Question

What is the linkage? Describe its types. Give factors affecting linkage.

Answer 1

Hint: Sexual reproduction involves the formation of male and female gametes. Due to the fusion of male and female gametes sexual reproduction results in the formation of offspring that are not identical to their parents. Linkage stands in direct contrast to the independent assortment of genes in which allelic genes segregate and assort independently.

Complete step-by-step answer:
Linkage is the close association of the non-allelic genes of a chromosome to be inherited together as a single block, rather than independently as individual units. Linkage involves two or more genes, located in the same chromosome in a linear order. The linkage may involve only the dominant gene, only excessive genes, or a group of both of them. Based on the genes involved, the linkage is classified into coupling linkage and repulsion linkage. Coupling linkage is the linkage either between dominant genes or between excessive genes. Repulsive linkage is the linkage between dominant and recessive genes. Based on chromosomes involved, linkage can be classified into autosomal and sex chromosome linkage. Based on crossing over Linkage is of two types, complete and incomplete linkage. This classification is based on the presence or absence of nonparental combinations of linked genes. In complete linkage, no crossing over occurs, no gene combinations are formed. But, in the incomplete linkage, choosing over may occur so that none parental gene combinations are formed.

Additional Information:
- The strength of linkage depends upon the distance between the linked genes, the lesser the distance, the higher the strength.
- Linkage causes a higher frequency of parental types than recombinant in crosses.
- Linkage is common for oligogenic and polymeric traits, and it is always more common for the former than for the latter.
- Linkage ensures that genes are transmitted not separately, but collectively as a single unit.

Note:
- Completely linked genes are very close to each other so that there is the least chance for their shuffling by crossing over.
- The linkage between the gene for the bent wing and those for shaven abdominal bristles in drosophila is an example of complete linkage.
- In drosophila complete linkage is observable for all the genes in all chromosomes, when the male parent is heterozygous. On the other hand, when a male parent is a homozygote, the incomplete linkage is observed.