The laws of genetics established by G. Mendel. Monohybrid and dihybrid crosses

Mendel’s first law.

The most amazing property of a living cell is its ability to transmit hereditary traits to offspring. The science that studies the material foundations of heredity and variability is called genetics. The founder of genetics is the Czech scientist Gregor Mendel. In 1865, Mendel formulated the laws that were then named after him.

Before Mendel, methods of crossing living beings were known, when the result was animals and plants with signs of both parents. Such living creatures were called and are called, to this day – hybrids. Previously, it was believed that in hybrids, the characteristics of the parents are simply and evenly mixed.

Mendel improved the hybridological method: firstly, he applied for the first time mathematical methods of data processing, which was not typical for biology in the 19th century; secondly, he very successfully chose the object of research – peas. Pea is a self-pollinator, so many of its varieties are very pure genetic lines.

In his experiments, Mendel used pea varieties that differed in well-defined features: the color and shape of the seeds, the length of the stem, the color and shape of the flowers, and others. The main thing is that he studied how each specific trait is inherited, and not all at once.

He discovered the following. In the first generation of hybrids (F1), all plants looked like one of the parents. For example, when crossing varieties with green and yellow seeds, all plants were yellow. The trait that appeared in the first generation of hybrids, Mendel called dominant , and which did not appear – recessive.

The meaning of these results became clear when Mendel obtained a second generation of hybrids (F2) by crossing plants of the first generation. At the same time, it turned out that in the second generation, 75% of the offspring showed dominant traits, and 25% – recessive. For each of the seven studied traits in the second generation of hybrids, the ratio of dominant to recessive traits was 3:1 . From crossing plants of the second generation with recessive traits in the third generation (F3), plants with only recessive traits were obtained. The progeny of plants with dominant traits split according to the manifestation of traits: 1/3 gave offspring exclusively with dominant traits, and 2/3 – mixed offspring, in which the ratio of the number of plants with dominant and recessive traits was 3:1.

Mendel’s first law is formulated as follows. In the offspring obtained from crossing hybrids of the first generation, the phenomenon of splitting is observed: a quarter of the hybrids have a recessive trait, three quarters are dominant.

Hypothesis of gamete purity.

In science, it is important not only to obtain data, but also to explain them correctly. The merit of G. Mendel is that he correctly interpreted the results obtained, ahead of contemporary science by 35 years. Mendel’s explanation is often called the “gamete purity hypothesis” , which emphasizes the main idea underlying this explanation. In modern terms, the main provisions of this hypothesis sound like this.

1. From generation to generation, not traits are transmitted, but genes that control their development.

2. The development of each trait is controlled by two genes: one from the father and the other from the mother. Two such genes are called alleles. Alleles can be identical, as is the case with parents descending from stable and proven over many generations of pure lines. Individuals whose allelic genes are the same are called homozygous. The genes of the hybrid are different, most often only one of them is manifested – dominant, less often intermediate inheritance is possible. Individuals whose allelic genes are different are called heterozygotes.

3. Two different hybrid genes – dominant and recessive – exist in it without merging, mixing or diluting. The transmission of genes to subsequent generations does not depend on whether the gene has carried out its action in the development of the individual or whether the trait controlled by it turned out to be suppressed.

4. During the formation of germ cells of a hybrid, only one gene from each allelic pair enters each gamete. Gametes with recessive and dominant genes are formed in equal numbers and have equal viability. The meeting and fusion of gametes during fertilization do not depend on the inclinations they bear.

The set of external features that manifest genes is called the phenotype, and the genetic constitution is called the genotype.

Mendel’s second law.

Mendel conducted experiments on crossing pea varieties that differ in more than one trait. If the first series of experiments is usually called monohybrid crossing, then these experiments are usually called dihybrid crossing. As in the first series of experiments, he used two pure lines, which, when self-pollinated, gave offspring identical to the parental form. One of the crossed lines had round yellow seeds, and the other had wrinkled green seeds. Since the genes that determine the round shape and yellow color of the seeds dominate, then all the offspring in the first generation of hybrids had yellow and round seeds. However, when hybrids of the first generation were crossed with each other in the second generation, splitting of characters occurred and, along with the original forms – yellow, smooth and green, wrinkled – mixed phenotypes appeared – yellow, wrinkled and green, smooth.

And Mendel was able to explain these results based on the assumption of the existence of genes. To do this, he had to make the assumption that during the formation of germ cells, the genes of different allelic pairs are transmitted independently of each other. The ratio between phenotypes in the second generation of dihybrid crossing was as follows: 9(round, yellow): 3(round, green): 3(wrinkled, yellow): 1(green, wrinkled).

This phenomenon of independent distribution is called Mendel’s second law. It is formulated like this. Cleavage in each pair of genes occurs independently of other pairs of genes.

Therefore, a dihybrid cross can be considered as two independent monohybrid. True, in the future we will be able to make sure that there are a lot of exceptions to this rule that confirm this law.

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