This law states that crossing individuals that differ in this trait (homozygous for different alleles) gives genetically homogeneous offspring (generation F 1), all individuals of which are heterozygous. All F 1 hybrids can have either the phenotype of one of the parents (complete dominance), as in Mendel's experiments, or, as was later discovered, an intermediate phenotype (incomplete dominance). Later it turned out that hybrids of the first generation F 1 can show signs of both parents (codominance). This law is based on the fact that when two forms homozygous for different alleles (AA and aa) are crossed, all their descendants are identical in genotype (heterozygous - Aa), and therefore in phenotype.

2.3. Law of splitting (Mendel's second law)

This law is called the law of (independent) splitting. Its essence is as follows. When an organism that is heterozygous for the trait under study forms germ cells - gametes, then one half of them carries one allele of a given gene, and the other half carries the other. Therefore, when such F1 hybrids are crossed among themselves, individuals with the phenotypes of both the original parental forms and F1 appear in certain proportions among the second-generation F2 hybrids.

This law is based on the regular behavior of a pair of homologous chromosomes (with alleles A and a), which ensures the formation of two types of gametes in F1 hybrids, as a result of which individuals of three possible genotypes are identified among F2 hybrids in the ratio 1AA: 2 Aa: 1aa. In other words, the "grandchildren" of the original forms - two homozygotes, phenotypically different from each other, give a phenotypic split in accordance with Mendel's second law.

However, this ratio may vary depending on the type of inheritance. So, in the case of complete dominance, 75% of individuals with a dominant and 25% with a recessive trait are distinguished, i.e. two phenotypes in a 3:1 ratio. With incomplete dominance and codominance, 50% of the second generation hybrids (F2) have the phenotype of the first generation hybrids and 25% each have the phenotypes of the original parental forms, i.e. a splitting of 1:2:1 is observed.

2.4. Law of independent combination (inheritance) of traits (Mendel's third law)

This law says that each pair of alternative traits behaves independently of each other in a number of generations, as a result of which, among the descendants of the first generation (i.e., in the F2 generation), individuals with new (compared to parental) traits appear in a certain ratio. combinations of features. For example, in the case of complete dominance when crossing the original forms that differ in two traits, in the next generation (F2), individuals with four phenotypes are revealed in a ratio of 9:3:3:1. At the same time, two phenotypes have “parental” combinations of traits, and the remaining two are new. This law is based on the independent behavior (splitting) of several pairs of homologous chromosomes. So, during dihybrid crossing, this leads to the formation of 4 types of gametes (AB, AB, AB, AB) in hybrids of the first generation (F 1), and after the formation of zygotes, to regular splitting according to the genotype and, accordingly, according to the phenotype in the next generation ( F2).

Paradoxically, in modern science, much attention is paid not so much to Mendel's third law in its original formulation, but to exceptions to it. The law of independent combination is not observed if the genes that control the traits under study are linked, i.e. are located next to each other on the same chromosome and are inherited as a connected pair of elements, and not as separate elements. Mendel's scientific intuition told him which traits should be chosen for his dihybrid experiments - he chose unlinked traits. If he had randomly selected traits controlled by linked genes, his results would have been different, since linked traits are not inherited independently of each other.

What is the importance of the exceptions to Mendel's law of independent combination? The fact is that it is these exceptions that make it possible to determine the chromosomal coordinates of genes (the so-called locus).

In cases where the heritability of a certain pair of genes does not obey the third law of Mendel, most likely these genes are inherited together and, therefore, are located on the chromosome in close proximity to each other. The dependent inheritance of genes is called linkage, and the statistical method used to analyze this inheritance is called the linkage method. However, under certain conditions, the patterns of inheritance of linked genes are violated. The main cause of these disorders is the phenomenon of crossing over, leading to recombination (recombination) of genes. The biological basis of recombination lies in the fact that in the process of gamete formation, homologous chromosomes, before separating, exchange their sections.

Crossing over is a probabilistic process, and the probability that a chromosome break will occur or not occur at a given particular site is determined by a number of factors, in particular, the physical distance between two loci of the same chromosome. Crossing over can also occur between neighboring loci, but its probability is much less than the probability of a gap (leading to the exchange of sites) between loci with a large distance between them.

