What did Mendel experiment with? Did the "father of genetics" Gregor Mendel violate scientific ethics

Mendel carried out all his experiments with two varieties of peas with yellow and green seeds, respectively. When these two varieties were crossed, all their offspring turned out to have yellow seeds, and this result did not depend on which variety the mother and father plants belonged to. Experience has shown that both parents are equally capable of passing on their hereditary traits to their children.

This was also confirmed in another experiment. Mendel crossed peas with wrinkled seeds with another variety with smooth seeds. As a result, the offspring turned out to be with smooth seeds. In each such experiment, one feature is found to prevail over the other. They called him dominant. It is he who appears in the offspring in the first generation. A trait that is extinguished by a dominant trait is called a recessive trait. In modern literature, other names are used: "dominant alleles" and "recessive alleles". The makings of traits are called genes. Mendel proposed to designate them with letters of the Latin alphabet.

Mendel's second law or splitting law

In the second generation of offspring, interesting patterns in the distribution of hereditary traits were observed. For experiments, seeds from the first generation (heterozygous individuals) were taken. In the case of pea seeds, it turned out that 75% of all plants were with yellow or smooth seeds and 25% with green and wrinkled, respectively. Mendel set up a lot of experiments and made sure that this ratio is exactly fulfilled. Recessive alleles appear only in the second generation of offspring. Splitting occurs in a ratio of 3 to 1.

Mendel's third law or the law of independent inheritance of traits

Mendel discovered his third law by examining two characteristics inherent in pea seeds (their wrinkling and color) in the second generation. Crossing homozygous plants with smooth yellow and wrinkled green, he found amazing phenomenon. In the offspring of such parents, individuals appeared that had characteristics that had never been observed in past generations. These were plants with yellow wrinkled seeds and green smooth ones. It turned out that with homozygous crossing, an independent combination and heredity of traits are observed. The combination happens randomly. The genes that determine these traits must be located on different chromosomes.

Ministry of Education and Science of the Russian Federation

Federal State Budgetary Educational Institution of Higher Professional Education "National Research Nuclear University "MEPhI"" Obninsk Institute of Atomic Energy - branch of NRNU MEPhI

Socio-economic faculty
Department of Economics, Economic-Mathematical Methods and Informatics

Essay

In the discipline "Concepts of modern natural science"

On the topic: "Mendel's experiments and modern understanding of heredity"

Performed:

2nd year student of the PIE-C10 group Churilina V.A.

Obninsk 2010

Genetics is a field of biology that studies heredity and variation. Man has always sought to control wildlife: the structural and functional organization of living beings, their individual development, adaptation to the environment, regulation of numbers, etc. Genetics came closest to solving these problems, revealing many patterns of heredity and variability of living organisms and setting them at the service of human society. This explains the key position of genetics among other biological disciplines.

The fact that organisms transmit signs and properties to their descendants, people intuitively knew for a long time. This knowledge has been used in agriculture when a peasant, wanting to get more grain, tried to leave for sowing the largest seeds from the most productive plants. Naturally, people could not understand the patterns of inheritance of traits for a long time. The first attempts to explain the fact that children usually resemble their parents were made by the great scientist and doctor Ancient Greece- Hippocrates. He said that the seed of a man and the seed of a woman, from which a child arises when merged, are produced in all parts of the parents' organism and therefore carry information about these parts. When the seed merges, there is a struggle between the signs of the father and the mother, and the sex of the child and who it will look like depends on who wins.

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. The laws of heredity, discovered by G. Mendel, laid the foundations for the formation of genetics as an independent science. The method of artificial hybridization was developed 100 years before the classical genetic work of Mendel, then the dominance of traits was discovered. Why is Gregor Mendel considered the founder of modern genetics?

G. Mendel possessed the most important qualities for a true scientist. Firstly, G. Mendel was able to formulate a specific question to which he would like to receive an answer, and, secondly, he knew how to correctly understand and interpret the results of experiments, i.e. was able to draw correct conclusions from the results of his experiments. G. Mendel summarized the results of many years of work in the publication “Experiments on Plant Hybrids”, which was published on February 8, 1865. This article outlined the main patterns of inheritance of traits that formed the basis of modern genetics. Thus, genetics is one of the few scientific disciplines that has an exact date of birth. However, the works of G. Mendel were ahead of their time; they were appreciated only after 35 years.

