Principles of Plant Breeding (2+1)

Genetic basis of heterosis

• There are two main theories of heterosis and inbreeding depression. 1. Dominant hypothesis 2. Over dominance hypothesis.

1. Dominant hypothesis :
First proposed by Davenport in 1908. It was later on expanded by Bruce, Keeble and Pellow.
According to this hypothesis at each locus the dominant allele has favourable effect, while the recessive allele has unfavourable effect. In heterozygous state, the deleterious effect of recessive alleles are masked by their dominant alleles. Inbreeding depression is produced by the harmful effects of recessive alleles which become homozygous due to inbreeding.
Two objections have been raised against the dominant hypothesis.

a) Failure of isolation of inbreds as vigorous as hybrids :
According to dominance hypothesis it is possible to isolate inbreds with all the dominant genes E.g. AA.
This inbreed should be as vigourous as that of hybrid. However in practice such inbreds were not isolated.

b) Symmetrical distribution in F2:
In F2dominant and recessive characters segregate in the ratio of 3:1. Quantitative characters, according to dominance hypothesis should not show symmetrical distribution. However, F2nearly always show symmetrical distribution.

Explanation for the two objections:
In 1917 Jones suggested that since quantitative characters are governed by many genes, they are likely to show linkage. In such a case inbreds containing all dominant genes cannot be isolated. So also the symmetrical distribution in F2is due to linkage. This explanation is often known as Dominance of Linked Genes Hypothesis.

2. Over dominance hypothesis:
This hypothesis was independently proposed by East and Shull in 1908. It is also known as single gene heterosisor super dominance theory. According to this hypothesis, heterozygotes or at least some of the loci are superior to both the homozygotes. Thus heterozygote Aa would be superior to AA and aa. In 1936 East proposed that at each locus there are several alleles a1a2a3a4etc, with increasingly different functions. Heterozygotes between more divergent alleles would be more heterotic. E.g. a1a4will be superior to a1a2or a2a4