Question

Overdominance hypothesis of heterosis given by

Answer 1

2) Overdominace Hypothesis:

It was independently proposed by Shull and East in 1908 and supported by Hull ( 1945). This theory is called by various names such as stimulation of heterozygosis, cumulative action of divergene alleles, single gene heterosis, super dominance and over dominance. Though this theory was proposed by Shull and East in 1908, the overdominace was coined by Hull in 1945 working on maize. This term is now in common use.

According to this hypothesis is the result of superiority of heterozygote over its both homozygous parents. Thus heterosis is directly proportional to the heterozygosis. The superiority of heterozygote over both homozygotes may arise either due to if 
1) Production of superior hybrid substances in heterozygote is completely different from either of the homozygous products or due to 
2) Greater buffering capacity in the heterozygote resulting from cumulative action of divergent alleles of stimulation of divergent alleles.

East in 1936 further elaborated this theory by proposing a series of alleles a1,a2,a3,a4 ----- of gradually increasing divergence in function. Thus a combination of more divergent alleles will exhibit higher heterosis than less divergent combinations. For example, combination of a1, a4 will exhibit higher heterosis as compared to combination as a1, a2, a3 and a4.

Overdominace has been reported in barley. In maize, available evidence suggest that if overdominace occurs, it is either infrequent in occurrence or small in magnitude. Dominance and overdominace hypothesis have some similarities and some dissimilarity.