspring of F 2 hybrids, the splitting by genotype and phenotype coincides (1:2:1). The law of purity of gametes This law reflects the essence of the process of formation of gametes in meiosis. Mendel suggested that hereditary factors (genes) do not mix during the formation of hybrids, but remain unchanged. In the body of the hybrid F, from crossing parents that differ in alternative traits, both factors are present - dominant and recessive. In the form of a trait, the dominant hereditary factor is manifested, while the recessive one is suppressed. Communication between generations during sexual reproduction is carried out through germ cells - gametes. Therefore, it must be assumed that each gamete carries only one factor of the pair. Then, during fertilization, the fusion of two gametes, each of which carries a recessive hereditary factor, will lead to the formation of an organism with a recessive trait that manifests itself phenotypically. The fusion of gametes that carry a dominant factor, or two gametes, one of which contains a dominant and the other a recessive factor, will lead to the development of an organism with a dominant trait. Thus, the appearance in the second generation (F 2) of a recessive trait of one of the parents (P) can take place only if two conditions are met: 1. If the hereditary factors remain unchanged in hybrids. 2. If germ cells contain only one hereditary factor from an allelic pair. The splitting of traits in the offspring when heterozygous individuals were crossed, Mendel explained by the fact that gametes are genetically pure, i.e. carry only one gene from an allelic pair. The law of gamete purity can be formulated as follows: during the formation of germ cells, only one gene from an allelic pair (from each allelic pair) enters each gamete. Cytological proof of the law of gamete purity is the behavior of the chromosome in meiosis: in the first meiotic division, homologous chromosomes enter different cells, and in the anaphase of the second, daughter chromosomes, which, due to crossing over, can contain different alleles of the same gene. It is known that in every cell of the body there is exactly the same diploid set of chromosomes. Two homologous chromosomes contain two identical allelic genes. The formation of genetically "pure" gametes is shown in the diagram in Figure 3.

Figure 3. Formation of "pure" gametes

When male and female gametes merge, a hybrid is formed that has a diploid set of chromosomes (see Fig. 4).

Figure 4. Hybrid formation

As can be seen from the diagram, the zygote receives half of the chromosomes from the paternal organism, and half from the maternal organism. During the formation of gametes in a hybrid, homologous chromosomes also enter different cells during the first meiotic division (see Fig. 5).

Figure 5. Formation of two varieties of gametes

Two varieties of gametes are formed for this allelic pair. Thus, the cytological basis of the law of gamete purity, as well as the splitting of traits in offspring during monohybrid crossing, is the divergence of homologous chromosomes and the formation of haploid cells in meiosis. Analyzing cross The hybridiological method developed by Mendel for studying heredity makes it possible to establish whether an organism is homozygous or heterozygous if it has a dominant phenotype for the gene under study. Is the breed pure? To do this, an individual with an unknown genotype and an organism homozygous for a recessive allele with a recessive phenotype are crossed. If the dominant individual is homozygous, the offspring from such a cross will be uniform and splitting will not occur (see Fig. 6).

Figure 6. Crossing of dominant individuals.

A different picture will be obtained if the organism under study is heterozygous (see Fig. 7).

Figure 7. Crossing of heterozygous individuals.

Cleavage will occur in a ratio of 1:1 by phenotype. Crossing this result is proof of the formation of two varieties of gametes in one of the parents, i.e. his heterozygosity is not a pure breed (see Fig. 8).

Figure 8. Cleavage will occur in a 1:1 ratio by phenotype.

test questions

1. Incomplete dominance and its manifestation in nature. 2. The essence of multiple allelism. 3. II-Mendel's law. splitting law. 4. The law of purity of gametes. 5. Cytological evidence for the law of purity of gametes. 6. Analyzing crossing, its essence and meaning.

Topic 4.2.2 III Mendel's law - the law of independent

combination of features

Terminology 1. Dihybrid cross- crossing on two contrasting traits. 2. Diheterozygous organisms- organisms heterozygous for two pairs of allelic genes. 3. Pannet grating- a graphical method for calculating the results of crossing. 4. Recombination- recombination of features. 5. Crossing over- the appearance of new signs with the overlap of chromosomes. 6. Morganida is the distance between genes. Dihybrid and polyhybrid crosses Organisms differ from each other in many ways. It is possible to establish patterns of inheritance of two or more pairs of alternative traits by dihybrid or polyhybrid crossing. For dihybrid crosses, Mendel used homozygous pea plants that differ in two pairs of traits - seed color (yellow and green) and seed shape (smooth and wrinkled). The dominant ones were yellow color (A) and smooth seed shape (B). Each plant produces one variety of gametes according to the studied alleles. When gametes merge, all offspring will be uniform (see Fig. 9).

Figure 9. Fusion of gametes

Organisms that are heterozygous for two pairs of allelic genes are called diheterozygous. During the formation of gametes in a hybrid, only one of each pair of allelic genes enters the gamete, while due to the accidental divergence of paternal and maternal chromosomes in the first division of meiosis, gene A can fall into one gamete with gene B or with gene b, just like gene a can unite in one gamete with gene B or with gene b (see Fig. 10).

Figure 10. Formation of gametes in a hybrid

Table 1.

