Types of linkage

 Linkage is generally classified on the basis of three criteria, viz:

• Presence or absence of crossing over,

• Genes involved, and

• The chromosome involved.

Based on crossing over

(i) Complete linkage- Linkage in which crossing over does not occur is known as complete linkage or absolute linkage. In other words, when only parental types are obtained from the testcross progeny, it refers to complete linkage. A good example of complete linkage is the Drosophila male and female silk moth.

(ii) Incomplete Linkage- If some frequency of crossing over also occurs between linked genes, it is known as incomplete linkage. To put it another way, when recombinations are also observed in the testcross progeny, besides parental combinations, it refers to incomplete linkage.

Incomplete linkage has been observed in maize, pea, Drosophila female, and several other organisms.

Based on genes involved

(i) Coupling linkage- It refers to linkage either between dominant genes or between recessive genes. Such linkage has been reported in pea, maize, and several other crops.

(ii) Repulsion linkage- It refers to the linkage of some dominant genes with some recessive genes. This type of linkage has also been observed in pea, maize, and several other crops.

Based on chromosomes involved

(i) Autosomal linkage- It refers to the linkage of such genes which are located in other than sex chromosomes (autosomes).

(ii) X-chromosomal linkage- It refers to the linkage of genes that are located in sex chromosomes.