In 1900 Three researchers (Hugo de Vries, Carl Erich Correns, Erich Cermak) independently rediscovered Mendel's laws on different objects. The results of the work of these researchers proved the correctness of the laws established at the time by G. Mendel. They honestly recognized his primacy in this matter and assigned the name of Mendel to these patterns. 1900 is considered the official birth date of the science of genetics.

Mendel set himself the goal of finding out the rules for the inheritance of individual traits in peas. The researcher conducted this work for 8 years, having studied more than 10,000 pea plants during this time.

Peas were convenient for various reasons. The offspring of this plant has a number of clearly distinguishable features - green or yellow cotyledons, smooth or, on the contrary, wrinkled seeds, swollen or constricted beans, long or short stem axis of the inflorescence, and so on. Transitional, half-hearted "blurred" signs were not. Each time it was possible to confidently say "yes" or "no", to deal with the alternative. And therefore there was no need to dispute Mendel's conclusions, to doubt them. And all the provisions of Mendel's theory have not been refuted by anyone and have deservedly become part of the golden fund of science.
In his works he used the hybridological method. The essence of this method is the crossing (i.e. hybridization) of organisms that are different in some way and in the subsequent analysis of the nature of the manifestation of these signs in the offspring.
Mendel was engaged in breeding peas, and it is to peas, scientific success and the rigor of Mendel's experiments that we owe the discovery of the basic laws of heredity: the law of uniformity of hybrids of the first generation, the law of splitting and the law of independent combination.
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 (F 1). 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.

FIRST LAW OF UNIFORMITY OF FIRST GENERATION HYBRIDS

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 hence in phenotype.

SECOND LAW OF SPLITTING
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 F 1 hybrids are crossed among themselves, individuals with phenotypes of both the original parental forms and F 1 appear in certain proportions among the second generation F 2 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 F 1 hybrids, as a result of which individuals of three possible genotypes are identified among F 2 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 (F 2) have the phenotype of the first generation hybrids and 25% each have the phenotypes of the original parental forms, i.e., 1:2:1 splitting is observed.

THE THIRD LAW OF INDEPENDENT COMBINATION (INHERITANCE) OF SIGNS
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 F 2 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 characteristics, in the next generation (F 2) 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, with dihybrid crossing, this leads to the formation of 4 types of gametes in hybrids of the first generation (F 1) (AB, AB, aB, av), 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, but modern science great 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.

THE SIGNIFICANCE OF MENDEL'S WORK FOR THE DEVELOPMENT OF GENETICS

In 1863 Mendel completed the experiments and in 1865 at two meetings of the Brunn Society of Naturalists reported the results of his work. In 1866, in the proceedings of the society, his article "Experiments on Plant Hybrids" was published, which laid the foundations of genetics as an independent science. This is a rare case in the history of knowledge when one article marks the birth of a new scientific discipline. Why is it considered so??
From the seven-year work of Mendel, which rightfully constitutes the foundation of genetics, the following consequences followed. Firstly, he created the scientific principles for describing and studying hybrids and their offspring (what forms to take in crossing, how to analyze in the first and second generations). Mendel developed and applied an algebraic system of symbols and designations for features, which was an important conceptual innovation. Secondly, Mendel formulated two basic principles, or the law of inheritance of traits in a number of generations, allowing predictions to be made. Finally, Mendel implicitly expressed the idea of ​​discreteness and binarity of hereditary inclinations: each trait is controlled by a maternal and paternal pair of inclinations (or genes, as they were later called), which are transmitted to hybrids through parent germ cells and do not disappear anywhere. The inclinations of traits do not affect each other, but diverge during the formation of germ cells and then freely combine in descendants (the laws of splitting and combining traits). The pairing of inclinations, the pairing of chromosomes, the double helix of DNA - this is the logical consequence and the main path for the development of genetics of the twentieth century based on the ideas of Mendel.

Conclusion

Mendelian theory of heredity, i.e. the totality of ideas about hereditary determinants and the nature of their transmission from parents to offspring, in its meaning is directly opposite to Domdelevsky theories, in particular the theory of pangenesis proposed by Darwin. In accordance with this theory, the signs of the parents are direct, i.e. from all parts of the body, are transmitted to offspring. Therefore, the nature of the attribute of the descendant should directly depend on the properties of the parent. This completely contradicts the conclusions made by Mendel: the determinants of heredity, i.e. genes are present in an organism relatively independently of itself. The nature of the traits (phenotype) is determined by their random combination. They are not modified by any parts of the body and are in a dominance-recession relationship. Thus, the Mendelian theory of heredity opposes the idea of ​​inheritance acquired during individual development signs.

