1 Mendel's law wording. Mendel's laws

The law of splitting Mendel planted the 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 (F2). Among them, not only yellow, but also green seeds were already encountered, that is, splitting occurred. At the same time, 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 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.

Laws of Mendel. fundamentals of genetics

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).

Attention

Mendel's first law. The law of uniformity of hybrids of the first generation Mendel took for crossing pea plants that differ in one trait (for example, in seed color).

Some had yellow seeds, others green. After cross-pollination, hybrids of the first generation (F1) are obtained.

All of them had yellow seeds, i.e., were uniform.

The phenotypic trait that determines the green color of the seeds has disappeared.

Mendel's second law.

Welcome

Info

Gregor Mendel is an Austrian botanist who studied and described the pattern of inheritance of traits.

Mendel's laws are the basis of genetics, which to this day play an important role in the study of the influence of heredity and the transmission of hereditary traits.
In his experiments, the scientist crossed different kinds peas that differ in one alternative feature: shade of flowers, smooth-wrinkled peas, stem height.
Besides, distinctive feature Mendel's experiments was the use of the so-called "clean lines", i.e.
offspring resulting from self-pollination of the parent plant. Mendel's laws, formulation and short 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.

Lecture No. 17. Basic concepts of genetics. laws of mendel

The expression of some genes can be highly dependent on environmental conditions. For example, some alleles appear phenotypically only at a certain temperature at a certain phase of an organism's development. This can also lead to violations of the Mendelian splitting.

Modifier genes and polygenes. In addition to the main gene that controls this sign, in the genotype there may be several more modifier genes that modify the expression of the main gene.

Important

Some traits may be determined not by one gene, but by a whole complex of genes, each of which contributes to the manifestation of a trait.

Such a trait is called polygenic. All this also introduces violations in the splitting of 3:1.

Mendel's laws

The state (allele) of a trait that appears in the first generation is called dominant, and the state (allele) that does not appear in the first generation of hybrids is called recessive. "Inclinations" of signs (according to modern terminology - genes) G.

Mendel proposed to denote by the letters of the Latin alphabet.

Conditions belonging to the same pair of traits are designated by the same letter, but the dominant allele is large, and the recessive allele is small.

Mendel's second law. When heterozygous hybrids of the first generation are crossed with each other (self-pollination or inbreeding), individuals with both dominant and recessive states of traits appear in the second generation, i.e. there is a split that occurs in certain relationships. So, in Mendel's experiments on 929 plants of the second generation, 705 with purple flowers and 224 with white flowers turned out to be.

one more step

Thus, peas with yellow seeds form only gametes containing the A allele.

Peas with green seeds form gametes containing the allele a.

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. Law of Independent Succession different signs Mendel carried out a dihybrid cross.

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All possible combinations of male and female gametes can be easily identified using the Punnett lattice, in which the gametes of one parent are written horizontally, and the gametes of the other parent are written vertically. The genotypes of the zygotes formed by the fusion of gametes are entered into the squares.

If we take into account the results of splitting for each pair of traits separately, it turns out that the ratio of the number of yellow seeds to the number of green ones and the ratio of smooth seeds to wrinkled ones 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.

i.e. regardless of the other pair of features.

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.

The laws of mendel are short and clear

The rediscovery of Mendel's laws by Hugo de Vries in Holland, Carl Correns in Germany and Erich Tschermak in Austria did not occur until 1900. At the same time, the archives were raised and the old works of Mendel were found.

At this time, the scientific world was already ready to accept genetics.

Her triumphal procession began. They checked the validity of Mendelian inheritance laws (Mendelization) on more and more new plants and animals and received invariable confirmations. All exceptions to the rules quickly developed into new phenomena of the general theory of heredity. At present, the three fundamental laws of genetics, the three laws of Mendel, are formulated as follows. Mendel's first law. Uniformity of hybrids of the first generation.

All signs of an organism can be in their dominant or recessive manifestation, which depends on the presence of alleles of a given gene.

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 to the description of the laws, it is necessary to introduce several concepts necessary for understanding this text: Dominant gene - a gene whose trait is manifested in the body.

