A Note on Fisher's Theory of the Origin of Dominance, and on a Correlation between Dominance and Linkage

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Abstract
The writer cites Fisher''s suggestion that a mutant gene usually produces at first a marked effect on the heterozygote, i.e., it is dominant or partially so. Since, however, the action of the new dominant is likely to be unfavorable to survival, its dominant behavior is a disability, and natural selection tends to eliminate it. This is brought about, according to Fisher, by prolonged selection of modifying genes which change the dominant to a recessive. Haldane, on the basis of statistical evidence, believes this explanation improbable. He considers it likely that genes are of the nature of catalysts, as held by Goldschmidt, and that most observable mutations therefore involve a reduction in activity of the gene concerned. On this assumption, he shows that it is advantageous for a species to possess wild-type genes with a double margin of safety. Then if a mutation (i.e., a reduction in catalytic activity) occurs in one member of a pair, the other member can still do its work adequately. If this is true, then allelomorphs having this excess catalytic power will be favored by natural selection when in the heterozygous state with one of the above mentioned types of mutation. It is this sort of selection rather than Fisher''s selection of modifying genes which is thought the more probable explanation of the dominance of the wild-type. Finally a theory of dominance must explain why in certain cases there are many more dominant mutants than recessives, and why this condition is correlated with a very small number of linkage groups as compared with the number of chromosomal pairs. Haldane believes this relationship is perhaps explainable by assuming that the dominant mutants are due not to single gene changes, but to the translocation or duplication of whole sections of chromosomes. He thinks the cytological work of Carothers on Orthoptera tends to support this view.

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