Processing the results of dihybrid crossing

AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

↓ → A - yellow color. a - green color. B is round. b - wrinkled form. Since many germ cells are formed in each organism, due to statistical patterns, four varieties of gametes are formed in the hybrid in the same amount (25% each) AB, Ab, aB, ab. During fertilization, each of the four types of gametes of one organism randomly meets any of the gametes of another organism. All possible combinations of male and female gametes can be easily identified using the Pannet grid. Parental gametes are drawn vertically and horizontally. The squares show the genotypes of zygotes resulting from the fusion of gametes. It can be seen that according to the phenotype, the offspring are divided into four groups: 9 yellow smooth, 3 yellow wrinkled, 3 green smooth, 1 yellow wrinkled. If we take into account the results of splitting for each pair of features separately, it turns out that the ratio of the number of smooth to the number of wrinkled for each pair is 3:1. Thus, in a dihybrid cross, each pair of traits, when split in the offspring, behaves in the same way as in a monohybrid cross, i.e. regardless of the other pair of features. During fertilization, gametes are combined according to the rules of random combinations, but with equal probability for each. In the resulting zygotes, various combinations of genes arise. An independent distribution of genes in the offspring and the emergence of various combinations of these genes during dihybrid crossing is possible only if the pairs of allelic genes are located in different pairs of homologous chromosomes. Mendel's third law, or the law of independent combination, can be formulated as follows: when two homozygous individuals are crossed, differing from each other in two pairs of alternative traits, genes and the corresponding traits are inherited independently of each other and are combined in all possible combinations. The third law is applicable only to the inheritance of allelic pairs located in different pairs of homologous chromosomes. The analysis of splitting is based on Mendel's laws and, in more complex cases, when individuals are distinguished by three or more pairs of signs. If the parental individuals differ in one pair of traits, in the second generation there is a splitting of traits in a ratio of 3:1, for a dihybrid cross it will be (3:1) 2 or 9:3:3:1, for a trihybrid (3:1) 3 and etc. You can also calculate the number of gamete varieties formed in hybrids using the formula 2 n , where n is the number of gene pairs by which parental individuals differ.

The laws of inheritance of traits by G. Mendel describe the primary principles of the transfer of hereditary characteristics from parent organisms to their children; these principles underlie classical genetics. These laws were discovered by Mendel as a result of crossing organisms (in this case, plants) with different genotypes. Usually describe one rule and two laws.

Rule of Uniformity for First Generation Hybrids

When crossing sowing peas with stable traits - purple and white flowers, Mendel noticed that the emerging hybrids were all with purple flowers, among them there was not a single white one. Mendel repeated the experiments more than once, using other signs. For example, if he crossed peas with yellow and green seeds, the offspring had yellow seeds, when he crossed peas with smooth and wrinkled seeds, the offspring had smooth seeds. The offspring from tall and low plants were tall.

So, first generation hybrids always acquire one of the parental traits. One sign (stronger, dominant) always suppresses another (weaker, recessive). Such a phenomenon is called complete dominance.

If we apply the above rule to a person, say, using the example of brown and blue eyes, then it is explained as follows. If in one homozygous parent in the genome both genes determine the brown color of the eyes (we denote such a genotype as AA), and in the other, also homozygous, both genes determine the blue color of the eyes (we denote such a genotype as aa), then the haploid gametes produced by them will always carry either the gene BUT, or a(see diagram below).

Scheme of the transfer of traits when crossing homozygous organisms

Then all children will have the genotype Ah, but everyone will have brown eyes because the brown eye gene is dominant over the blue eye gene.

Now consider what happens if heterozygous organisms (or first-generation hybrids) are crossed. In this case, it will feature splitting in certain quantitative terms.

The law of feature splitting, or Mendel's First Law

If a heterozygous descendants of the first generation, identical in terms of the studied trait, are crossed among themselves, then in the second generation, the traits of both parents appear in a certain numerical ratio: 3/4 individuals will have a dominant trait, 1/4 will have a recessive trait(see diagram below).

The scheme of inheritance of traits when crossing heterozygous organisms

The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some are recessive, is called splitting. As we understand, the recessive trait in the hybrids of the first generation did not disappear, but was only suppressed and appeared in the second hybrid generation. Mendel was the first to understand that during the formation of hybrids, hereditary factors do not mix and do not “blur”, but remain unchanged. In a hybrid organism, both factors (genes) are present, but only the dominant hereditary factor manifests itself as a trait.

Communication between generations during sexual reproduction is carried out through germ cells, each gamete carries only one factor from a pair. The fusion of two gametes, each carrying one recessive hereditary factor, will result in an organism with a recessive trait. The fusion of gametes, each of which carries a dominant factor, or two gametes, one of which contains a dominant and the other a recessive factor, leads to the development of an organism with a dominant trait.

Mendel explained the splitting during crossing of heterozygous individuals by the fact that gametes carry only one gene from an allelic pair ( law of gamete purity). Indeed, this is possible only if the genes remain unchanged and the gametes contain only one gene from a pair. It is convenient to study feature relationships using the so-called Punnett lattice:

	A (0.5)	a (0.5)
A (0.5)	AA (0.25)	Aa (0.25)
a (0.5)	Aa (0.25)	aa (0.25)

Due to the statistical probability, with a sufficiently large number of gametes in the offspring, 25% of the genotypes will be homozygous dominant, 50% - heterozygous, 25% - homozygous recessive, i.e., a mathematical ratio is established AA:2Ah:1aa. Accordingly, according to the phenotype, the offspring of the second generation during monohybrid crossing is distributed in a ratio of 3: 1 - 3 parts of individuals with a dominant trait, 1 part of individuals with a recessive one.