Mendel's experiments served as the basis for the development of modern genetics - a science that studies the two main properties of an organism - heredity and variability. He managed to identify patterns of inheritance thanks to fundamentally new methodological approaches:

1) Mendel successfully chose the object of study;

2) he analyzed the inheritance of individual traits in the offspring of crossed plants that differ in one, two, or three pairs of contrasting alternative traits. In each generation, records were kept separately for each pair of these traits;

3) he not only recorded the results obtained, but also carried out their mathematical processing.

The listed simple methods of research constituted a fundamentally new, hybridological method for studying inheritance, which became the basis for further research in genetics.

Humanity can be proud of the outstanding achievements of geneticists. In particular, the Human Genome program was completed, as a result of which the corresponding heredity code was deciphered. The genomes of a number of other organisms have also been deciphered.
The second outstanding event in genetics is the discovery of the leading role of regulatory systems in the chemically reshaped development of living systems and the shaping process caused by it. Cascades of genes triggered by specialized genes - "masters" and implementing programs for the development of various regions of the living system have been identified.
Based on the achievements of genetics, jointly with molecular biology and experimental embryology, animal cloning has become possible, which does not bring much practical benefit, but allows solving important and urgent fundamental problems.
etc.................

Gregor Mendel, peas and probability theory

The fundamental work of Gregor Mendel, devoted to the inheritance of traits in plants, "Experiments on plant hybrids", was published in 1865, but actually went unnoticed. His work was appreciated by biologists only at the beginning of the 20th century, when Mendel's laws were rediscovered. Mendel's conclusions did not influence the development of contemporary science: evolutionists did not use them in constructing their theories. Why do we consider Mendel the founder of the theory of heredity? Is it only for the observance of historical justice?

To understand this, let's follow the course of his experiments.

The phenomenon of heredity (the transmission of traits from parents to offspring) has been known since time immemorial. It's no secret that children look like their parents. Gregor Mendel knew this too. What if the kids don't look like their parents? After all, there are known cases of the birth of a blue-eyed child from brown-eyed parents! The temptation is great to explain this by adultery, but, for example, experiments with artificial pollination of plants show that the offspring of the first generation may be unlike any of the parents. And it's all fair here. Therefore, the traits of offspring are not simply the sum of the traits of their parents. What happens? Children can be anything? Also no. So is there any pattern at all in inheritance? And can we predict the set of traits (phenotype) of offspring, knowing the phenotypes of the parents?

Such reasoning led Mendel to formulate the research problem. And if a problem is posed, you can move on to solving it. But how? What should be the method? To come up with a method - this is what Mendel did brilliantly.

The natural desire of a scientist in the study of any phenomenon is to discover a pattern. Mendel decided to observe a phenomenon of interest to him - heredity - in peas.

It must be said that peas were not chosen by Mendel by chance. View Pisum sativum L.. very useful for studying heredity. Firstly, it is easy to grow and the entire life cycle is fast. Secondly, it is prone to self-pollination, and without self-pollination, as we shall see below, Mendel's experiments would have been impossible.

But what, in fact, should be paid attention to when observing in order to identify a pattern and not get lost in the chaos of data?

First of all, the trait, the inheritance of which is observed, must be clearly distinguished visually. The easiest way is to take a sign that manifests itself in two versions. Mendel chose the color of the cotyledons. The cotyledons of pea seeds can be either green or yellow. Such manifestations of the trait are clearly distinguishable and clearly divide all seeds into two groups.

Mendel's experiments: A- yellow and green pea seeds; b– smooth and wrinkled pea seeds

In addition, one must be sure that the observed pattern of inheritance is the result of crossing plants with different manifestations of the selected trait, and not caused by some other circumstances (how, strictly speaking, he could know that the color of the cotyledons does not depend, for example, on temperature, under which peas grew?). How to achieve this?

Mendel cultivated two lines of peas, one producing only green seeds and the other producing only yellow seeds. Moreover, for many generations in these lines, the pattern of inheritance has not changed. In such cases (when there is no variability in a number of generations), it is said that a pure line is used.

Pea plants on which G. Mendel experimented

Mendel did not know all the factors affecting heredity, so he made a non-standard logical move. He studied the results of crossing plants with cotyledons of the same color (in this case, the descendants are an exact copy of the parents). After that, he crossed plants with cotyledons of different colors (one green, the other yellow), but under the same conditions. This gave him reason to argue that the differences that would appear in the pattern of inheritance were caused by the different phenotypes of the parents in these two crosses, and not by any other factor.

