The law of genes: the phenomenon of gene separation

Mendel devoted himself to the research of animal and plant hybridization experiments from his youth, and achieved great results.

Mendel (1822— 1884), Austrian. I have loved natural science since I was a child. Due to family difficulties, I joined a monastery and became a monk at the age of 2 1. 185 1 year, Mendel went to Vienna to study natural science and mathematics. The study of these courses. This played an important role in his later research work. Three years later, he returned to the monastery. At that time, the experimental research on the hybridization between plants and animals carried out by the scientific community aroused his great interest, so he used a small garden in the monastery to plant peas, chamomile, mirabilis jalapa, raspberry, corn and other plants. Various plant hybridization experiments were carried out, among which the most outstanding achievement was pea hybridization experiment. After eight years of painstaking research, he read the paper "Plant Hybridization Experiment" in the local natural science research society 1865, and put forward the separation phenomenon and the law of free combination. However, people did not pay due attention to Mendel's research results and this epoch-making paper. It was not until 1900 that three botanists confirmed Mendel's discovery with different plants, and these achievements were recognized by the scientific community. Since then, genetics as a science was born and developed rapidly. Mendel mainly used peas as experimental materials in hybridization experiments, because peas are self-pollinated plants and pollinated by flowers, that is, peas were pollinated before flowering. Therefore, in the natural state, peas can avoid the interference of exotic pollen grains and remain pure. Therefore, the results of artificial hybridization of peas are reliable and easy to analyze.

Mendel chose peas as the experimental material for another reason, because he found that some varieties of peas have the characteristics of easy distinction during cultivation. For example, peas have high stems (height 1.5m ~ 2.0m) and short stems (height about 0.3m). Some seeds are round, others are wrinkled. In this way, the different types of expression of the same trait of an organism are called relative traits. Mendel also found that these traits of peas can be stably passed on to future generations. Using these easily identifiable and stable traits to cross pea varieties, the test results are easy to observe and analyze. After careful observation, Mendel selected 7 pairs of pea related characters for hybridization test.

Mendel noticed that there were many pairs of related traits among different varieties of peas at the same time, but in order to facilitate the analysis, he only studied each pair of related traits separately at first. Mendel used purebred high-stem peas and purebred short-stem peas as parents (denoted by P). No matter whether the tall-stem pea is used as the female parent (orthogonal) or the male parent (backcross), the first generation after hybridization (referred to as offspring, represented by F 1) is always tall.

Why didn't dwarf plants appear in the first generation? What will happen to future generations if they are allowed to pollinate themselves with tall stems? These problems aroused Mendel's great interest, and he used his first generation of plants for selfing. It can be seen that in the second generation (referred to as the second generation, denoted by F2) plants, there are not only tall stems, but also short stems. The above experimental results aroused Mendel's thinking. He thought that the dwarf character did not disappear in the first generation, but just disappeared. Therefore, Mendel called the traits appearing in the first generation of hybrid seeds dominant traits, such as tall stems; Traits that are not revealed are called recessive traits, such as short stems. Mendel didn't just observe the genetic performance of offspring, but made a further statistical analysis of their genetic traits. He found that among 1 0,064 second-generation pea plants, 787 were tall and 277 were short, and the ratio of tall to short stems was close to 3: 1. This phenomenon, which shows both dominant and recessive traits in hybrid offspring, is called trait segregation in genetics.

Mendel made six other hybridization experiments of relative traits, observed the hybridization of thousands of peas, and made a statistical analysis of the experimental results of each pair of relative traits, and finally got the same results as the above experiments: the first generation only showed dominant traits; In the second generation, the phenomenon of character separation appeared, and the quantitative ratio of dominant characters to recessive characters was close to 3: 1. In the above pea hybridization experiment, why did the first generation only show dominant characters, but the second generation showed character separation? Why is the separation ratio close to 3∶ 1?

Mendel believed that biological traits were controlled by genetic factors (later renamed genes). Genes that control dominant traits (such as tall stems) are dominant genes, which are represented by capital letters (such as D); Genes that control recessive traits (such as short stems) are recessive genes, which are represented by lowercase English letters (such as D). In somatic cells of organisms, genes controlling traits exist in pairs. For example, the somatic cells of purebred high-stem peas contain the pairing gene dd, and the somatic cells of purebred short-stem peas contain the pairing gene DD. When an organism forms a germ cell-gamete, the paired genes are separated from each other and enter different gametes respectively. Therefore, the gamete of purebred high-stem pea contains only one dominant gene D; The gamete of pure dwarf pea contains only one recessive gene D. During fertilization, the male and female gametes combine and the genes in the zygote pair again. For example, in F 1 somatic cells, gene D and gene D combine to form Dd. Here, F 1(Dd) only shows tall stems due to the dominant effect of gene D on gene D.