It should not be forgotten that the distribution of genes and their entry into gametes is of a probabilistic nature. Mendel's approach to the analysis of descendants was quantitative, statistical: all descendants with a given trait state (for example, smooth or wrinkled peas) were combined into one group, their number was counted, which was compared with the number of descendants with a different trait state (wrinkled peas). This pairwise analysis ensured the success of his observations. In the case of a person, it can be very difficult to observe such a distribution - it is necessary that one pair of parents have at least a dozen children, which is a rather rare occurrence in modern society. So it may well happen that one single child is born to brown-eyed parents, and that blue-eyed one, which, at first glance, violates all the laws of genetics. At the same time, if you experiment with Drosophila or laboratory mice, Mendelian laws are quite easy to observe.

It should be said that in a certain sense Mendel was lucky - from the very beginning he chose a suitable plant as an object - colored peas. If he came across, for example, such plants as the night beauty or snapdragon, then the result would be unpredictable. The fact is that in snapdragons, heterozygous plants obtained by crossing homozygous plants with red and white flowers have pink flowers. However, none of the alleles can be called either dominant or recessive. This phenomenon can be explained by the fact that complex biochemical processes due to different work of alleles do not necessarily lead to alternative mutually exclusive results. The result can be intermediate, depending on the characteristics of the metabolism in a given organism, in which there are always many options, shunting mechanisms or parallel processes with various external manifestations.

This phenomenon is called incomplete dominance or codominance, it is quite common, including in humans. An example is the human blood type system MN (note in passing that this is only one of the systems, there are many classifications of blood groups). At one time, Landsteiner and Levin explained this phenomenon by the fact that erythrocytes can carry on their surface either one antigen (M), or the other (N), or both together (MN). If in the first two cases we are dealing with homozygotes (MM and NN), then in the heterozygous state (MN) both alleles manifest themselves, while both appear (dominate), hence the name - codominance.

The Law of Independent Inheritance of Characters, or Mendel's Second Law

This law describes the distribution of features under the so-called dihybrid and polyhybrid crossing, i.e. when the crossed individuals differ in two or more characteristics. In Mendel's experiments, plants were crossed that differed in several pairs of traits, such as: 1) white and purple flowers, and 2) yellow or green seeds. At the same time, the inheritance of each trait followed the first two laws, and the traits were combined independently of each other. As expected, the first generation after crossing had a dominant phenotype in all respects. The second generation followed the formula 9:3:3:1, that is, 9/16 copies were with purple flowers and yellow peas, 3/16 with white flowers and yellow peas, another 3/16 with purple flowers and green peas, and, finally, 1/16 - with white flowers and green peas. This was because Mendel successfully chose traits whose genes were on different pea chromosomes. Mendel's second law is fulfilled only in cases where the analyzed pairs of genes are located on different chromosomes. According to the gamete frequency rule, traits are combined independently of each other, and if they are on different chromosomes, then the inheritance of traits occurs independently.

The 1st and 2nd laws of Mendel are universal, but exceptions are constantly found from the 3rd law. The reason for this becomes clear if we remember that there are many genes in one chromosome (in humans, from several hundred to a thousand or more). If the genes are on the same chromosome, then linked inheritance. In this case, signs are transmitted in pairs or groups. Genes located on the same chromosome are called in genetics clutch groups. Most often, traits determined by genes that are close to each other on the chromosome are transmitted together. Such genes are called closely linked. At the same time, genes located far from each other are sometimes linked together. The reason for this different behavior of genes is a special phenomenon exchange of material between chromosomes during gamete formation, in particular, at the stage of prophase of the first division of meiosis.

This phenomenon was studied in detail by Barbara McClintock (Nobel Prize in Physiology or Medicine in 1983) and was called crossing over. Crossing over is nothing more than the exchange of homologous regions between chromosomes. It turns out that each particular chromosome does not remain unchanged during transmission from generation to generation, it can “take with it” a homologous section from its paired chromosome, giving that, in turn, a section of its DNA.

In the case of humans, it is quite difficult to establish the linkage of genes, as well as to identify crossing over due to the impossibility of arbitrary crossings (you can’t force people to give offspring in accordance with some scientific tasks!), Therefore, such data were obtained mainly on plants, insects and animals . Nevertheless, thanks to the study of large families in which there are several generations, examples of autosomal linkage (ie, joint transmission of genes located on autosomes) are known in humans. For example, there is a close linkage between the genes that control the Rh factor (Rh) and the MNS blood group antigen system. In humans, cases of linkage of certain traits with sex, that is, in connection with the sex chromosomes, are better known.

Crossing over generally enhances combinative variability, i.e., contributes to a greater diversity of human genotypes. In this regard, this process is of great importance for. Using the fact that the farther apart the genes are located on the same chromosome, the more they are subject to crossing over, Alfred Sturtevant built the first maps of Drosophila chromosomes. Today, complete physical maps of all human chromosomes have been obtained, that is, it is known in what sequence and which genes are located on them.

Mendel's laws

Scheme of the first and second law of Mendel. 1) A plant with white flowers (two copies of the recessive allele w) is crossed with a plant with red flowers (two copies of the dominant allele R). 2) All descendant plants have red flowers and the same Rw genotype. 3) At self-fertilization, 3/4 of the plants of the second generation have red flowers (RR + 2Rw genotypes) and 1/4 have white flowers (ww).

Mendel's laws- these are the principles of the transmission of hereditary traits from parent organisms to their descendants, arising from the experiments of Gregor Mendel. These principles formed the basis for classical genetics and were subsequently explained as a consequence of the molecular mechanisms of heredity. Although three laws are usually described in Russian-language textbooks, the "first law" was not discovered by Mendel. Of particular importance among the regularities discovered by Mendel is the “gamete purity hypothesis”.