It is denoted by a capital letter: 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 - a gene in the phenotype is not manifested, although it is present in the genotype. Capitalized letter a,b. Heterozygous - a hybrid in whose genotype (set of genes) there is both a dominant and a recessive gene for some trait.
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. Thus, Mendel's third law is formulated as follows: when two homozygous individuals are crossed, differing from each other in two or more pairs of alternative traits, genes and their corresponding traits are inherited independently of each other. Recessives flew. Mendel obtained the same numerical ratios when splitting the alleles of many pairs of traits. This in particular implied the same survival of individuals of all genotypes, but this may not be the case.

In the 50s and 60s of the 19th century, the Austrian biologist and monk Gregor Mendel conducted experiments on crossing peas. As a result of statistical data processing, Mendel not only established, but was also able to explain a number of genetic patterns. This despite the fact that at that time they did not know anything about DNA and genes as carriers of hereditary information. Gregor Mendel is considered the father of genetics.

Even before Mendel, a number of scientists in early XIX For centuries, it has been noted that the hybrids of some plants show the trait of only one parent. But only Mendel guessed to investigate the statistical relationships of hybrids in a series of several generations. In addition, he was lucky with the choice of an object for experiments - peas. Mendel studied seven traits of this plant, and almost all of them were inherited as being on different chromosomes and complete dominance was observed. If there were linked traits, as well as those inherited by the type of incomplete dominance or codominance, etc., then this would confuse the research of the scientist.

The patterns of inheritance established by Mendel are now called the first, second and third laws of Mendel. Mendel's first law is the law of uniformity of hybrids of the first generation.

Mendel carried out a monohybrid cross. He took pure lines that differed only in one alternative pair of features. For example, plants with yellow and green seeds (or smooth and wrinkled, or high and low stems, or axillary and apical flowers, etc.) cross-pollinated pure lines and obtained first-generation hybrids. (The designation of generations F 1, F 2 was introduced at the beginning of the 20th century.) All F 1 hybrids showed a sign of only one of the parents. Mendel called this trait dominant. In other words, all first-generation hybrids were uniform.

The second, recessive, trait disappeared in the first generation. However, it appeared in the second generation. And it required some explanation.

Based on the results of two crosses (F 1 and F 2), Mendel realized that two factors are responsible for each trait in plants. In pure lines, they were also paired, but identical in essence. Hybrids of the first generation received one factor from each parent. These factors did not merge, but remained separate from each other, but only one (which turned out to be dominant) could manifest itself.

Mendel's first law is not always formulated as the law of uniformity of hybrids of the first generation. There is also a similar expression: PThe characteristics of an organism are determined by a pair of factors,and ingametes by one factorfor every sign. (These "factors" of Mendel are now called genes.) Indeed, an important conclusion that could be drawn from Mendel's experiments is that organisms contain two carriers of information about each trait, transmit through gametes to descendants by one factor, and in organism, the factors that caused the same symptom do not mix with each other.

A deeper genetic, as well as cytological and molecular explanation of Mendel's laws was received later. Exceptions to the laws were identified and also explained.

Pure lines are homozygotes. They have the same pair of alleles under study (for example, AA or aa). Acting as a parent (P), one plant forms gametes containing only the A gene, and the other - only the a gene. The hybrids of the first generation (F 1) obtained from their crossing are heterozygotes, since they have the Aa genotype, which, with complete dominance, also manifests itself phenotypically as the homozygous AA genotype. It is this pattern that describes Mendel's first law.

In the diagram below, w is the gene responsible for White color flower, R - for red (this trait is dominant). The black lines indicate different variants gamete encounters. They are all equally incredible. (Such a "drawing" of the meeting of gametes will be important in explaining Mendel's second law.) In any case (at any meeting of parental gametes), the same genotypes are formed in hybrids of the first generation - Rw.

Introduction.

Genetics is a science that studies the laws of heredity and variability of living organisms.

