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.

Figure 1.2 Crossing diagram of field pea. In the parental generation (P), individuals for red (homozygous dominant) or white (homozygous recessive) flowers are crossed with each other. The F1 (first filial generation) resulting from thus cross are uniform and phenotypically identical to the homozygously dominant individual (red flowers). In the F2 generation, which is created by crossing two individuals of the F1 generation, there are 75% individuals with a dominant phenotype (red flowers) and 25% individuals with a recessive phenotype (white flowers). This is a classic 3:1 ratio of red:white flowers.

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