This pattern is used in the preparation of genetic maps of chromosomes (mapping). The distance between two loci is estimated by counting the number of recombinations per 100 gametes. This distance is considered a unit of gene length and is called centimorgan in honor of the geneticist T. Morgan, who first described groups of linked genes in the fruit fly Drosophila, a favorite object of geneticists. If two loci are located at a considerable distance from each other, then the gap between them will occur as often as when these loci are located on different chromosomes.

Using the patterns of reorganization of genetic material during recombination, scientists have developed a statistical method of analysis called linkage analysis.

Mendel's laws in their classical form operate under certain conditions. These include:

1) homozygosity of the initial crossed forms;

2) the formation of gametes of hybrids of all possible types in equal proportions (provided by the correct course of meiosis; the same viability of gametes of all types; the equal probability of meeting any gametes during fertilization);

3) the same viability of zygotes of all types.

Violation of these conditions can lead either to the absence of splitting in the second generation, or to splitting in the first generation; or to a distortion of the ratio of different genotypes and phenotypes. Mendel's laws are universal for all diploid organisms that reproduce sexually. In general, they are valid for autosomal genes with complete penetrance (i.e., 100% frequency of manifestation of the analyzed trait; 100% penetrance implies that the trait is expressed in all carriers of the allele that determines the development of this trait) and constant expressivity (i.e., constant the severity of the symptom); constant expressivity implies that the phenotypic expression of a trait is the same or approximately the same in all carriers of the allele that determines the development of this trait.

Knowledge and application of Mendel's laws is of great importance in medical genetic counseling and determining the genotype of phenotypically "healthy" people whose relatives suffered from hereditary diseases, as well as in determining the risk of developing these diseases in relatives of patients.

Mendel's third law is the law of independent distribution of features. By this is meant that each gene of one allele pair can be in a gamete with any other gene from another allele pair. For example, if an organism is heterozygous for two genes under study (AaBb), then it forms the following types of gametes: AB, Ab, aB, ab. That is, for example, gene A can be in the same gamete with both gene B and b. The same applies to other genes (their arbitrary combination with non-allelic genes).

Mendel's third law is manifesting itself in a dihybrid cross(especially with trihybrid and polyhybrid), when pure lines differ in two studied features. Mendel crossed a variety of peas with smooth yellow seeds with a variety that had green wrinkled seeds, and obtained exclusively yellow smooth seeds F 1 . Then he grew F 1 plants from seeds, allowed them to self-pollinate and received F 2 seeds. And here he observed splitting: plants appeared with both green and wrinkled seeds. The most surprising thing was that among the hybrids of the second generation were not only plants with yellow smooth and green wrinkled seeds. There were also yellow wrinkled and green smooth seeds, i.e., a recombination of characters occurred, and such combinations were obtained that were not found in the original parental forms.

Analyzing the quantitative ratio of different seeds F 2 , Mendel found the following:

    If we consider each trait separately, then it split in a ratio of 3: 1, as in a monohybrid cross. That is, for every three yellow seeds there was one green, and for every 3 smooth seeds - 1 wrinkled.

    Plants with new combinations of traits appeared.

    The phenotype ratio was 9:3:3:1, where for nine yellow smooth pea seeds, there were three yellow wrinkled, three green smooth, and one green wrinkled.

Mendel's third law is well illustrated by the Punnett lattice. Here, in the headings of the rows and columns, possible gametes of the parents (in this case, hybrids of the first generation) are written. The probability of producing each type of gamete is ¼. It is also equally probable that they combine into one zygote.


We see that four phenotypes are formed, two of which did not previously exist. The ratio of phenotypes is 9:3:3:1. The number of different genotypes and their ratio is more complex:

It turns out 9 different genotypes. Their ratio: 4: 2: 2: 2: 2: 1: 1: 1: 1. At the same time, heterozygotes are more common, and homozygotes are less common.

If we return to the fact that each trait is inherited independently, and for each there is a splitting of 3: 1, then we can calculate the probability of phenotypes for two traits of different alleles, multiplying the probability of occurrence of each allele (i.e., it is not necessary to use the Punnett lattice). So, the probability of smooth yellow seeds will be equal to ¾ × ¾ = 9/16, smooth green - ¾ × ¼ = 3/16, wrinkled yellow - ¼ × ¾ = 3/16, wrinkled green - ¼ × ¼ = 1/16. Thus, we get the same ratio of phenotypes: 9:3:3:1.

