From the moment a person began to become aware of himself, he had the question “Why do children look like their parents, although they never completely copy them?” In ancient times, the theory of pangenesis arose, one of the proponents of which was Aristotle. According to it, the seed is formed in all members of the body, after which the blood flow is transmitted to the genitals. The similarity between parents and offspring was explained by the fact that the seed reflects the characteristics of those parts of the body in which it was formed. This theory dominated science until the 19th century. Its adherent was the creator of the first evolutionary theory, Jean Baptiste de Lamarck. He considered pangenesis to be the main mechanism of evolution, explaining the inheritance by offspring of all characteristics acquired by parents during their lives.

In the mid-19th century, German zoologist August Weismann formulated the germplasm theory. According to Weisaman, there are two types of plasma in the body: germinal (sex cells and the cells from which they are formed) and somatic (all other cells). Germplasm remains unchanged and is passed on from generation to generation, while somatic plasma is created by the germplasm and serves to protect it and also promote reproduction.

However, none of these theories provided an answer to the question of the mechanisms and patterns of inheritance of traits. The basic laws of inheritance were discovered by the monk of the Augustinian monastery in the city of Brunne (modern Brno) Gregor Johann Mendel. From 1856 to 1866 he conducted experiments with garden peas (Pisum sativum), trying to find out how its characteristics are inherited. Mendel's experiments are still a model of scientific research.

It must be said that long before Mendel, many scientists tried to understand the meaning and mechanism of inheritance of traits in living organisms. To do this, they crossed both plants and animals, after which they assessed the similarity of parents and offspring. However, no patterns could be derived from the results obtained. The fact is that some characteristics were common in the descendants with one of the parents, others - with the other, others were common with both, fourths appeared only in the parents, and fifths - only in the descendants.

Mendel was the first to realize that all attention needed to be concentrated on one characteristic by which the organisms of the parents clearly differed from each other. That is why he chose garden peas as a research object, since there were a huge number of its varieties. Mendel received seeds of various varieties from European seed growers. After which, from all the diversity, he selected varieties that clearly differ in one characteristic.

However, before crossing plants with each other, Mendel bred each variety separately for two years to make sure that the trait he chose was constantly inherited from generation to generation. In essence, Mendel developed pure lines of pea varieties with which he had to work.

Another important feature of Mendel's experiments was a strict quantitative approach. In each new experiment, he counted the number of offspring of different types, trying to understand whether carriers of one or another trait from each pair were reproduced with the same frequency.

Finally, Mendel very cleverly staged the crossing experiment. It is known that peas are a self-pollinating plant. In order to carry out cross-pollination, Mendel opened the buds and removed the stamens with unripe pollen. After that, he pollinated these flowers with pollen from another plant.

It turned out that all the descendants had yellow peas in their pods, regardless of whether the mother or father plant had the same yellow peas. The opposite trait, the green color of peas, did not appear in the descendants of the first generation. Thus, all first-generation hybrids turn out to be uniform.

Mendel found that all 7 pairs of his chosen traits behaved in this way - in the first generation of descendants, only one of the two alternative ones manifested itself. Mendel called such traits dominant, and the opposite ones - recessive.

By growing plants from the resulting hybrid seeds, Mendel allowed them to self-pollinate. It turned out that in the second generation of descendants there were plants with both yellow and green seeds. Moreover, peas of different colors were often found in one “pod”. Mendel calculated that for every 6,022 yellow peas there are 2,001 green peas, which is a ratio of 3:1 (more precisely 3.0095:1).

Similar relationships were obtained in experiments with other traits. In the second generation, three quarters of the plants had a dominant trait and only one quarter had a recessive trait. Thus, the recessive trait reappeared after a generation.

Table 1. Results of G. Mendel’s experiments on crossing pea varieties that differ in one trait

After this, Mendel germinated the seeds of the second generation of hybrid plants and gave them the opportunity to self-pollinate. This allowed him to determine whether the characteristics of the descendants of the second generation were preserved in the future or not. It turned out that plants with green seeds were bred in purity, that is, they always produced plants with the same green seeds. But plants with yellow seeds turned out to be heterogeneous. About a third of plants with yellow seeds were always bred pure, that is, in all subsequent generations, their descendants had only yellow seeds. The offspring of the remaining 2/3 of the plants with yellow seeds produced both yellow and green peas, the ratio of which was approximately 3:1.

