![]() This hypothesis was given by Shull (1903) and East (1908) independently. According to him trait like yield is governed by large number of genes or polygenes, which exhibit continuous variation resulting in symmetrical distribution of genes. This objection was removed by Collins (1921). If heterosis is due to dominance of independent factors, the F 2 distribution curve for heterotic character should be skewed (slanting) rather than smooth and symmetrical.īut the curve of F 2 is always, smooth and symmetrical and not skewed. It is already studied that in F 2 dominant and recessive characters segregate in the ratio of 3:1. He suggested that there may be a linkage between some favourable dominant genes and some un-favourable recessive genes and as a result it is not possible to obtain true breeding homozygous individual for all dominant genes in F 2 generation. ![]() Jones (1917) in this modified theory entitled “Dominance of Linked Genes Hypothesis” provided explanation for this. However, such hybrids have not been isolated. Such inbreeds would be as vigorous as the F 1 hybrids. ![]() If the above hypothesis is true, it should be possible to isolate inbreeds with all dominant genes. Failure in the isolation of inbreeds as vigorous as hybrids: It can be further divided into two types:ġ. True heterosis (euheterosis) and pseudo-heterosis. ![]() Smith (1955) opined that the use of heterosis and hybrid vigour as synonyms is highly desirable on the basis of their long usage. However, Whaley (1944) was of the opinion that it would be more appropriate to term the developed superiority of the hybrids as hybrid vigour and to refer to the mechanism by which the superiority is developed as heterosis. On the basis of this explanation it was justified to use the term hybrid vigour as synonym of heterosis. It is generally agreed that hybrid vigour describes only superiority of the hybrid over the parents while heterosis describes the other situation as well i.e., crossing over may result in weak hybrids e.g., many hybrids in tomato are earlier (vegetative phase is replaced by reproductive phase).Įarliness in many crops is agriculturally desirable so, it is argued that F, shows faster development in which vegetative phase is replaced by the reproductive phase more quickly than in the parents. Hybrid vigour is used as synonym of heterosis. ![]()
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