The laws of inheritance of G. Mendel for monohybrid crossing are preserved in the case of a more complex dihybrid. With this type of interaction, parent forms differ in two pairs of contrasting features.
Let's consider dihybrid crossing and confirmation of G. Mendel's laws on an example. They crossed two varieties of peas: with white flowers and a normal corolla and with purple flowers and an elongated corolla. All individuals of the first generation had white flowers with a normal corolla. From this we conclude that the white color (let's denote it C) and the normal length (let's write E) are dominant characters, and the purple color (c) and the elongated corolla (e) are recessive. During self-pollination of plants of the first generation, splitting occurs. For better clarity, we will draw up a crossover scheme.
First cross: P1 CCE x cce
G 2Сс and 2Eee
F1 Csee
Second cross (self-pollination of F1 hybrids): P2 Ccee x Ccee. Dihybrid crossing goes with the formation of 16 types of zygotes. Each gamete will contain 1 representative from the C-c gene pair and the E-e pair. At the same time, gene Cit can be combined with E or e with equal probability. In turn, c can combine with E or e. As a result, the CcEe hybrid forms 4 types of gametes with equal frequency: CE, Ce, cE, ce. Together, they form the following organisms: 9 whites with a normal corolla, 3 whites with an elongated corolla, 3 purples with a normal corolla and 1 purple with an elongated corolla.
In the second generation, as a result of crossing, in addition to hybrids that look similar to the parental forms, forms with a new combination of traits (combinative or hereditary variability) are formed. This phenomenon plays an important role in evolution, gives new combinations of adaptive traits. It is also actively used in breeding, where the crossing of plants and animals of improved varieties and breeds makes it possible to breed new species.
The number of phenotypes in F2 is less than the number of genotypes. This is due to the fact that different combinations of gametes can give the same morphological characters. So, we get splitting by phenotype - 9:3:3:1.
Such a dihybrid crossing is possible if the dominant genes are located on non-homologous chromosomes. The cytological basis of such fusion and redistribution is meiosis and fertilization. G. Mendel noticed that with such interaction of genes, each pair of traits is inherited independently of one another, freely combined in all possible combinations (independent inheritance).
All patterns of inheritance that G. Mendel established for mono- and dihybridcrossings are also characteristic of more complex combinations. So, polyhybrid crossing occurs when the organisms taken for this differ in three or more contrasting traits. This fusion of gametes and the redistribution of genetic information are based on the laws of splitting and independent inheritance of traits.
From the foregoing, we conclude that a dihybrid cross is, in fact, two independently running simple crosses, where one alternative trait (monohybrid) is taken into account. This is true for both plants and animals.
