## Experiments with peas

Mendel's exact thinking was significantly manifested in the methodology of his pea crossing experiments, as he was the first to realise that the laws of nature are of a mathematical nature. A very important discovery in his work was that the plant does not have to be examined as a whole, but the focus is on individual characteristics/properties. Mendel chose seven different pea characteristics for his experiments (Table 1.1), but he had no idea that each of these traits is governed by one gene with two variants, which was the basic premise of his success.

Table 1.1 Pea characteristics. Mendel selected several characteristics to study in his peas, which are all governed by a single gene with two variants, dominant or recessive, which give rise to different traits.

| Dominant trait | Recessive trait
Seeds | Yellow colour | Green colour
with a smooth surface | with a wrinkled surface
Flowers | Red | White
terminally located | axially located
Pods | Inflated | Constricted
green colour | yellow colour
Plant | Tall | Short

At the time, it was believed that a plant organism develops only from an egg cell without being fertilised by a pollen cell. Mendel guessed that what determines the hereditary characteristics of the offspring must be stored in the germ cells (the egg and pollen cells) of both parents, and this information is combined during fertilisation. He therefore decided to make crosses between different plants and analyse their offspring.

In the first generation, which Mendel designated as the parental generation (P), he crossed a red-flowered plant with a white-flowered one. He made crossings in both directions – that is, the red-flowered plant was the donor of the female reproductive cell (ovum) and the white-flowered plant was the donor of the male (pollen) cell, while in another cross it was the other way around. All individuals of the first generation of offspring (first filial generation – F1) had a red flower, i.e., the parental characteristics did not mix and only one trait was visible. The F1 plants were self-pollinated and the seeds of the F1 generation collected and sown by Mendel. The resulting plants of the new generation, which he called the F2 generation, or the second filial generation, were again crossed with each other. It was surprising that white flowers appeared in the F2 generation, a feature that did not appear in the F1 generation. The ratio between red-flowered and white-flowered plants was approximately 3:1 (Figure 1.2). Such a procedure was repeated for several generations in a row with other traits, with the ratio of characteristics in the P, F1 and F2 generations being key and forming the basis of Mendel's laws.

Having carried out hundreds of such crosses, Mendel came to the following conclusions:

- The external appearance of the organism (phenotype) is conditioned by paired elements, which Mendel named hereditary factors. Today we know that these hereditary factors are our genes. In a diploid organism (such as humans) the individual will contain one set of chromosomes from each parent, with each gene represented in two forms – alleles - where the individual receives one from each parent after the fusion of sex cells.
- The hereditary factor/allele for white flower colour did not disappear in the F1 generation (Figure 1.2), but its expression was suppressed by the allele for red flower colour. The red colouring is therefore a dominant trait, conditioned by the dominant allele of the gene. The white colour of the flower, on the other hand, is a recessive trait, for which the recessive allele is responsible. If an individual received the same alleles from both parents, they are termed a homozygote. If both alleles are dominant, the individual is a dominant homozygote (i.e., red flowers), while if both alleles are recessive, the offspring is a recessive homozygote (as in the case of white flowers). If an individual received different alleles of a gene (one dominant and the other recessive), the offspring is called heterozygous.
- During the formation of gametes (sex cells), the number of chromosomes is reduced to half the original number, so that the two alleles of one gene segregate - they separate into two different sex cells. As a result, the gamete contains only half of the parent's genetic information. During fertilization, during which the gametes of the parents fuse, the original number of chromosomes, which is typical for each species, is restored.