The third law of Mendel is explained by the independent divergence of homologous chromosomes of different pairs during the first division of meiosis. The chromosome containing the A gene can go into the same cell with the same probability both with the chromosome containing the B gene and with the chromosome containing the b gene. The chromosome with the A gene is not tied to the chromosome with the B gene in any way, although they were both inherited from the same parent. We can say that as a result of meiosis, the chromosomes are mixed. The number of their different combinations is calculated by the formula 2 n , where n is the number of chromosomes of the haploid set. So, if a species has three pairs of chromosomes, then the number of their different combinations will be 8 (2 3).

When the law of independent inheritance of traits does not apply

Mendel's third law, or the law of independent inheritance of traits, is valid only for genes located on different chromosomes or located on the same chromosome, but far enough apart.

Basically, if the genes are on the same chromosome, then they are inherited together, that is, they show linkage with each other, and the law of independent inheritance of traits is no longer valid.

For example, if the genes responsible for the color and shape of pea seeds were on the same chromosome, then the first-generation hybrids could form gametes of only two types (AB and ab), since the parental chromosomes diverge independently during meiosis, but not individual genes. In this case, in the second generation there would be a 3:1 split (three yellow smooth to one wrinkled green).

However, everything is not so simple. Due to the existence in nature of conjugation (rapprochement) of chromosomes and crossing over (exchange of parts of chromosomes), genes located in homologous chromosomes also recombine. So, if a chromosome with genes AB in the process of crossing over exchanges a site with gene B with a homologous chromosome whose site contains gene b, then new gametes (Ab and, for example, aB) can be obtained. The percentage of such recombinant gametes will be less than if the genes were located on different chromosomes. In this case, the probability of crossing over depends on the remoteness of the genes on the chromosome: the farther, the greater the probability.

Mendel's laws - these are the principles of transmission of hereditary traits from parent organisms to their descendants, arising from experiments Gregor Mendel . These principles formed the basis for the 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 "hypothesis of the purity of gametes."

Mendel's laws


The law of uniformity of hybrids of the first generation

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

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 conceptclean line regarding the trait under study - in modern language this means homozygosity 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 clean lines 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).

The law of feature splitting

Definition

The law of splitting, or the second law mendel: when two heterozygous descendants of the first generation are crossed with each other in the second generation, splitting is observed in a certain numerical ratio: by phenotype 3:1, by genotype 1:2:1.

Crossing organisms of two pure lines, differing in the manifestations of one studied trait, for which they are responsible alleles one gene is calledmonohybrid cross .

The phenomenon in which crossing heterozygous individuals leads to the formation of offspring, some of which carry a dominant trait, and some - a recessive one, 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 falls into 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).

Purity hypothesis gametes . Mendel suggested that during the formation of hybrids, hereditary factors do not mix, but remain unchanged. The hybrid has both factors - dominant and recessive, but the manifestation of the trait determines the dominanthereditary factor , recessive is suppressed. Communication between generationssexual reproduction carried out through the sex cells - gametes . Therefore, it must be assumed that each gamete carries only one factor of the pair. Then at 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, manifested phenotypically . 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, will lead to the development of an organism with a dominant trait. Thus, the appearance in the second generation of a recessive trait of one of the parents can only be under two conditions: 1) if the hereditary factors remain unchanged in hybrids; 2) if germ cells contain only one hereditary factor from allelic couples. Mendel explained the splitting of offspring during the crossing of heterozygous individuals by the fact that gametes are genetically pure, that is, they carry only one gene from an allelic pair. The hypothesis (now called the law) of the purity of gametes can be formulated as follows: during the formation of germ cells, only one allele from a pair of alleles of a given gene enters each gamete.

It is known that in each cell organism in most cases there is exactly the same diploid set of chromosomes. Two homologous Chromosomes usually each contain one allele of a given gene. Genetically "pure" gametes are formed as follows:

The diagram shows the meiosis of a cell with a diploid set 2n=4 (two pairs of homologous chromosomes). Paternal and maternal chromosomes are marked with different colors.

