Theories of Heterosis
There are two main theories which have been used to explain the mechanism of heterosis. One is the dominance hypothesis and the second is overdominace hypothesis. The epistasis is also considered to be associated with heterosis. Thus, there are three possible genetic causes of heterosis viz.
1) Dominance,
2) Over dominance
3) Epistasis
These are briefly discussed below:
1) Dominance Hypothesis:
This theory was proposed by Davenport (1908) Bruce (1910) and Keeble and Pellew (1910). This is the most widely accepted hypothesis of heterosis. According to this hypothesis, heterosis is the result of the superiority of dominant alleles, when recessive alleles are deleterious; here the deterious recessive genes of one parent are hidden by the dominant genes of another parent and the hybrid exhibits heterosis. Both the parents differ for dominant genes. Suppose genetic constitution of one parent is AABBccdd and that of another as aabbCCDD. A hybrid between these two parents will have four dominant genes and exhibit superiority over both the parents which have two dominant genes each. Thus heterosis is directly proportional to the number of dominant genes contributed by each parent.
AABBccdd X aabbCCDD ——-) AaBbCcDd
Parent 1 Parent 2 Hybrid
Objections:
There are two objections to dominant gene hypothesis.
i) If it is true it is should be possible to obtain pure heterotic individuals in F2 which are homozygous for all the dominant genes. Jones (1917) provided explanation for this. He suggested that there may be linkage between some favourable dominant genes and some unfavourable recessive genes and as a result. It is not possible to obtain true breeding homozygous individual for all dominant genes to F2 generation. He proposed dominance of linked gene hypothesis to explain heterosis.
ii) If the heterosis is due to dominance, the F2 curve should be skews towards dominant genes, but the curve of F2 is found always smooth and symmetrical not skewed. Collins (1921) provided explanation for the objections. He suggested that trial like yield is governed by large number of genes or poly genes, which exhibit continuous variation resulting in symmetrical distribution of genes.
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.
3) EPISTASIS:
Epistasis refers to interaction between alleles of two or more different loci. It is also known as non-allelic interaction. The non-allelic interaction is of three type’s viz. additive X additive, dominance X dominance and heterosis has positive association with the presence and magnitude of non allelic interaction. Epistasis, particularly that involves dominance effects (dominance X dominance) may contribute to heterosis. This has been observed in cotton and maize. Epistasis can be detected or estimated by various biometrical models.