Man has long noted three phenomena related to heredity: first, the similarity of the signs of descendants and parents; secondly, the differences between some (sometimes many) traits of descendants from the corresponding parental traits; thirdly, the emergence in the offspring of traits that were only in distant ancestors. The continuity of traits between generations is ensured by the process of fertilization. From time immemorial, man spontaneously used the properties of heredity for practical purposes - to breed varieties of cultivated plants and breeds of domestic animals.

The first ideas about the mechanism of heredity were expressed by the ancient Greek scientists Democritus, Hippocrates, Plato, Aristotle. Author of the first scientific theory evolution of J.-B. Lamarck used the ideas of ancient Greek scientists to explain what he postulated at the turn of the 18th-19th centuries. the principle of transferring new traits acquired during the life of an individual to offspring. Charles Darwin put forward the theory of pangenesis, which explained the inheritance of acquired traits

Charles Darwin defined heredity as a property of all living organisms to pass on their characteristics and properties from generation to generation, and variability as a property of all living organisms to acquire new features in the process of individual development.

Inheritance of traits is carried out through reproduction. In sexual reproduction, new generations arise as a result of fertilization. The material foundations of heredity are contained in the germ cells. With asexual or vegetative reproduction, a new generation develops either from unicellular spores or from multicellular formations. And with these forms of reproduction, the connection between generations is carried out through cells, which contain the material foundations of heredity (elementary units of heredity) - genes - are sections of the DNA of chromosomes.

The totality of genes that an organism receives from its parents makes up its genotype. The combination of external and internal features is the phenotype. The phenotype develops as a result of the interaction of the genotype and environmental conditions. One way or another, the basis remains the signs that carry the genes.

The patterns by which signs are passed from generation to generation were first discovered by the great Czech scientist Gregor Mendel. He discovered and formulated the three laws of inheritance, which formed the basis of modern genetics.

The life and scientific research of Gregor Johann Mendel.

Moravian monk and plant geneticist. Johann Mendel was born in 1822 in the town of Heinzendorf (now Ginchice in the Czech Republic), where his father owned a small peasant allotment. Gregor Mendel, according to those who knew him, was indeed a kind and pleasant man. After receiving his primary education at the local village school and later, after graduating from the College of Piarists in Leipnik, in 1834 he was admitted to the Troppaun Imperial-Royal Gymnasium in the first grammar class. Four years later, Johann's parents, as a result of a confluence of many unfortunate events that quickly followed one another, were completely deprived of the opportunity to reimburse the necessary expenses associated with their studies, and their son, then only 16 years old, was forced to take care of his own maintenance completely independently. . In 1843, Mendel was admitted to the Augustinian monastery of St. Thomas in Altbrunn, where he took the name Gregor. In 1846, Mendel also listened to lectures on housekeeping, horticulture and viticulture at the Philosophical Institute in Brunn. In 1848, having completed his theology course, Mendel received permission with deep respect to study for his Ph.D. When in the following year he strengthened his intention to take the examination, he was given an order to take the place of the supplent of the imperial-royal gymnasium in Znaim, which he followed with joy.

In 1851, the abbot of the monastery sent Mendel to study at the University of Vienna, where he, among other things, studied botany. After graduating from university, Mendel taught science at a local school. Thanks to this step, his financial situation has changed radically. In the salutary well-being of the physical existence, so necessary for every occupation, both courage and strength returned to him with deep reverence, and during the trial year he studied the prescribed classical subjects with great diligence and love. In his spare hours he was engaged in a small botanical and mineralogical collection, provided in the monastery at his disposal. His passion for the field of natural science became the greater, the more opportunities he got to give himself to it. Although the one mentioned in these studies was devoid of any guidance, and the path of the autodidact here, as in no other science, is difficult and leads slowly to the goal, nevertheless, during that time, Mendel acquired such a love for the study of nature that he no longer spared his strength. to fill his changed gaps by self-study and following the advice of people with practical experience. On April 3, 1851, the "teaching corps" of the school decided to invite Mr. Gregor Mendel, canon of the monastery of St. Thomas, to temporarily fill the professorial position. Gregor Mendel's pomological successes qualified him for a star title and for a temporary position as supplent in natural history in the preparatory class at the Technical School. In the first semester of study, he studied only ten hours a week and only with Doppler. In the second semester, he studied a week for twenty hours. Of these, ten - physics with Doppler, five a week - zoology with Rudolf Kner. Eleven hours a week - botany with Professor Fenzl: in addition to lectures on morphology and taxonomy, he also took a special workshop on the description and definition of plants. In the third semester, he signed up for thirty-two hours a week: ten hours of physics with Doppler, ten hours of chemistry with Rottenbacher: general chemistry, medicinal chemistry, pharmacological chemistry, and a workshop in analytical chemistry. Five - for zoology at Kner. Six hours of classes with Unger, one of the first cytologists in the world. In his laboratories, he studied plant anatomy and physiology and took a workshop on microscopy techniques. And yet - once a week at the Department of Mathematics - a workshop on logarithm and trigonometry.