When F 1(Dd) selfs to produce gametes, similarly, gene D and gene D will be separated, so that F 1 produces two kinds of male gametes and female gametes: one contains gene D and the other contains gene D, and the number of these two gametes is equal. In the process of fertilization, male and female gametes combine randomly, and three gene combinations appear in F2: dd, Dd and Dd, and the quantitative ratio between them is close to 1: 2: 1. Due to the dominant effect of gene D on gene D, F2 has only two types of characters: tall stem and short stem, and the quantitative ratio between these two types of characters is close to 3: 1.

There are three gene combinations dd, dd and Dd in F2 generation in the cross test of related characters of pea tall stem and short stem. Plants with Dd and DD gene combinations are individuals developed from zygotes with the same genes, which are called homozygotes, while plants with DD gene combinations are individuals developed from zygotes with different genes, which are called heterozygotes. Homozygotes can be inherited stably, and their self-bred offspring will not be separated again; Heterozygotes can not be inherited stably, and their inbred offspring will also have character separation. 【 Definition 】 When parents with two (or more) pairs of relative traits cross, when offspring produce gametes, genes on non-homologous chromosomes are free to combine while alleles are separated.

During meiosis, heterozygotes produce gametes.

[Scope of application] Unconnected genes. For other complete linkage, partial linkage and so-called false linkage genes, the law of linkage exchange is followed.

Non-allelic free combination. That is to say, the separation or combination of alleles on one pair of chromosomes and alleles on another pair of chromosomes do not interfere with each other and are assigned to gametes independently. So it is also called the law of independent distribution. Gregor Johann Mendel, the founder of [Discovering Man] theory, put forward and preliminarily verified it as a hypothesis in 1856- 1864.

cross experiment

Mendel took two groups of peas with different relative characters as the research object. One parent is a dominant yellow round grain (marked as yyrr), and the other parent is a recessive green wrinkled grain (marked as YyRr). A hybrid yellow round grain (marked as YYRR) was obtained from the offspring of F 1. If it is self-pollination (self-pollination), there will be obvious separation and free combination in F2 generation. Among the 556 F2 seeds counted by * * *, there are four different expression types, the number of which is as follows: If the proportion of the 32 green crisps with the least number is 1, the digital ratio of the four phenotypes of F2 is about 9∶3∶3∶ 1. It can be seen from the results of the above-mentioned pea hybridization experiment that in the four types of F2, there are two parents' original combinations, namely, yellow round grains and green round grains, and two new combinations, namely, yellow round grains and green round grains, which are different from the parents' types, and the results show the free combination of different relative characters.

In order to prove that two pairs of F 1 hybrids with related traits did produce four different gametes with the same number, Mendel also verified it by means of test crossing. The hybrid of F 1 (YyRr) is crossed with the double recessive parent (YyRr). Because the double recessive parent can only produce one kind of gamete (yr) containing two recessive genes, the offspring produced by test crossing can not only show the type of hybrid gamete, but also reflect the proportion of all kinds of gametes. In other words, if the hybrid F 1 can produce four different types of offspring after test crossing with double recessive parents, and the proportion is equal, then it is proved that when the hybrid F 1 forms a gamete, its genes are combined with each other according to the law of free combination. The actual test results, whether orthogonal or crossing, obtained four different types of offspring with similar numbers, and the ratio was 1: 1: 1, which was completely in line with the expected results. This proves that the male-female hybrid F 1 does produce four gametes with equal number when forming gametes, thus verifying the correctness of the law of free combination. 【 the theoretical significance of the law of free combination 】 is:

It can explain why there are so many kinds of creatures in nature and why there are no two identical individuals in the world. For example, human fingerprints, no two people in the world have exactly the same fingerprints. One of the reasons for biological variation is that in sexual reproduction, gene recombination produces a variety of offspring.

The practical significance of the law of free combination lies in:

It has a great guiding role in hybrid breeding, because through hybridization, gene recombination can produce new types different from parents, which is conducive to artificial breeding of new varieties. For example, one wheat variety is lodging-resistant, but not rust-resistant, and another variety is rust-resistant and prone to lodging. After hybridization, the second generation may have a new type that is resistant to rust and does not lodging. Through artificial selection, new varieties that meet human requirements can be obtained.

In medical practice, people can analyze the simultaneous occurrence of two genetic diseases in a family according to the law of free combination of genes, infer the genotype, phenotype and occurrence probability of offspring, and provide theoretical basis for the prediction and diagnosis of genetic diseases.