In the process of gamete formation in a hybrid, homologous chromosomes during the first meiotic division enter different cells. The fusion of male and female gametes results in a zygote with a diploid set of chromosomes. At the same time, the zygote receives half of the chromosomes from the paternal organism, half from the maternal one. For a given pair of chromosomes (and a given pair of alleles), two varieties of gametes are formed. At fertilization, gametes carrying the same or different alleles randomly meet each other. By virtue of statistical probability with a sufficiently large number of gametes in the offspring 25% genotypes will be homozygous dominant, 50% - heterozygous, 25% - homozygous recessive, that is, the ratio 1AA: 2Aa: 1aa is established (splitting according to the genotype 1: 2: 1). Accordingly, according to the phenotype, the offspring of the second generation during monohybrid crossing is distributed in a ratio of 3: 1 (3/4 individuals with a dominant trait, 1/4 individuals with a recessive one). Thus, in a monohybrid cross cytological the basis for the splitting of traits is the divergence of homologous chromosomes and the formation of haploid germ cells in meiosis.

Law of independent inheritance of traits

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 in different pairs of homologous chromosomes peas. 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 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:

  1. A large number of crosses (a large number of offspring) are being studied.
  2. Gametes containing alleles A and a are formed in equal numbers (have equal viability).
  3. There is no selective fertilization: gametes containing any allele fuse with each other with equal probability.
  4. Zygotes (embryos) with different genotypes are equally viable.

Conditions for fulfilling the law of independent inheritance

  1. All the conditions necessary to fulfill the law of splitting.
  2. The location of the genes responsible for the studied traits in different pairs of chromosomes (non-linkage).

Conditions for the fulfillment of the law of purity of gametes

  1. Normal course of meiosis. As a result of nondisjunction of chromosomes, both homologous chromosomes from a pair can get into one gamete. In this case, the gamete will carry a pair of alleles of all the genes that are contained in this pair of chromosomes.

Gregor Mendel in the 19th century, conducting research on peas, identified three main patterns of inheritance of traits, which are called the three laws of Mendel. The first two laws relate to monohybrid crossing (when parental forms are taken that differ in only one trait), the third law was revealed during dihybrid crossing (parental forms are examined according to two different traits).

Mendel's first law. The law of uniformity of hybrids of the first generation

Mendel took pea plants for crossing, differing in one trait (for example, in seed color). Some had yellow seeds, others green. After cross-pollination, hybrids of the first generation (F 1) are obtained. All of them had yellow seeds, that is, they were uniform. The phenotypic trait that determines the green color of the seeds has disappeared.

Mendel's second law. splitting law

Mendel planted hybrids of the first generation of peas (which were all yellow) and allowed them to self-pollinate. As a result, seeds were obtained, which are hybrids of the second generation (F 2). Among them, not only yellow, but also green seeds were already encountered, that is, splitting occurred. The ratio of yellow to green seeds was 3:1.

The appearance of green seeds in the second generation proved that this trait did not disappear or dissolve in the hybrids of the first generation, but existed in a discrete state, but was simply suppressed. The concepts of the dominant and recessive allele of a gene were introduced into science (Mendel called them differently). The dominant allele overrides the recessive one.

A pure line of yellow peas has two dominant alleles, AA. A pure line of green peas has two recessive alleles - aa. In meiosis, only one allele enters each gamete. Thus, yellow seeded peas only produce gametes containing the A allele. Green seeded peas produce gametes containing the a allele. When crossed, they produce Aa hybrids (first generation). Since the dominant allele in this case completely suppresses the recessive one, the yellow color of the seeds was observed in all hybrids of the first generation.

First generation hybrids already produce gametes A and a. During self-pollination, randomly combining with each other, they form the genotypes AA, Aa, aa. Moreover, the heterozygous Aa genotype will occur twice as often (since Aa and aA) than each homozygous one (AA and aa). Thus we get 1AA: 2Aa: 1aa. Since Aa produces yellow seeds like AA, it turns out that for 3 yellows there is 1 green.

