Complete and Incomplete Linkage

Complete Linkage:

In complete linkage, the linked genes are transmitted together to the offspring and only parental combinations of genes appear for two or more or for several generations. It means in complete linkage linked genes don’t separate to form nonparental combinations during gametogenesis and produce two types of gametes having genes in parental combinations only. The gametes are called non-crossover gametes.

When genes are very closely placed in the chromosome, they are always transmitted together and show complete linkage. However, complete linkage is rare. In Drosophila, all the mutant alleles present on the fourth chromosome show complete linkage. Also when F1 heterozygous males are test crossed with a double recessive female. They produce only parental combinations showing complete linkage.

Incomplete Linkage:

Linkage is incomplete when new or nonparental combinations of linked genes are also formed. It means linked genes separate in certain cases as a result of crossing over. In incomplete linkage, the homologous chromosomes undergo breakage and reunion under meiosis at the time of gamete formation. As a result of the exchange, crossover gametes or recombinant gametes are formed along with non-crossover gametes. Incomplete linkage is very common and has been studied in almost all organisms.