In nature, there are two types of inheritance of several genes: independent and linked.
Independent inheritance occurs when genes that determine non-allelic traits are located on different pairs of chromosomes. In this case, inheritance obeys Mendel’s third law: there is a combination of genes and traits in all possible combinations. When analyzing the crossing of diheterozygotes, 4 variants of phenotypes appear in equal proportions.
Example: inheritance of color and seed shape traits in peas.
As a result of crossing of diheterozygous AaBb plants with recessive aabb digomozygotes, four phenotypes in the same numbers are observed in the offspring.
Linked inheritance occurs when the genes responsible for different traits are located on the same pair of homologous chromosomes. The clutch can be complete or incomplete.
When fully linked, genes located on the same chromosome are inherited together.
In this case, crossing of a diheterozygote 14-05-2018 12-22-43.jpg and a recessive digomozygote 14-05-2018 12-23-40.jpg leads to the emergence of two phenotypes, completely repeating the phenotypes of the parents.
Example: crossing a recessive dihomozygous female Drosophila with a diheterozygous male.
When crossing a female recessive in both traits, having a dark body and short wings, with a diheterozygous dominant male, 50% of gray flies with long wings and 50% of flies with a dark body and short wings were formed.
Incomplete linkage of genes is observed if genes are located far from each other on the chromosome. When a diheterozygote and a recessive homozygote are crossed, 4 classes of different phenotypes are obtained. In this case, the formation of new genotypes, completely different from the parental ones, occurs.
In this case, crossing over interferes with the formation of gametes.
Example: crossing a diheterozygous female Drosophila with a dihomozygous male.
If a dihybrid female is crossed with a homozygous recessive male, then the result is offspring: 41.5% – gray with long wings, 41.5% – gray with short wings, 8.5% – dark with long wings, 8.5% – dark with short wings.
It was found that the smaller the distance between the studied genes in the parental chromosome, the higher the probability of their complete linked inheritance. Accordingly, the farther from each other they are located, the more often the intersection occurs during meiosis.