Mendel obtained similar results for other pairs of traits. In all cases, carriers of recessive traits from among the second generation hybrids were bred pure. Carriers of dominant traits were of two types: a third of them were always bred pure, while in the offspring of the remaining 2/3, dominant and recessive traits were found in a ratio of 3: 1.

Explaining the results of his experiments, Mendel made the following assumption. Alternative characteristics are determined by certain factors that are transmitted from parents to offspring with gametes. Each factor exists in two alternative forms, which provide one of the possible manifestations of the trait. The fact that in the offspring of hybrids of the first and subsequent generations there are carriers of both parental characteristics allowed Mendel to draw a very important conclusion: “The two factors that determine the alternative manifestations of a trait in no way merge with each other, but remain separate throughout the life of the individual and When gametes are formed, they diverge into different gametes.” Subsequently, this statement was called Mendel's law of splitting.

Mendel not only carried out his experiments brilliantly, but also tested his assumptions. To do this, he crossed first-generation hybrid plants with a recessive parent plant. As a result of such crossing, plants with a dominant and recessive trait were found to be in approximately equal proportions (i.e., 1:1). This proved the validity of the conclusions drawn. The method used by Mendel to check the results of crossing is widely used today and is called analyzing crossing.

In the spring of 1865, Mendel reported the results of his experiments at a meeting of the Brunn Society of Naturalists. Oddly enough, he was not asked a single question, and the report itself did not arouse much interest. A year later, his article was published in the journal “News of the Natural History Society of Brunn”. However, like the report, it did not arouse interest among scientists. It so happened that the outstanding discovery was forgotten until the beginning of the 20th century. In 1900, three scientists independently of each other: the Dutchman Hugo de Vries, the German Karl Correns and the Austrian Erich Tsermak, having conducted their own experiments, obtained the same results as Mendel. To our credit, all three unconditionally recognized Mendel's priority in this discovery.

QUESTIONS AND TASKS FOR REVIEW

Question 1. Who was the discoverer of the patterns of inheritance of traits?

The discoverer of the laws of inheritance of traits was Gregor Mendel.

Question 2. On what plants did G. Mendel conduct experiments?

G. Mendel very successfully chose the object for his experiments. Peas are easy to grow in the conditions of the Czech Republic; they reproduce several times a year, pea varieties differ from each other in a number of clearly distinguishable characteristics, and, finally, in nature peas are self-pollinating, but in an experiment self-pollination is easy to prevent, and the researcher can pollinate the plant with pollen from another plant.

Question 3. Thanks to what techniques did G. Mendel manage to reveal the laws of inheritance of traits?

In carrying out his classical experiments, Mendel followed several rules. Firstly, he used plants that differed from each other in a small number of characteristics. Secondly, the scientist worked only with plants of pure lines. So, in plants of one line the seeds were always green, and in the other - yellow. Mendel first developed pure lines by self-pollinating pea plants.

Mendel conducted experiments simultaneously with several parent pairs of peas; the plants of each pair belonged to two different pure lines. This allowed him to get more experimental material.

When processing the data obtained, Mendel used quantitative methods, accurately counting how many plants with a given trait (for example, seeds with yellow and green colors) appeared in the offspring.

QUESTIONS AND TASKS FOR DISCUSSION

Question 1. What features of pea plants allowed G. Mendel to classify the organisms he took for hybridization as pure lines?

Peas are easy to grow in the conditions of the Czech Republic; they reproduce several times a year, pea varieties differ from each other in a number of clearly distinguishable characteristics, and, finally, in nature peas are self-pollinating, but in an experiment self-pollination is easy to prevent, and the researcher can pollinate the plant with pollen from another plant.

Question 2. What is the essence of the hybridological method developed by G. Mendel?

The essence of the hybridological method is the crossing (hybridization) of organisms that differ from each other in one or more characteristics. The basis of G. Mendel’s hybridological method is the following techniques and objects:

1) analysis of inheritance was carried out according to individual distinct characteristics;

2) studying the nature of the transmission of traits to the descendants of the first and subsequent generations;

3) quantitative accounting of the distribution of heritable traits in individuals in hybrid generations (statistics);

4) peas were chosen as the object of research - a plant in which both natural self-pollination and artificial cross-pollination are possible.