Mendel's third law. The law of independent inheritance of different traits

Mendel carried out a dihybrid crossing, that is, he took pea plants for crossing, which differ in two ways (for example, in the color and wrinkling of the seeds). One pure line of peas had yellow and smooth seeds, while the second line had green and wrinkled ones. All of their first generation hybrids had yellow and smooth seeds.

In the second generation, as expected, splitting occurred (a part of the seeds showed a green color and wrinkling). However, plants were observed not only with yellow smooth and green wrinkled seeds, but also with yellow wrinkled and green smooth ones. In other words, there was a recombination of characters, indicating that the inheritance of the color and shape of the seeds occurs independently of each other.

Indeed, if the genes for seed color are located in one pair of homologous chromosomes, and the genes that determine the shape are in the other, then during meiosis they can be combined independently of each other. As a result, gametes can contain both alleles for yellow and smooth (AB), and yellow and wrinkled (Ab), as well as green smooth (aB) and green wrinkled (ab). When gametes are combined with each other, nine types of second-generation hybrids are formed with different probability: AABB, AABb, AaBB, AaBb, AAbb, Aabb, aaBB, aaBb, aabb. In this case, according to the phenotype, splitting into four types will be observed in the ratio 9 (yellow smooth) : 3 (yellow wrinkled) : 3 (green smooth) : 1 (green wrinkled). For clarity and detailed analysis, a Punnett lattice is built.

Gregor Mendel - an Austrian botanist who studied and described Mendel's Laws - these are still playing an important role in studying the influence of heredity and the transmission of hereditary traits.

In his experiments, the scientist crossed different types of peas that differ in one alternative feature: the shade of flowers, smooth-wrinkled peas, and the height of the stem. In addition, a distinctive feature of Mendel's experiments was the use of so-called "clean lines", i.e. offspring resulting from self-pollination of the parent plant. Mendel's laws, formulation and brief description will be discussed below.

For many years, studying and meticulously preparing an experiment with peas: protecting flowers from external pollination with special bags, the Austrian scientist achieved incredible results at that time. A thorough and lengthy analysis of the data obtained allowed the researcher to derive the laws of heredity, which later became known as Mendel's Laws.

Before proceeding with the description of the laws, it is necessary to introduce several concepts necessary for understanding this text:

dominant gene- a gene whose trait is expressed in the body. It is designated A, B. When crossing, such a trait is considered conditionally stronger, i.e. it will always appear if the second parent plant has conditionally less weak signs. This is what Mendel's laws prove.

recessive gene - the gene is not expressed in the phenotype, although it is present in the genotype. It is denoted by the capital letter a,b.

Heterozygous - a hybrid in whose genotype (set of genes) there is both a dominant and some trait. (Aa or Bb)

Homozygous - hybrid , possessing exclusively dominant or only recessive genes responsible for a certain trait. (AA or bb)

Mendel's Laws, briefly formulated, will be considered below.

Mendel's first law, also known as the law of uniformity of hybrids, can be formulated as follows: the first generation of hybrids resulting from crossing pure lines of paternal and maternal plants has no phenotypic (i.e. external) differences in the studied trait. In other words, all daughter plants have the same shade of flowers, stem height, smoothness or roughness of peas. Moreover, the manifested trait phenotypically exactly corresponds to the original trait of one of the parents.

Mendel's second law or the law of splitting says: the offspring from heterozygous hybrids of the first generation during self-pollination or inbreeding has both recessive and dominant traits. Moreover, splitting occurs according to the following principle: 75% - plants with a dominant trait, the remaining 25% - with a recessive one. Simply put, if the parent plants had red flowers (dominant trait) and yellow flowers (recessive trait), then 3/4 of the daughter plants will have red flowers, and the rest will have yellow flowers.

Third And last Mendel's law, which is also called in general terms, means the following: when crossing homozygous plants with 2 or more different traits (that is, for example, a tall plant with red flowers (AABB) and a short plant with yellow flowers (aabb), the studied traits (stem height and flower shade) are inherited independently, i.e., the result of crossing can be tall plants with yellow flowers (Aabb) or short plants with red flowers (aaBb).

Mendel's laws, discovered in the middle of the 19th century, gained recognition much later. On their basis, all modern genetics was built, and after it - selection. In addition, Mendel's laws are a confirmation of the great diversity of species that exist today.