Story

At the beginning of the 19th century, J. Goss, experimenting with peas, showed that when plants with greenish-blue peas and yellowish-white ones were crossed, yellow-white ones were obtained in the first generation. However, during the second generation, recessive traits that did not appear in hybrids of the first generation, and later called by Mendel, reappeared, and plants with them did not give splitting during self-pollination.

O. Sarzhe, conducting experiments on melons, compared them according to individual characteristics (pulp, peel, etc.) also established the absence of mixing of characteristics that did not disappear among the descendants, but only redistributed among them. S. Noden, crossing different types of dope, discovered the predominance of signs of dope Datula tatula above Datura stramonium, and it did not depend on which plant is the mother and which is the father.

Thus, by the middle of the 19th century, the phenomenon of dominance, the uniformity of hybrids in the first generation (all hybrids of the first generation are similar to each other), splitting and combinatorics of characters in the second generation, were discovered. Nevertheless, Mendel, highly appreciating the work of his predecessors, pointed out that they had not found a universal law for the formation and development of hybrids, and their experiments did not have sufficient reliability to determine numerical ratios. Finding such a reliable method and mathematical analysis of the results, which helped create the theory of heredity, is the main merit of Mendel.

Methods and course of Mendel's work

• Mendel studied how individual traits are inherited.
• Mendel chose from all the signs only alternative ones - those that had two clearly different options for his varieties (seeds are either smooth or wrinkled; there are no intermediate options). Such a conscious narrowing of the research task made it possible to clearly establish the general patterns of inheritance.
• Mendel planned and carried out a massive experiment. He received 34 varieties of peas from seed companies, from which he selected 22 "pure" (not splitting according to the studied characteristics during self-pollination) varieties. Then he carried out artificial hybridization of varieties, and the resulting hybrids crossed with each other. He studied the inheritance of seven traits, studying a total of about 20,000 second-generation hybrids. The experiment was facilitated by the successful choice of the object: the pea is normally a self-pollinator, but it is easy to carry out artificial hybridization.
• Mendel was one of the first in biology to use precise quantitative methods to analyze data. Based on his knowledge of probability theory, he realized the need to analyze a large number of crosses to eliminate the role of random deviations.

The manifestation in hybrids of the trait of only one of the parents Mendel called dominance.

The law of uniformity of hybrids of the first generation(Mendel's first law) - when crossing two homozygous organisms belonging to different pure lines and differing from each other in one pair of alternative manifestations of the trait, the entire first generation of hybrids (F1) will be uniform and will carry the manifestation of the trait of one of the parents.

This law is also known as "the law of trait dominance". Its formulation is based on the concept clean line regarding the trait under study - in modern language, this means the homozygosity of individuals for this trait. Mendel, on the other hand, formulated the purity of a trait as the absence of manifestations of opposite traits in all descendants in several generations of a given individual during self-pollination.

When crossing pure lines of peas with purple flowers and peas with white flowers, Mendel noticed that the ascended descendants of plants were all with purple flowers, among them there was not a single white one. Mendel repeated the experiment more than once, using other signs. If he crossed peas with yellow and green seeds, all the descendants had yellow seeds. If he crossed peas with smooth and wrinkled seeds, the offspring had smooth seeds. The offspring from tall and low plants were tall. So, hybrids of the first generation are always uniform in this trait and acquire the trait of one of the parents. This sign (stronger, dominant), always suppressed the other ( recessive).

Co-dominance and incomplete dominance

Some opposite signs are not in relation to complete dominance (when one always suppresses the other in heterozygous individuals), but in relation to incomplete dominance. For example, when crossing pure snapdragon lines with purple and white flowers, first-generation individuals have pink flowers. When crossing pure lines of black and white Andalusian chickens, gray chickens are born in the first generation. With incomplete dominance, heterozygotes have signs intermediate between those of recessive and dominant homozygotes.

The phenomenon in which the crossing of heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some of which are recessive, is called splitting. Therefore, splitting is the distribution of dominant and recessive traits among offspring in a certain numerical ratio. The recessive trait in hybrids of the first generation does not disappear, but is only suppressed and manifests itself in the second hybrid generation.

Explanation

The law of purity of gametes: only one allele from a pair of alleles of a given gene of the parent individual enters each gamete.

Normally, the gamete is always pure from the second gene of the allelic pair. This fact, which could not be firmly established in Mendel's time, is also called the hypothesis of gamete purity. Later this hypothesis was confirmed by cytological observations. Of all the patterns of inheritance established by Mendel, this "Law" is the most general in nature (it is carried out under the widest range of conditions).

Law of independent inheritance of traits

Illustration of independent trait inheritance

Definition

Law of Independent Succession(Mendel's third law) - when crossing two homozygous individuals that differ from each other in two (or more) pairs of alternative traits, genes and their corresponding traits are inherited independently of each other and are combined in all possible combinations (as in monohybrid crossing). When plants differing in several characters, such as white and purple flowers and yellow or green peas, were crossed, the inheritance of each of the characters followed the first two laws, and in the offspring they were combined in such a way as if their inheritance occurred independently of each other. The first generation after crossing had a dominant phenotype in all respects. In the second generation, a splitting of phenotypes was observed according to the formula 9:3:3:1, that is, 9:16 were with purple flowers and yellow peas, 3:16 with white flowers and yellow peas, 3:16 with purple flowers and green peas, 1 :16 with white flowers and green peas.