1850, life was going well. Mendel could already support himself, and was highly respected by his colleagues, for he was good at his duties, and was very pleasant to talk to. The students loved him.

In 1851, Gregor Mendel swung at the cardinal question of biology - the problem of variability and heredity. It was then that he began to conduct experiments on the directed cultivation of plants. Mendel delivered various plants from the far and near neighborhoods of Brunn. He cultivated plants in groups in a part of the monastery garden specially designated for each of them under various external conditions. He was engaged in painstaking meteorological observations. Gregor made most of his experiments and observations with peas, which, beginning in 1854, he sown every spring every year in a small garden under the windows of the prelature. On peas, it was not difficult to set up a clear hybridization experiment. To do this, you just need to open a large, albeit not yet ripened flower with tweezers, cut off the anthers, and independently predetermine a “pair” for it to cross. Since self-pollination is excluded, pea varieties are, as a rule, “pure lines” with constant traits that do not change from generation to generation, which are outlined very clearly. Mendel singled out the features that determined intervarietal differences: the color of the peel of mature grains and, separately, unripe grains, the shape of mature peas, the color of the “protein” (endosperm), the length of the stem axis, the location and color of the buds. He used more than thirty varieties in the experiment, and each of the varieties was previously subjected to a two-year test for “constancy”, for “constancy of characteristics”, for “purity of blood” - in 1854 and in 1855. Eight years there were experiments with peas. Hundreds of times in eight blooms, with his own hands, he carefully cut off the anthers and, picking up pollen from the stamens of a flower of a different variety on tweezers, applied it to the stigma of the pistil. For ten thousand plants obtained as a result of crosses and from self-pollinated hybrids, ten thousand passports were issued. The records are accurate: when the parent plant was grown, which flowers it had, whose pollen was fertilized, which peas - yellow or green, smooth or wrinkled - were obtained, which flowers - coloring along the edges, coloring in the center - bloomed when the seeds were received. , how many of them are yellow, how many are green, round, wrinkled, how many of them are selected for planting, when they are planted, and so on.

The result of his research was the report "Experiments on plant hybrids", which was read by the Brunn naturalist in 1865. The report says: “The reason for setting up the experiments to which this article is devoted was the artificial crossing of ornamental plants, carried out in order to obtain new forms that differ in color. To set up further experiments in order to trace the development of hybrids in their offspring, the conspicuous regularity with which the hybrid forms constantly returned to their parental forms gave impetus. As often happens in the history of science, Mendel's work did not immediately receive due recognition from his contemporaries. The results of his experiments were published at a meeting of the Society of Natural Sciences of the city of Brunn, and then published in the journal of this Society, but Mendel's ideas did not find support at that time. An issue of the journal describing Mendel's revolutionary work has been collecting dust in libraries for thirty years. It was only at the end of the 19th century that scientists dealing with the problems of heredity discovered the works of Mendel, and he was able to receive (posthumously) the well-deserved recognition.

The improvement of the hybridiological method allowed G. Mendel to identify a number of important patterns of inheritance of traits in peas, which, as it turned out later, are true for all diploid organisms that reproduce sexually.