Question 1. Define the concepts of “heredity” and “variability”.
Heredity- this is the ability of living organisms to transmit their characteristics, properties and developmental characteristics to the next generation. It ensures the material and functional continuity of generations and is the reason that the new generation is similar to the previous one. The inheritance of traits is based on the transmission of genetic material to offspring.
Variability- this is the ability of living organisms to exist in various forms, that is, to acquire, in the process of individual development, characteristics that differ from the qualities of other individuals of the same species, including their parents. Variability can be determined by the characteristics of an individual’s genes, their combination, etc., or maybe by the interaction of the individual and the environment. In the latter case, even genetically identical organisms are capable of acquiring different characteristics and properties during the process of ontogenesis.

Question 2. Who first discovered the patterns of inheritance of traits?
The first person to discover the laws of inheritance of traits was the Austrian scientist Gregor Mendel (1822-1884). As a monk at the monastery in Brunn (Brno, modern Czech Republic), he crossed different varieties of peas for eight years (1856-1863). In 1865, G. Mendel reported on the results of his experiments at a meeting of the Society of Natural Scientists of Brünn. The work was appreciated only after 1900, when three botanists (Hugo de Vries in Holland, Karl Correns in Germany and Erich Tsermak in Austria) independently rediscovered the patterns of inheritance.

Question 3. On what plants did Mendel conduct his experiments?
Mendel conducted experiments on different varieties of seed peas. For his experiments, he used 22 varieties of peas, differing in seven characteristics. In total, during his research he studied more than ten thousand plants.

Question 4. Thanks to what features of the organization of work did Mendel manage to discover the laws of inheritance of traits?
Gregor Mendel managed to discover the laws of inheritance of traits thanks to the following features of his work:
the experimental plant was peas - an unpretentious plant that has great fertility and produces several harvests per year;
Peas are a self-pollinating plant, which avoids accidental ingress of foreign pollen. Mendel, during cross-pollination experiments, removed the stamens and used a brush to transfer pollen from one parent plant to the pistil of another;
Mendel studied qualitative, clearly distinguishable traits, each of which was controlled by a single gene;
When processing data, the scientist kept strict quantitative records of all plants and seeds.

Gregor Mendel, peas and probability theory

Gregor Mendel's fundamental work on the inheritance of traits in plants, “Experiments on Plant Hybrids,” was published in 1865, but virtually 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 doctrine of heredity?

Is it only to maintain historical justice?

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

The phenomenon of heredity (transmission of characteristics from parents to offspring) has been known since time immemorial. It's no secret that children look like their parents. Gregor Mendel also knew this. What if children don't look like their parents? After all, there are known cases of the birth of a blue-eyed child from brown-eyed parents! It is tempting to explain this as marital infidelity, but, for example, experiments with artificial pollination of plants show that the descendants of the first generation may be unlike either parent. And here everything is definitely fair. Consequently, the characteristics of offspring are not simply the sum of the characteristics of their parents.

What happens? Can children be anything they want? 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? Similar reasoning led Mendel to pose the research problem. And if a problem is posed, you can move on to solving it. But how? What should be the method?

But what exactly should you pay attention to when making observations in order to identify a pattern and not get lost in the chaos of data?

First of all, the trait whose inheritance is observed must be clearly distinguishable visually. The easiest way is to take a sign that appears in two variants.

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 a consequence of crossing plants with different manifestations of the selected trait, and not caused by some other circumstances (from which, strictly speaking, he could know that the color of the cotyledons does not depend, for example, on temperature, at which conditions did the peas grow?). How to achieve this?

Mendel grew two lines of peas, one of which produced only green seeds, and the other only yellow ones. Moreover, over many generations in these lines the pattern of inheritance did not change. In such cases (when there is no variability in a number of generations), they say that a pure line was used.

Pea plants on which G. Mendel conducted experiments

Mendel did not know all the factors influencing 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 this, he crossed plants with cotyledons of different colors (one had green, the other yellow), but under the same conditions. This gave him grounds to argue that the differences that would appear in the pattern of inheritance were caused by the different phenotypes of the parents in the two crosses and not by any other factor.

These are the results Mendel obtained. 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 were obtained with green cotyledons. This manifestation of a trait, when predominantly one of the variants is observed, Mendel called dominant (an alternative manifestation, respectively, recessive), and this result was called law of uniformity of first generation hybrids , or .