Explanation

Mendel came across traits whose genes were located in different pairs of homologous pea chromosomes. During meiosis, homologous chromosomes of different pairs combine in gametes randomly. If the paternal chromosome of the first pair got into the gamete, then both the paternal and maternal chromosomes of the second pair can get into this gamete with equal probability. Therefore, traits whose genes are located in different pairs of homologous chromosomes are combined independently of each other. (Subsequently, it turned out that of the seven pairs of traits studied by Mendel in peas, in which the diploid number of chromosomes is 2n = 14, the genes responsible for one of the pairs of traits were located on the same chromosome. However, Mendel did not find a violation of the law of independent inheritance, so how linkage between these genes was not observed due to the large distance between them).

The main provisions of Mendel's theory of heredity

In the modern interpretation, these provisions are as follows:

• Discrete (separate, non-mixing) hereditary factors - genes - are responsible for hereditary traits (the term "gene" was proposed in 1909 by W. Johannsen)
• Each diploid organism contains a pair of alleles of a given gene responsible for a given trait; one of them is received from the father, the other - from the mother.
• Hereditary factors are passed on to offspring through germ cells. During the formation of gametes, only one allele from each pair gets into each of them (gametes are “pure” in the sense that they do not contain the second allele).

Conditions for the implementation of Mendel's laws

According to Mendel's laws, only monogenic traits are inherited. If more than one gene is responsible for a phenotypic trait (and there are an absolute majority of such traits), it has a more complex inheritance pattern.

Conditions for the fulfillment of the law of splitting in monohybrid crossing

Splitting 3: 1 by phenotype and 1: 2: 1 by genotype is performed approximately and only under the following conditions.

The patterns of inheritance were formulated in 1865 by Gregory Mendel in his work "Experiments on Plant Hybrids". In his experiments, he crossed different varieties of peas (Czech Republic / Austria-Hungary). In 1900, the patterns of inheritance were rediscovered by Correns, Cermak and Gogo de Vries.

The first and second laws of Mendel are based on monohybrid crosses, and the third - on di and polyhybrid. Monohybrid crossing occurs on one pair of alternative traits, dihybrid in two pairs polyhybrid - more than two. The success of Mendel is due to the peculiarities of the applied hybridological method:

The analysis begins with crossing pure lines: homozygous individuals.

Separate alternative mutually exclusive signs are analyzed.

Accurate quantitative accounting of descendants with different combinations of traits

The inheritance of the analyzed traits can be traced in a number of generations.

The rule for writing out gametes according to the formula 2n , where n is the number of heterozygotes: for monohybrids - 2 varieties of gametes, for dihybrids - 4, for trihybrids - 8.

Mendel's 1st law: "The law of uniformity of hybrids of the 1st generation"

When crossing homozygous individuals analyzed for one pair of alternative traits, hybrids of the 1st generation show only dominant traits and uniformity in phenotype and genotype is observed.

In his experiments, Mendel crossed pure lines of pea plants with yellow (AA) and green (aa) seeds. It turned out that all descendants in the first generation are identical in genotype (heterozygous) and phenotype (yellow).

2nd Mendel's Law: "The Law of Splitting"

When crossing heterozygous hybrids of the 1st generation, analyzed by one pair of alternative traits, the hybrids of the second generation show splitting according to the phenotype 3:1, and according to the genotype 1:2:1

In his experiments, Mendel crossed the hybrids obtained in the first experiment (Aa) with each other. It turned out that in the second generation, the suppressed recessive trait reappeared. The data of this experiment testify to the splitting of the recessive trait: it is not lost, but appears again in the next generation.

Cytological basis of Mendel's 2nd law

The cytological foundations of Mendel's 2nd law are revealed in hypothesis of "purity of gametes" . Crossing schemes show that each trait is determined by a combination of two allelic genes. When heterozygous hybrids are formed, allelic genes do not mix, but remain unchanged. As a result meiosis in gametogenesis, only 1 of a pair of homologous chromosomes enters each gamete. Therefore, only one of the pair of allelic genes, i.e. the gamete is pure relative to another allelic gene.

3rd Mendel's Law: "The Law of Independent Combination of Features"

When crossing homozygous organisms analyzed for two or more pairs of alternative traits, in hybrids of its 3rd generation (obtained by crossing hybrids of the 2nd generation), an independent combination of traits and their corresponding genes of different allelic pairs is observed.

To study the patterns of inheritance plants , differing in one pair of alternative signs, Mendel used monohybrid cross . Then he moved on to experiments on crossing plants that differ in two pairs of alternative traits: dihybrid cross , where he used homozygous pea plants that differ in color and shape of seeds. As a result of crossing smooth (B) and yellow (A) with wrinkled (b) and green (a), in the first generation all plants were with yellow smooth seeds.

Thus, the law of uniformity of the first generation is manifested not only in mono, but also in polyhybrid crossing, if the parental individuals are homozygous.

During fertilization, a diploid zygote is formed due to the fusion of different varieties of gametes. English geneticist Bennet to facilitate the calculation of options for their combination, he proposed writing in the form gratings - tables with the number of rows and columns according to the number of types of gametes formed by crossing individuals.

Analyzing cross

Since individuals with a dominant trait in the phenotype may have a different genotype (Aa and AA), Mendel proposed to cross this organism with recessive homozygote .

A homozygous individual will give uniform generation,

and heterozygous - split by phenotype and genotype 1:1.

Mogran's chromosome theory. Linked inheritance

Establishing patterns of inheritance, Mendel crossed pea plants. Thus, his experiments were carried out at the organismal level. The development of the microscope at the beginning of the 20th century made it possible to identify cells - the material carrier of hereditary inf, transferring research to the cellular level. Based on the results of numerous experiments with fruit flies, in 1911 Thomas Morgan formulated the main provisions of the chromosome theory of heredity .

The genes on a chromosome are arranged in a linear fashion loci . Allelic genes occupy the same loci of homologous chromosomes.