Describing the results of crossings, Mendel himself did not interpret the facts he established as certain laws. But after their rediscovery and confirmation on plant and animal objects, these phenomena, which repeat under certain conditions, began to be called the laws of inheritance of traits in hybrids.

Some researchers distinguish not three, but two laws of Mendel. At the same time, some scientists combine the first and second laws, believing that the first law is part of the second and describes the genotypes and phenotypes of the offspring of the first generation (F1). Other researchers combine the second and third laws into one, believing that the "law of independent combination" is in essence the "law of independence of splitting" that occurs simultaneously in different pairs of alleles. However, in the domestic literature we are talking about the three laws of Mendel.

Mendel's great scientific success was that his chosen seven traits were determined by genes on different chromosomes, which ruled out possible linked inheritance. He found that:

1) In hybrids of the first generation, there is a sign of only one parental form, while the other “disappears”. This is the law of uniformity of hybrids of the first generation.

2) In the second generation, splitting is observed: three-quarters of the offspring have the trait of hybrids of the first generation, and a quarter have the trait that “disappeared” in the first generation. This is the law of splitting.

3) Each pair of traits is inherited independently of the other pair. This is the law of independent inheritance.

Of course, Mendel did not know that these provisions would eventually be called the first, second and third laws of Mendel.

Modern formulation of laws

Mendel's first law

The law of uniformity of hybrids of the first generation - 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 of a clean line with respect to the feature under study - on modern language this means that individuals are homozygous for this trait.

Mendel's second law

The law of splitting - when two heterozygous descendants of the first generation are crossed among themselves 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.

The phenomenon in which the crossing of 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 (recombination) 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.

The splitting of offspring when crossing heterozygous individuals is explained by the fact that gametes are genetically pure, that is, they 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 allele from a pair of alleles of a given gene enters each gamete. The cytological basis for the splitting of signs is the divergence of homologous chromosomes and the formation of haploid germ cells in meiosis (Fig. 4).

Fig.4.

The example illustrates the crossing of plants with smooth and wrinkled seeds. Only two pairs of chromosomes are depicted, one of these pairs contains the gene responsible for the shape of the seeds. In plants with smooth seeds, meiosis results in gametes with the smooth (R) allele, while in plants with wrinkled seeds, gametes with the wrinkled (r) allele. F1 hybrids of the first generation have one chromosome with the smooth allele and one with the wrinkled allele. Meiosis in F1 results in the formation of an equal number of gametes with R and with r. The random pairing of these gametes during fertilization leads in the F2 generation to the appearance of individuals with smooth and wrinkled peas in a ratio of 3: 1.

Mendel's third law

The law of independent inheritance - when crossing two 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).

Mendeleev's law of independent inheritance can be explained by the movement of chromosomes during meiosis (Fig. 5). During the formation of gametes, the distribution between them of alleles from a given pair of homologous chromosomes occurs independently of the distribution of alleles from other pairs. It is the random arrangement of homologous chromosomes at the spindle equator in metaphase I of meiosis and their subsequent arrangement in anaphase I that leads to the diversity of allele recombination in gametes. The number of possible combinations of alleles in male or female gametes can be determined by the general formula 2n, where n is the haploid number of chromosomes. In humans, n=23, and the possible number of different combinations is 223=8,388,608.

Fig.5. Explanation of the Mendelian law of independent distribution of factors (alleles) R, r, Y, y as a result of independent divergence of different pairs of homologous chromosomes in meiosis. Crossing plants that differ in the shape and color of seeds (smooth yellow H green wrinkled) gives hybrid plants in which the chromosomes of one homologous pair contain the R and r alleles, and the other homologous pair contains the Y and y alleles. In metaphase I of meiosis, the chromosomes received from each of the parents can with equal probability move either to the same spindle pole (left figure) or to different ones (right figure). In the first case, gametes appear containing the same combinations of genes (YR and yr) as in the parents, in the second case, alternative combinations of genes (Yr and yR). As a result, four types of gametes are formed with a probability of 1/4, a random combination of these types leads to a splitting of the offspring 9:3:3:1, as observed by Mendel.

