G. Mendel’s inheritance laws for monohybrid crosses are also preserved in the case of a more complex dihybrid cross . With this type of interaction, the parent forms differ in two pairs of contrasting characters.
Consider the crossbreeding and confirmation of the laws of G. Mendel on an example. Two varieties of peas were crossed: 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 (denote it by C) and the normal length (we write E) are the dominant features, and the purple color (c) and the elongated corolla (e) are recessive. With self-pollination of plants of the first generation, splitting occurs. For better visualization, we will make a crossing scheme.
First crossing: P1 CCEE x see
G 2 and 2
F1 csee
The second cross (self-pollination of F1 hybrids): P2 x . Hybrid hybridization leads to the formation of 16 types of zygotes. Each gamete will contain 1 representative from a pair of Cc genes and a pair of Ee. In this case, the C gene with equal probability can combine with E or e. In turn, c can combine with E or e. As a result, the CCEe hybrid forms 4 types of gametes with the same frequency: CE, Ce, ce, ce. Combining with each other, they form such organisms: 9 white with a normal corolla, 3 white with an elongated corolla, 3 purple with a normal corolla and 1 purple with an elongated corolla.
In the second generation, as a result of interbreeding, in addition to hybrids that look similar to parental forms, forms with a new combination of characters (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 the phenotype cleavage - 9: 3: 3: 1.
Such a hybrid cross is possible if the
dominant genes are located on non-homologous chromosomes. The cytological basis of such a merger and redistribution is meiosis and fertilization. G. Mendel noted that with such an
interaction of genes, each pair of characters is inherited independently of one another, freely combining in all possible combinations (independent inheritance).
All the laws of inheritance that G. Mendel established for mono- and dihybrid crosses are also characteristic of more complex combinations. So, polyhybrid hybridization occurs when the organisms taken for this differ in three or more contrasting characters. Such fusion of gametes and redistribution of genetic information are based on the laws of splitting and independent inheritance of characters.
From the foregoing, we conclude that dihybrid crossing is, in fact, two independently occurring simple crosses, where one alternative trait (monohybrid) is taken into account. This is true for both plants and animals.