Here are the results obtained by Mendel.

In the descendants of the first generation from crossing plants with yellow and green cotyledons, only one of two alternative manifestations of the trait was observed - all seeds turned out with green cotyledons. Such a manifestation of a trait, when one of the variants is predominantly observed, Mendel called dominant (alternative manifestation, respectively, recessive), and this result was called law of uniformity of hybrids of the first generation , or Mendel's first law .

In the second generation, obtained by self-pollination, seeds appeared with both green and yellow cotyledons, and in a ratio of 3:1.
This ratio is called splitting law , or Mendel's second law.
But the experiment does not end with the results. There is still such an important stage as their interpretation, i.e., understanding the results obtained from the point of view of already accumulated knowledge.

What did Mendel know about the mechanisms of inheritance? Never mind. At the time of Mendel (mid-19th century), no genes and chromosomes were yet known. Even the idea of ​​the cellular structure of all living things was not yet universally recognized. For example, many scientists (including Darwin) believed that the inherited manifestations of traits constitute a continuous series. This means, for example, that when a red poppy is crossed with a yellow poppy, the offspring must be orange.

Mendel, in principle, could not know the biological nature of inheritance. What did his experiments give? At a qualitative level, it turns out that the descendants really are anything and there is no pattern. What about quantitative? And what, in this case, can the quantitative evaluation of the results of the experiment say at all?

Fortunately for science, Gregor Mendel was not just an inquisitive Czech monk. In his youth, he was very interested in physics, he received a good physical education. Mendel also studied mathematics, including the beginnings of the theory of probability developed by Blaise Pascal in the middle of the 17th century. (What does the theory of probability have to do with it will become clear below.)

Memorial bronze plaque dedicated to G. Mendel, opened in Brno in 1910

How did Mendel interpret his results? He quite logically assumed that there was some real substance (he called it a hereditary factor) that determines the color of the cotyledons. Suppose the presence of a hereditary factor A defines green color cotyledons, and the presence of a hereditary factor A - yellow. Then, naturally, plants with green cotyledons contain and inherit the factor A , and with yellow - the factor A . But why, then, among the descendants of plants with green cotyledons, there are plants with yellow cotyledons?
Mendel suggested that each plant carries a pair of hereditary factors responsible for a given trait. Moreover, if there is a factor A factor A no longer appears (green color dominates over yellow).
It must be said that after the remarkable works of Carl Linnaeus, European scientists had a fairly good idea of ​​the process of sexual reproduction in plants. In particular, it was clear that something from the mother passes into the daughter organism, and something from the father. It was not clear what and how.
Mendel suggested that during reproduction, the hereditary factors of the maternal and paternal organisms are combined with each other at random, but in such a way that one factor from the father and the other from the mother enters the daughter organism. This is, frankly, a rather bold assumption, and any skeptical scientist (and a scientist must be a skeptic) will wonder why, in fact, Mendel built his theory on this.
This is where probability theory comes into play. If hereditary factors are combined with each other at random, i.e. independently, is the probability of getting into the daughter organism of each factor from the mother or from the father the same?
Accordingly, according to the multiplication theorem, the probability of the formation of a specific combination of factors in the daughter organism is: 1/2 x 1/2 = 1/4.
Obviously combinations are possible. AA, Ah, aa, aa . With what frequency do they appear? It depends on the ratio of the factors A And A presented to parents. Let us consider the course of experience from these positions.
First, Mendel took two lines of peas. In one of them, yellow cotyledons did not appear under any circumstances. So the factor A was absent in it, and all plants carried a combination AA (in cases where an organism carries two identical alleles, it is called homozygous ). Similarly, all plants of the second line carried the combination aa .
What happens when crossing? From one of the parents with probability 1 comes a factor A , and from the other with probability 1 - the factor A . Then they give a combination with a probability of 1x1=1 Ah (an organism that carries different alleles of the same gene is called heterozygous ). This perfectly explains the law of uniformity of hybrids of the first generation. All of them have green cotyledons.
During self-pollination, from each of the parents of the first generation, with a probability of 1/2 (presumably), either the factor A , or a factor A . This means that all combinations are equally likely. What should be the proportion of offspring with yellow cotyledons in this case? Obviously one quarter. But this is the result of Mendel's experiment: splitting according to the 3:1 phenotype! Therefore, the assumption of equiprobable outcomes in self-pollination was correct!
The theory proposed by Mendel to explain the phenomena of heredity is based on rigorous mathematical calculations and is of a fundamental nature. It can even be said that, in terms of severity, Mendel's laws are more similar to the laws of mathematics than biology. For a long time (and still) the development of genetics consisted in testing the applicability of these laws to a particular case.