MENDEL'S LAWS MENDEL'S LAWS

established by G. Mendel patterns of distribution in the offspring of inheritances, signs. The basis for the formulation of M. h. served as a long-term (1856-63) experiments on crossing several. pea varieties. G. Mendel's contemporaries were unable to appreciate the importance of his conclusions (his work was reported in 1865 and published in 1866), and only in 1900 these regularities were rediscovered and correctly assessed independently by K. Correns, E. Chermak and X .De Vries. The use of strict methods for selecting the source material, specials, contributed to the identification of these patterns. crossbreeding schemes and taking into account the results of experiments. Recognition of justice and value of M. h. in the beginning. 20th century associated with the definition advances in cytology and the formation of the nuclear hypothesis of heredity. The mechanisms underlying M. h. were elucidated through the study of the formation of germ cells, in particular the behavior of chromosomes in meiosis, and the proof of the chromosome theory of heredity.

law of uniformity hybrids of the first generation, or the first law of Mendel, states that the offspring of the first generation from crossing stable forms that differ in one trait have the same phenotype for this trait. Moreover, all hybrids can have the phenotype of one of the parents (complete dominance), as was the case in Mendel's experiments, or, as was later discovered, an intermediate phenotype (incomplete dominance). Later it turned out that hybrids of the first generation 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 hence in phenotype.

splitting law, or Mendel's second law, states that when crossing hybrids of the first generation among themselves among hybrids of the second generation in a certain. ratios, individuals with the phenotypes of the original parental forms and hybrids of the first generation appear. So, in the case of complete dominance, 75% of individuals with a dominant and 25% with a recessive trait are detected, that is, two phenotypes in a ratio of 3:1 (Fig. 1). With incomplete dominance and codominating, 50% of the second generation hybrids have the phenotype of the first generation hybrids and 25% each have the phenotypes of the original parental forms, i.e., they observe a splitting of 1:2:1. The second 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 hybrids of the first generation, as a result of which individuals of three possible genotypes are identified among hybrids of the second generation in the ratio 1AA: 2Aa: 1aa . Specific types of interaction of alleles give phenotype splits in accordance with Mendel's second law.

The law of independent combination (inheritance) of features, or Mendel's third law, states that each pair of alternative signs behaves independently of each other in a number of generations, as a result of which among the descendants of the second generation in a certain. ratio, individuals with new (in relation to the parental) combinations of traits appear. For example, when crossing initial forms that differ in two traits, individuals with four phenotypes are revealed in the second generation in a ratio of 9:3:3:1 (the case of complete dominance). At the same time, two phenotypes have “parental” combinations of traits, and the remaining two are new. This law is based on independent behavior (splitting) of several. pairs of homologous chromosomes (Fig. 2). For example, during dihybrid crossing, this leads to the formation of 4 types of gametes in hybrids of the first generation (AB, Ab, aB, ab) and, after the formation of zygotes, a regular splitting according to the genotype and, accordingly, according to the phenotype.

As one of the M. z. in the genetic literature often mention the law of purity of gametes. However, despite the fundamental nature of this law (which is confirmed by the results of tetrad analysis), it does not apply to the inheritance of traits and, moreover, was formulated not by Mendel, but by W. Batson (in 1902).

For M.'s identification h. in their classic the form requires: homozygosity of the original forms, the formation of gametes in hybrids of all possible types in equal proportions, which is ensured by the correct course of meiosis; equal viability of gametes of all types, equal probability of meeting any types of gametes during fertilization; 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 decomp. geno- and phenotypes. M. h., revealing the discrete, corpuscular nature of heredity, have a universal character for all diploid organisms that reproduce sexually. For polyploids, fundamentally the same patterns of inheritance are revealed, however, the numerical ratios of geno- and phenotypic. classes differ from those of diploids. The ratio of classes also changes in diploids in the case of gene linkage ("violation" of Mendel's third law). In general, M. h. valid for autosomal genes with complete penetrance and constant expressivity. When genes are localized in the sex chromosomes or in the DNA of organelles (plastids, mitochondria), the results of reciprocal crossings may differ and do not follow M. z., which is not observed for genes located in autosomes. M. h. were of great importance - it was on their basis that the intensive development of genetics took place at the first stage. They served as the basis for the assumption of the existence in cells (gametes) of inheritances, factors that control the development of traits. From M. h. it follows that these factors (genes) are relatively constant, although they may be in decomp. states, couples in somatic. cells and single in gametes, discrete and can behave independently in relation to each other. All this served at one time as a serious argument against the theories of "fused" heredity and was confirmed experimentally.