Genes located on the same chromosome form clutch group and are predominantly inherited together. The number of linkage groups is equal to n sets of chromosomes.

between homologous chromosomes crossing over - exchange of sites, which can disrupt the linkage of genes. The probability that genes will remain linked is directly proportional to the distance between them: the closer the genes are located on the chromosome, the higher the probability of their linkage. This distance is calculated in morganides: 1 morganide corresponds to 1% of the formation of crossover gametes.

For his experiments, Morgan used fruit flies that differ in 2 pairs of traits: gray (B) and black (b); the length of the wings is normal (V) and short (v).

1) Dihybrid cross – first, homozygous individuals of AABB and aabb were crossed. Thus, results similar to Mendel were obtained: all individuals with a gray body and normal wings.

2) Analyzing cross was carried out with the aim of breeding the genotype of hybrids of the 1st generation. A diheterozygous male was crossed with a recessive dihomozygous female. According to Mendel's 3rd law, one could expect the appearance of 4 phenotypes due to an independent combination of traits: sn (BbVv), chk (bbvv), sk (Bbvv), ch (bbVv) in a ratio of 1:1:1:1. However, only 2 combinations were obtained: sn (BbVv) hk (bbvv).

Thus, in the second generation, only original phenotypes in a ratio of 1:1.

Such a deviation from the free combination of traits is due to the fact that the genes that determine body color and wing length in fruit flies are located in on the same chromosome and are inherited in a linked fashion . It turns out that a diheterozygous male gives only 2 varieties of non-crossover gametes, and not 4, as in the case of dihybrid crossing of organisms with unlinked traits.

3) Analyzing reciprocal crossing - a system of crosses in which genotypically different parental individuals are used once as a maternal form, another time as a paternal form.

This time, Morgan used a diheterozygous female and a homozygous recessive male. Thus, 4 phenotypes were obtained, however, their ratio did not correspond to that observed in Mendel with an independent combination of traits. The number of sn and chk was 83% of the total offspring, and the number of sk and chk was only 17%.

Linkage between genes located on the same chromosome is broken as a result of crossing over . If the chromosome break point lies between linked genes, then the linkage is broken, and one of them passes into a homologous chromosome. So, in addition to two varieties non-crossover gametes , two more varieties are formed crossover gametes in which chromosomes have exchanged homologous regions. Crossover individuals develop from them upon merging. According to the position of the chromosome theory, the distance between the genes that determine body color and wing length in Drosophila is 17 morganids - 17% of crossover gametes and 83% of non-crossover ones.

Allelic interaction of genes

1) Incomplete dominance: when crossing homozygous sweet pea plants with red and white flowers, all offspring in the first generation have pink flowers - an intermediate form. In the second generation, phenotype splitting corresponds to genotype splitting in relation to 1cr: 2rose: 1bel.

2) Overdominance : at the dominant allele in a heterozygote the trait is more pronounced than in the homozygote. At the same time, the heterozygous organism Aa has better fitness than both types of homozygotes.

Sickle cell anemia is caused by a mutant s allele. In areas where malaria is prevalent, Ss heterozygotes are more resistant to malaria than SS homozygotes.

3) Codominance : in the phenotype of heterozygotes, both allelic genes are manifested, as a result of which a new trait is formed. But it is impossible to call one allele dominant and the other recessive, since they equally affect the phenotype.

Formation of the 4th blood group in humans. Allele Ia determines the presence of antigen a on erythrocytes, allele Ib - the presence of antigen b. The presence of both alleles in the genotype causes the formation of both antigens on erythrocytes.

4) Multiple alleles: there are more than two allelic genes in a population. Such genes arise as a result of mutation of the same locus of the chromosome. In addition to the dominant and recessive genes, there are intermediate alleles , which behave as recessive in relation to the dominant, and as dominant in relation to the recessive. Each diploid individual can have no more than two allelic genes, but their number in a population is not limited. The more allelic genes, the more options for their combinations. All alleles of one gene are designated by one letter with different indices: A1, A2, a3, etc.

In guinea pigs, the coat color is determined by 5 alleles of one locus, which in various combinations give 11 color options. In humans, according to the type of multiple alleles, blood groups are inherited according to the ABO system. Three genes Io, Ia, Ib determine the inheritance of 4 human blood groups (genes Ia Ib are dominant in relation to Io).

Non-allelic interaction of genes

1) Complimentary or complementary interaction of genes - a phenomenon in which two non-allelic dominant or recessive gene give new feature . This interaction of genes is observed when the forms of the crest are inherited in chickens:

A pea-shaped (A-cc); B- rose-shaped (aaB-); AB walnut; aavb leafy.

When crossing chickens with pea and rose combs, all 1st generation hybrids will have a walnut comb. When crossing dihybrids of the 1st generation with walnut combs, in the 2nd generation individuals with all types of combs appear in the ratio 9op: 3rose: 3gor: 1 leaf. However, unlike Mendel's 3rd Law segregation, there is no 3:1 segregation of each allele. In other cases of complementarity, perhaps 9:7 and 9:6:1.

2) epistasis or epistatic interaction of genes - suppression the action of the genes of one allele by the genes of another. The suppressor gene is a suppressor or inhibitor.