Tasks

1. In pumpkin, the white color of the fruit dominates over the yellow.

A. Parent plants are homozygous and have white and yellow fruits. What will be the result of crossing a first generation hybrid with its white parent? What about the yellow parent?
B. When crossing a white pumpkin with a yellow one, offspring are obtained, half of which have white fruits, and half have yellow ones. What are the genotypes of the parents?
Q. Is it possible to get yellow fruits when crossing a white pumpkin and its white descendant from the previous question?
D. Crossing white and yellow gourds produced only white fruits. What offspring will two such white pumpkins produce when crossed with each other?

2. Black females from two different groups of mice were crossed with brown males. From the first group, 50% black and 50% brown mice were obtained. From the second group received 100% black mice. Explain the results of the experiments.

3. . Mr. Brown bought a black bull from Mr. Smith for his black herd. Alas, among the 22 calves born, 5 turned out to be red. Mr. Brown made a claim against Mr. Smith. “Yes, my bull let us down,” said Mr. Smith, “but he is only half to blame. Half of the blame is borne by your cows.” “Nonsense!” said Mr. Brown indignantly, “my cows have nothing to do with it!” Who is right in this dispute?

Here we are talking about the work of Linnaeus " Sexum Plantarum"("Sex in Plants"), dedicated to the sexual reproduction of plants. This work, published in 1760, described the process of reproduction in such detail that for a long time it was banned at St. Petersburg University as immoral.

genetics. Garden peas were chosen as the object for experiments, since there are many of its varieties that clearly differ in a number of ways; plants are easy to grow and cross. Mendel's success is due to careful planning and careful execution of experiments, as well as the presence a large number experiments to obtain statistically reliable information.

For his first experiments, Mendel chose plants that clearly differed in any pair of characters, for example, in the arrangement of flowers (“axillary” or “apical”). Growing plants of each type for several generations, Mendel was convinced of their suitability for the experiment. Mendel crossed - pollinated plants of one type with pollen from plants of another type. A number of precautions (for example, removing stamens from flowers that were subsequently pollinated, and putting caps on flowers to avoid additional pollination from other plants) made it possible to obtain reliable results. In all cases, plants with axillary flowers grew from seeds collected from these hybrids. The trait "axillary flowers" observed in hybrids of the first generation was called dominant, the trait "apical flowers" - recessive.

Further, the plants of the first hybrid generation were given the opportunity to self-pollinate. In the second hybrid generation, axillary flowers were formed in some plants, and apical flowers in the other part. Mendel suggested that the trait "apical flowers" was also present in the first generation, but in a latent form. In all such experiments carried out with any pair of traits, about three-quarters of the second generation hybrids had a trait that also appeared in the first generation of hybrids (it was called dominant), and a quarter of the second generation offspring had a trait that did not appear in the first generation hybrids (recessive ). It is important that the more experiments were performed, the closer the result was to the ratio of 3: 1.

Based on this series of experiments, the following conclusions were drawn:

Parental plants had two identical "factors" (for example, "axillary flowers" or "apical flowers").

Hybrids of the first generation received one factor from each parent, and these factors did not merge, but retained their individuality.

Thus, it was formulated splitting law (Mendel's first law).

So, each trait of an organism is controlled by a pair of gene variants (or, as they say, sometimes alleles). If the genotype of an organism contains alleles of both types, then one of them (dominant) will manifest itself, completely suppressing the other (recessive). During meiosis, each pair of alleles is split, and only one allele can be transmitted with each gamete as a discrete, unchanging quantity. The transmission of genes to descendants is in full accordance with the theory of probability. The probability that a gamete derived from a first-generation hybrid will carry the dominant allele is 1/2. The probability of each of the four combinations at fertilization will be 1/4; of these, three combinations will contain the dominant allele and result in individuals with the dominant trait. The first of these combinations contains exclusively dominant alleles - AA (they say that it is homozygous for the dominant allele), and the other two contain one dominant and one recessive allele - Aa (heterozygous). The fourth combination will contain only recessive alleles; they will match offspring with a recessive trait (that is, they will be homozygous for the recessive allele).

