1. Further experiments are described on the mode of inheritance and distribution in six species of New World cottons of a pair of duplicate factors for chlorophyll deficiency.
2. Duplication of factors is considered to have taken place through polyploidy with subsequent mutation of one or other of the members constituting the pair in some of the species.Barbadense andDarwinii have become monomeric through the loss of factorC hb whilepurpurascens, hirsutum, andtaitense, when monomeric, are shown to have lostC ha .
3. The experimental data support Haldane’s view that in polyploid species one member of a pair of duplicate genes may mutate without disadvantage, provided its functions can be performed by a gene in one of the other sets of chromosomes.
4. The taxonomic and evolutionary significance of the results is discussed and it is suggested that the extent to which the dimeric condition is converted to the monomeric in polyploid species may provide some indication of the age of the species. In an old series of allopolyploids such as the New World species ofGossypium are considered to be, cases of dimery might be expected only infrequently, and usually only in interspecific crosses.
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