In the second generation, obtained through self-pollination, seeds with both green and yellow cotyledons appeared, and in a ratio of 3:1.
This ratio is called law of splitting law of uniformity of first generation hybrids Mendel's second law.
But the experiment does not end with obtaining results. There is also 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. In Mendel's time (mid-19th century), no genes or chromosomes were known.

Even the idea of ​​the cellular structure of all living things was not yet generally accepted. For example, many scientists (including Darwin) believed that heritable manifestations of traits constitute a continuous series. This means, for example, that when a red poppy is crossed with a yellow poppy, the offspring should be orange.

Mendel, in principle, could not know the biological nature of inheritance. What did his experiments yield? At a qualitative level, it turns out that descendants really can be anything and there is no pattern. What about quantitative?

And what can a quantitative assessment of the experimental results even say in this case?

Fortunately for science, Gregor Mendel was not just an inquisitive Czech monk. In his youth he was very interested in physics and received a good physics education. Mendel also studied mathematics, including the beginnings of probability theory, developed by Blaise Pascal in the mid-17th century. (What the probability theory has to do with this 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 determined the color of the cotyledons. Suppose the presence of a hereditary factor Mendel's experiments: A Memorial bronze plaque dedicated to G. Mendel, opened in Brno in 1910. determines the green color of the cotyledons, and the presence of a hereditary factor Mendel's experiments: - yellow. Then, naturally, plants with green cotyledons contain and inherit the factor
, and with yellow – factor Memorial bronze plaque dedicated to G. Mendel, opened in Brno in 1910. . Mendel's experiments: But why then, among the descendants of plants with green cotyledons, are there plants with yellow cotyledons?
It must be said that after the remarkable works of Carl Linnaeus, European scientists had a fairly good understanding of the process of sexual reproduction in plants. In particular, it was clear that something from the mother, and something from the father, passes into the daughter organism. It was just 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 the daughter organism receives one factor from the father and another from the mother. This, frankly speaking, is a rather bold assumption, and any skeptical scientist (and a scientist must be a skeptic) will wonder why, in fact, Mendel based his theory on this.
This is where probability theory comes into play. If hereditary factors are combined with each other at random, i.e.
Regardless, is the probability of each factor entering the daughter organism 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 a daughter organism is equal to: 1/2 x1/2 = 1/4. Obviously, combinations are possible, AA, Ahh, aA ahh Memorial bronze plaque dedicated to G. Mendel, opened in Brno in 1910. . With what frequency do they appear? It depends on the ratio of factors Mendel's experiments: And
presented to parents. Let us consider the course of experiment from these positions. Mendel's experiments: Mendel first took two lines of peas. In one of them, yellow cotyledons did not appear under any circumstances. So the factor Obviously, combinations are possible was absent from it, and all plants carried a combination (in cases where an organism carries two identical alleles, it is called homozygous aA .
). Memorial bronze plaque dedicated to G. Mendel, opened in Brno in 1910. Similarly, all plants of the second line carried the combination Mendel's experiments: What happens during crossing? The factor comes from one of the parents with probability 1 AA , and from the other with probability 1 – factor . Then they give a combination with a probability of 1x1=1
(an organism carrying different alleles of the same gene is called Memorial bronze plaque dedicated to G. Mendel, opened in Brno in 1910. heterozygous Mendel's experiments: ). This perfectly explains the law of uniformity of first-generation hybrids. They all have green cotyledons.
The theory proposed by Mendel to explain the phenomena of heredity is based on strict mathematical calculations and is fundamental in nature. One could even say that in terms of severity, Mendel's laws are more similar to the laws of mathematics than of biology. For a long time (and still) the development of genetics consisted of testing the applicability of these laws to a particular case.

Tasks

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

A. The parent plants are homozygous and had white and yellow fruits. What fruits will be obtained from crossing a first generation hybrid with its white parent? What about the yellow parent?
B. When a white pumpkin is crossed with a yellow one, offspring are obtained, half of which have white fruits, and half of which have yellow fruits. What are the genotypes of the parents?
Q. Is it possible to get yellow fruits by crossing a white pumpkin and its white offspring from the previous question?
D. Crossing white and yellow pumpkins produced only white fruits. What kind of 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. The first group produced 50% black and 50% brown mice. The second group produced 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 me down,” said Mr. Smith, “but he’s only half to blame. Your cows bear half the blame.” “Nonsense!,” Mr. Brown was indignant, “my cows have nothing to do with it!” Who is right in this debate? Here we are talking about the work of Linnaeus "