Dominant epistasis - dominant suppressor gene: inheritance of feather color in chickens. C - pigment synthesis, I - suppressor gene. Chickens with the C-II genotype will be dyed. The remaining individuals will be white, since in the presence of a dominant suppressor gene, the suppressed color gene does not appear, or the gene responsible for pigment synthesis (ccii) is absent. In the case of crossing dihybrids, splitting in the second generation will be 13:3 or 12:3:1.

recessive epistasis - the suppressor genome is a recessive gene, for example, the inheritance of the color of mice. B - synthesis of gray pigment, b - black; A contributes to the manifestation of color, and - suppresses it. Epistasis will only occur in cases where there are two aa suppressor genes in the genotype. When crossing dihybrid individuals with recessive epistasis, splitting in the second generation is 9:3:4.

Bombay Phenomenon manifested in the inheritance of blood groups according to the ABO system. A woman with blood group 1 (IoIo), who married a man with blood group 2 (IaIo), gave birth to two girls with 4 (IaIb) and 1 (IoIo) groups. This is explained by the fact that their mother possessed the Ib allele, but its action was suppressed by a rare recessive gene, which, in the homozygous state, exerted its epistatic effect. As a result, the woman phenotypically manifested 1st group.

3) Polymerism - the same trait is determined by several alleys. At the same time, dominant genes from different allelic pairs affect the degree of manifestation of one trait. It depends on the number of dominant genes in the genotype (the more dominant genes, the more pronounced the trait) and on the influence of environmental conditions.

Polymeric genes are usually denoted by one letter of the Latin alphabet with numerical indices A 1 A 2 a 3, etc. They define polygenic traits . This is how many quantitative and some qualitative traits are inherited in animals and humans: height, weight, skin color. Inheritance of the color of wheat grains: each of the dominant genes determines the red color, the recessive genes determine the white color. With an increase in the number of dominant genes, the color intensity increases. And only if the organism is homozygous for all pairs of recessive genes, the grains are not colored. So when crossing dihybrids, splitting in relation to 15okr: 1bel.

4) Pleiotropy One gene affects several traits. The phenomenon was described by Mendel, who found that hereditary factors in pea plants can determine several traits: red flowers, gray seeds, and a pink spot at the base of the leaves. It often extends to evolutionarily important traits: fertility, longevity, the ability to survive in extreme environmental conditions.

In some cases, a pleiotropic gene is dominant in relation to one trait, and recessive in relation to another. If a pleiotropic gene is only dominant or only recessive in relation to all the traits it defines, then the nature of inheritance is similar to the patterns of Mendel's laws.

A kind of splitting is observed when one of the traits is recessive or lethal (homozygous leads to death). For example, the black wool of Karakul sheep and the development of the scar are determined by one gene, and the gray wool and the underdeveloped scar are determined by the gene allelic to it. Gray dominates over black, norm over anomaly. Homozygous individuals for the gene of underdevelopment of the scar and gray color die, therefore, when heterozygous individuals are crossed, a quarter of the offspring (gray homozygotes) are not viable. Splitting in the ratio 2:1.

penetrance and expressivity

The genotype of an individual determines only the potential for the development of a trait: the realization of a gene into a trait depends on the influence of other genes and environmental conditions, so the same hereditary information manifests itself differently under different conditions. Consequently, it is not a ready-made attribute that is inherited, but the type of reaction to the action of the environment.

Penetrance - penetration of a gene into a trait. It is expressed as a percentage of the number of individuals carrying a trait to the total number of gene carriers potentially capable of being realized in this trait. Complete penetrance (100%) - all carriers of the gene have a phenotypic manifestation of the trait. Incomplete - the action of the gene is not manifested in all carriers.

If a gene has broken into a trait, it is penetrant, but it can manifest itself in different ways. expressiveness - the degree of expression of the sign. A gene that causes a decrease in the number of facets of the eye in Drosophila has a different expressivity. Homozygotes have a different number of facets, up to their complete absence.

Penetrance and expressivity depend on the influence of other genes and the environment.

Variability

Variability - the ability to acquire new features under the influence of external and internal environmental factors (morphological, physiological, biochemical). The variety of individuals of the same species is associated with variability, which serves as material for evolutionary processes. The unity of heredity and variability is a condition for ongoing biological evolution. There are several types:

1) Hereditary, genotypic, indeterminate, individual

It is hereditary in nature, and is due to the recombination of genes in the genotype and mutations, is inherited. There are combinative and mutational

2) Non-hereditary, modification, phenotypic, group, specific

Modification variability - evolutionarily fixed adaptive reactions of the body in response to changes in environmental conditions, a consequence of the interaction of the environment and the genotype. It is not inherited, as it does not lead to a change in the genotype. Unlike mutations, many modifications are reversible: sunburn, milk yield of cows, etc.

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USE section: 3.5. Patterns of heredity, their cytological basis. The patterns of inheritance established by G. Mendel, their cytological foundations (mono- and dihybrid crossing) ...

Mendel, conducting experiments on crossing different varieties of peas, he established a number of laws of inheritance that laid the foundation for genetics. He developed hybrid-logical method of inheritance analysis signs of organisms. This method involves crossing individuals with alternative traits; analysis of the studied traits in hybrids without taking into account the rest; quantitative accounting of hybrids.

Conducting monohybrid crossing (crossing for one pair of alternative traits), Mendel established law of uniformity first generation.

Key points hybridological method

• For crossing, organisms are taken whose ancestors in a number of generations did not give splitting according to selected traits, that is, pure lines.
• Organisms differ in one or two pairs of alternative traits.
• An individual analysis of the offspring of each cross is carried out.
• Statistical processing of results is used.

■ The first law of G. Mendel

When two homozygous individuals are crossed, differing from each other in one pair of alternative traits, all offspring in the first generation are uniform both in phenotype and genotype.

■ Second lawG.mendel

When hybrids of the first generation (two heterozygous individuals) are crossed, a splitting of 3: 1 occurs in the second. Along with the dominant, a recessive trait also appears.

