Phases of linkage

 There are two phases of linkage, viz., coupling phase and repulsion phase. These phases were given by Bateson and Punett (1905), but they could not give a proper interpretation of these terms. Later on, Morgan (1910) based on his studies with Drosophila explained that coupling and repulsion are the two aspects of the same phenomenon what we call linkage.

i. Coupling phase

The linkage between two or more either dominant (AB) or recessive (ab) alleles is referred to as coupling. A good example of coupling was reported by Hutchinson in maize for the genes governing the color of seed (colored and colorless) and shape of seed (full and shrunken). The colored seed is governed by a dominant gene (C) and the full seed is also governed by a dominant gene (S). He made a cross between plants having colored full seeds (CCSS) and colorless shrunken seeds (ccss). The F1 seeds were colored full. When the F1 was test crossed with the double recessive parent the following results were obtained instead of 1:1:1:1 ratio.

This indicates that parental combinations are higher than recombination’s, indicating the presence of linkage. The parental combinations occurred in 96.4% instead of 50% and re-combinations were 3.6% instead of 50% in this case. There are several other cases of coupling in other plant species.

ii. Repulsion phase

The linkage of the dominant allele with that of the recessive allele (Ab or aB) is known as repulsion. Hutchinson also observed the repulsion phase of linkage in maize. He observed this type of linkage when he made a cross between plants having colored shrunken seeds (Cs) with those having colorless full seeds (cS). In F1 the seeds were colored full. By crossing F1 with the double recessive parent, the following results were obtained instead of the 1:1:1:1 ratio.

Again, parental combinations were higher (97.1%) than expected (50%) and recombinations were lower (2.9%) than expected (50%). Thus, in both cases, linked genes tend to remain together during hereditary transmission. Haldane (1942) used the terms cis and trans for coupling and repulsion, respectively.