.(Source: "Biological Encyclopedic Dictionary." Chief editor M. S. Gilyarov; Editorial board: A. A. Babaev, G. G. Vinberg, G. A. Zavarzin and others - 2nd ed., corrected . - M .: Sov. Encyclopedia, 1986.)

The main patterns of inheritance, discovered by G. Mendel. In 1856-1863. Mendel carried out extensive, carefully planned experiments on the hybridization of pea plants. For crosses, he selected constant varieties (pure lines), each of which, during self-pollination, steadily reproduced the same traits in generations. Varieties differed by alternative (mutually exclusive) variants of any trait controlled by a pair of allelic genes ( alleles). For example, color (yellow or green) and shape (smooth or wrinkled) of seeds, stem length (long or short), etc. To analyze the results of crossings, Mendel applied mathematical methods, which allowed him to discover a number of patterns in the distribution of parental traits in offspring. Traditionally, in genetics, Mendel's three laws are accepted, although he himself formulated only the law of independent combination. The first law, or the law of uniformity of hybrids of the first generation, states that when organisms that differ in allelic traits are crossed, only one of them appears in the first generation of hybrids - dominant, and the alternative, recessive, remains hidden (see. dominance, recessiveness). For example, when crossing homozygous (pure) varieties of peas with yellow and green seed color, all first-generation hybrids had yellow color. This means that yellow color is a dominant trait, and green is a recessive one. This law was originally called the law of dominance. Soon, its violation was discovered - an intermediate manifestation of both traits, or incomplete dominance, in which, however, the uniformity of hybrids is preserved. Therefore, the modern name of the law is more accurate.
The second law, or the law of splitting, states that when two hybrids of the first generation are crossed with each other (or when they self-pollinate), in the second generation both signs of the original parental forms appear in a certain ratio. In the case of yellow and green seeds, their ratio was 3:1, i.e. splitting according to phenotype it happens that in 75% of plants the seed color is dominant yellow, in 25% - recessive green. This splitting is based on the formation by heterozygous hybrids of the first generation in equal proportions of haploid gametes with dominant and recessive alleles. At the fusion of gametes in hybrids of the 2nd generation, 4 genotype- two homozygous, carrying only dominant and only recessive alleles, and two heterozygous, as in hybrids of the 1st generation. Therefore, splitting according to the genotype 1:2:1 gives a splitting according to the phenotype 3:1 (yellow color is provided by one dominant homozygote and two heterozygotes, green color is provided by one recessive homozygote).
The third law, or the law of independent combination, states that when homozygous individuals are crossed, differing in two or more pairs of alternative traits, each of these pairs (and pairs of allelic genes) behaves independently of other pairs, i.e. both genes and the traits corresponding to them are inherited in the offspring independently and are freely combined in all possible combinations. It is based on the law of splitting and is performed if pairs of allelic genes are located on different homologous chromosomes.
Often, as one of Mendel's laws, the law of gamete purity is also given, stating that only one allelic gene enters each sex cell. But this law was not formulated by Mendel.
Misunderstood by his contemporaries, Mendel discovered the discrete ("corpuscular") nature of heredity and showed the fallacy of ideas about "fused" heredity. After the rediscovery of forgotten laws, Mendel's experimentally based teaching was called Mendelism. His justice has been confirmed chromosome theory of heredity.

.(Source: "Biology. Modern Illustrated Encyclopedia." Editor-in-Chief A.P. Gorkin; M.: Rosmen, 2006.)

See what "MENDEL'S LAWS" are in other dictionaries:

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