Cytological bases of monohybrid crossing

To explain the results of his research, Mendel proposed the “gamete purity” hypothesis. He suggested that alternative traits are determined by hereditary inclinations (factors) that are passed from parents to offspring with gametes. Each gamete contains one factor from a pair.

The development of genetics has confirmed Mendel’s assumptions. The nature of hereditary inclinations was established. They began to be called genes.

Communication between generations during sexual reproduction is carried out through gametes, which carry genes that determine the development of a particular trait. During the formation of gametes, each of them gets one of the homologous chromosomes, and, therefore, one gene from a pair.

In the somatic cells of a diploid organism, these inclinations are paired: one received from the paternal organism, and the other from the maternal.

Mendel proposed to designate the dominant hereditary inclinations in capital letters, and the corresponding recessive inclinations – in capital letters.

Each gene has two states – A and a. They form one pair and are located in the same loci (sections) of homologous chromosomes.

Let us present the results of Mendel’s experiments on monohybrid crossing of peas in the form of a diagram.

In the parental generation, the maternal and paternal forms are homozygous for the trait under study, therefore, they form gametes only with the A allele or only with a.

When fertilized, these gametes form a zygote, which has both the dominant and recessive Aa alleles. As a result, all F1 hybrids are uniform in this trait, since the dominant allele suppresses the effect of the recessive allele.

First generation hybrids are heterozygous and form two types of gametes, carrying alleles A and a. When they self-pollinate in F2, a genotype splitting is obtained with respect to 1AA: 2Aa: 1aa, i.e., one fourth of the hybrids is homozygous for dominant alleles, half is heterozygous, and one fourth is homozygous for recessive alleles.

Since the genotypes AA and Aa correspond to the same phenotype (yellow color of seeds), the phenotype splitting will be as follows – 3 yellow: 1 green.

This means that in the second generation, the genotype cleavage is 1: 2: 1, and the phenotype is 3: 1.

A fourth of the offspring (25%) are dominant homozygotes, half (50%) are heterozygotes, and a fourth (25%) are recessive homozygotes.

Three parts of the offspring (75%) get a dominant trait, one part (25%) – a recessive one.