Analyzing cross- crossing, in which an individual with an unknown genotype to be established (AA or Aa) is crossed with a recessive homozygote (aa). If all the offspring from the total crossing will be uniform, the organism under study has the AA genotype. If 1:1 phenotype splitting is observed in the offspring of Sudet, the organism under study is heterozygous Aa.

■ The thirdlawG.mendel

When crossing homozygous individuals that differ in two or more pairs of alternative traits, each trait is inherited independently of the others, combining in all possible combinations.

In his experiments, Mendel used different crossing methods : monohybrid, dihybrid and polyhybrid. At the last crossing, individuals differ in more than two pairs of characters. In all cases, the law of uniformity of the first generation, the law of splitting characteristics in the second generation, and the law of independent inheritance are observed.

Law of independent inheritance: each pair of traits is inherited independently of each other. In the offspring, there is a splitting according to the phenotype 3: 1 for each pair of traits. The law of independent inheritance is valid only if the genes of the pairs of traits under consideration lie in different pairs of homologous chromosomes. Homologous chromosomes are similar in shape, size, and gene linkage groups.

The behavior of any pairs of nonhomologous chromosomes in meiosis does not depend on each other. Discrepancy: them to cell poles is random. Independent inheritance is of great importance for evolution; because it is the source of combinative heredity.

TABLE: all patterns of inheritance

This is a biology abstract for grades 10-11 on the topic “Patterns of heredity. Morgan's laws". Choose a next action:

We paid attention to the fact that heredity and inheritance are two different phenomena that not everyone strictly distinguishes.

Heredity there is a process of material and functional discrete continuity between generations of cells and organisms. It is based on the exact reproduction of hereditarily significant structures.

Inheritance - the process of transferring hereditarily determined signs and properties of an organism and a cell in the process of reproduction. The study of inheritance allows you to reveal the essence of heredity. Therefore, it is necessary to strictly separate these two phenomena.

The patterns of splitting and independent combination that we have considered are related to the study of inheritance, and not heredity. Wrong when " splitting law" and " law of independent combination of trait-genes are interpreted as the laws of heredity. The laws discovered by Mendel are the laws of inheritance.

At the time of Mendel, it was believed that when crossing, parental traits are inherited in the offspring together ("fused heredity") or mosaic - some traits are inherited from the mother, others from the father ("mixed heredity"). Such ideas were based on the belief that in the offspring the heredity of the parents mixes, merges, and dissolves. This notion was wrong. It did not make it possible to scientifically argue the theory of natural selection, and in fact, if during crossing, hereditary adaptive traits in the offspring were not preserved, but "dissolved", then natural selection would work in vain. To free his theory of natural selection from such difficulties, Darwin put forward the theory of the hereditary determination of a trait by individual units - the theory of pangenesis. However, she did not give a correct answer to the question.

Mendel's success is due to the discovery of a method of genetic analysis of individual pairs of hereditary traits; Mendel developed method of discrete analysis of trait inheritance and essentially created the scientific foundations of genetics, discovering the following phenomena:

1. each hereditary trait is determined by a separate hereditary factor, deposit; in the modern view, these inclinations correspond to genes: “one gene - one trait”, “one gene - one enzyme”;
2. genes are preserved in their pure form in a number of generations, without losing their individuality: this was proof of the basic position of genetics: the gene is relatively constant;
3. both sexes equally participate in the transmission of their hereditary properties to offspring;
4. reduplication of an equal number of genes and their reduction in male and female germ cells; this position was a genetic prediction of the existence of meiosis;
5. hereditary inclinations are paired, one is maternal, the other is paternal; one of them may be dominant, the other recessive; this provision corresponds to the discovery of the principle of allelism: a gene is represented by at least two alleles.

Thus, Mendel, having discovered the method of genetic analysis of the inheritance of individual pairs of traits (rather than a combination of traits) and establishing the laws of inheritance, for the first time postulated and experimentally proved the principle of discrete (genetic) determination of hereditary traits.

Based on the foregoing, it seems useful to us to distinguish between the laws directly formulated by Mendel and related to the process of inheritance, and the principles of heredity that follow from the work of Mendel.

The laws of inheritance include the law of splitting of hereditary traits in the offspring of a hybrid and the law of independent combination of hereditary traits. These two laws reflect the process of transmission of hereditary information in cell generations during sexual reproduction. Their discovery was the first actual proof of the existence of heredity as a phenomenon.

The laws of heredity have a different content, and they are formulated in the following form:

First law- the law of discrete (genetic) hereditary determination of traits; it underlies the theory of the gene.

Second law- the law of the relative constancy of the hereditary unit - the gene.

third law- the law of the allelic state of the gene (dominance and recessiveness).

It is these laws that represent the main result of Mendel's work, since it is they that reflect the essence of heredity.

Mendelian laws of inheritance and laws of heredity are the main content of genetics. Their discovery gave modern natural science a unit of measurement of life processes - the gene, and thereby created the possibility of combining the natural sciences - biology, physics, chemistry and mathematics with the aim of analyzing biological processes.

In the future, when defining the hereditary unit, we will use only the term "gene". The concepts of "hereditary factor" and "hereditary inclination" are cumbersome, and, moreover, the time has probably come when the hereditary factor and the gene should be distinguished and put into each of these concepts its own content. By the concept of "gene" we mean hereinafter an indivisible functionally integral unit of heredity, which determines a hereditary trait. The term "hereditary factor" should be interpreted in a broader sense as a complex of a number of genes and cytoplasmic influences on a hereditary trait.

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