Homozygous individuals do not split during subsequent self-pollination (they give uniform offspring). In the offspring of self-pollinating heterozygous individuals, splitting according to outward signs in the same 3:1 ratio.

A gene is usually denoted by the first letter that begins the name of the dominant allele of that gene (for example, A). In this case, the dominant allele is denoted capital letter(A) and the recessive is lowercase (a).

The hybrid of the first generation in the described experiments is heterozygous in its genotype, but has a dominant phenotype (that is, it has a dominant trait). In the second generation, individuals with a dominant phenotype may have both homozygous and heterozygous genotypes. To find out the genotype of a second generation hybrid in one cross, it is necessary to perform a back (analyzing) cross with an individual homozygous for the recessive allele of the gene under study. If all the descendants from this crossing show a dominant phenotype, then the individual with the determined genotype was homozygous for the dominant trait. If individuals appear with both dominant and recessive traits (in an approximate ratio of 1:1), then the individual under study was heterozygous.

In the described experiments, monohybrid cross- individuals were taken that differed only in one trait. Later, Mendel turned to the study dihybrid crossing when, using the same method, experiments were carried out on pure-bred (homozygous) individuals that differ in two characteristics (for example, yellow and green seeds, wrinkled and smooth seeds). As a result, in the second generation, individuals with seeds four types: yellow and smooth, yellow and wrinkled, green and smooth, green and wrinkled. The ratio of different phenotypes in the second generation was approximately 9: 3: 3: 1. At the same time, for each pair of traits, the ratio was approximately 3: 1. Based on this, Mendel deduced principle of independent distribution (Mendel's second law).

It is convenient to write down the dihybrid crossing scheme in a special table - the so-called the Punnett lattice; at the same time, the number of possible errors in determining the genotype of the offspring is minimized. All genotypes male gametes are entered in the headings of the vertical columns, and all genotypes of female gametes - in the headings of the horizontal ones. If we return to the example with pea seeds, we can find out that the probability of occurrence in the second generation of individuals with smooth seeds (dominant allele) is 3/4, with wrinkled seeds - 1/4 (recessive allele), with yellow seeds - 3/4 (dominant allele) and with green seeds - 1/4 (recessive allele). Thus, the probabilities of combining alleles in the genotype are equal.

Gregor Mendel (1822 - 1884 ) is an outstanding Czech scientist. Founder of genetics. For the first time discovered the existence of hereditary factors, later called genes.

Gregor Mendel experimented with peas. Among the large number of varieties, he chose for the first experiment two that differ in one trait. Seeds of one variety of peas were yellow, and the other - green. It is known that peas, as a rule, reproduce by self-pollination and therefore there is no variability in seed color within a variety. Using this property of peas, G. Mendel produced artificial pollination by crossing varieties that differ in seed color (yellow and green). Regardless of which variety the mother plants belonged to, the hybrid seeds turned out to be only yellow.
Consequently, the hybrids of the first generation had a trait of only one parent. G. Mendel called such signs dominant. The signs that do not appear in hybrids of the first generation, he called recessive. In experiments with peas, the trait of yellow seed color dominated over green color. Thus, G. Mendel found in the offspring of hybrids uniformity of the first generation, i.e. all hybrid seeds had the same color. In experiments where the crossing varieties differed in other traits, the same results were obtained: the uniformity of the first generation and the dominance of one trait over another.

Segregation of traits in hybrids of the second generation. Mendel's first law.

From hybrid pea seeds, G. Mendel grew plants that, by self-pollination, produced seeds of the second generation. Among them were not only yellow seeds, but also green ones. In total, he received 6022 yellow and 2001 green seeds. Moreover, ¾ of the seeds of hybrids of the second generation had a yellow color and ¼ - green. Consequently, the ratio of the number of descendants of the second generation with a dominant trait to the number of descendants with a recessive trait turned out to be 3: 1. He called this phenomenon splitting signs.

Similar results in the second generation were given by numerous experiments on the hybridological analysis of other pairs of traits. Based on the results obtained, G. Mendel formulated his first law - splitting law. In the offspring obtained from crossing individuals of hybrids of the first generation, a splitting phenomenon is observed: ¼ individuals from hybrids of the second generation carry recessive sign, ¾ - dominant.

Dihybrid cross. Mendel's second law.

A cross that involves two pairs of alleles is called dihybrid cross.

The wording of Mendel's second law is: splitting for each pair of genes proceeds independently of other pairs of genes.