Crossing in which the parental individuals differ in two pairs of alleles is called dihybrid.
Hybrids that are heterozygous for two genes are called diheterozygous. Their genotype is AaBb.
The patterns of inheritance of several pairs of traits were studied by G. Mendel. For the dihybrid crossing, he used pure lines of peas, differing in two pairs of traits: yellow smooth seeds and green wrinkled ones.
All hybrids of the first generation had yellow smooth seeds, i.e. uniformity of the first generation was observed.
Genes that determine the development of different pairs of traits are called non-allelic and are designated by different letters of the Latin alphabet.
Let’s designate alleles of yellow color A, green color – a, smooth – B, wrinkled – b.
Parent plants in this case have genotypes AABB and aabb, and hybrids F1 – AaBb, that is, they are diheterozygous.
In the second generation, after self-pollination of the F1 hybrids, wrinkled and green seeds reappeared.
In this case, four phenotypic groups were obtained in the following ratio: 315 yellow smooth, 101 yellow wrinkled, 108 green smooth, 32 green wrinkled seeds.
This is very close to the 9: 3: 3: 1 ratio.
From 556 seeds, Mendel received 423 smooth and 133 wrinkled, 416 yellow and 140 green. For each pair of traits, a 3: 1 splitting is observed. The same splitting is observed with monohybrid crosses.
It can be concluded that in this case there are two processes of monohybrid crossing, which do not affect each other in any way. Individual pairs of traits are inherited independently. This is the essence of Mendel’s third law – the law of independent inheritance of traits.
When crossing individuals that differ from each other in two or more pairs of alternative traits, genes and their corresponding traits are inherited independently of each other and are combined in all possible combinations.
Mendel’s third law is fulfilled only for genes localized in different pairs of homologous chromosomes.