On the occurrence and significance of deleterious genes in cotton

An account is given of the genetics of deleterious characters in cottons. Six such characters, all apparently due to single genes, have been recorded among Old World cottons. Genetic studies on five such characters in New World cottons have been published elsewhere. Further data are given on chlorophyll deficiency, and accounts are presented of the genetics of nine characters that have not been reported previously. Of the fourteen deleterious characters in New World cottons for which data are available, three have a three-factor, five a duplicate-factor, and six a single-factor basis. A remarkably close parallelism exists between the deleterious characters recorded from the Old World diploid and New World polyploid cottons. The importance of gene duplication in making possible the survival of otherwise deleterious mutant genes is noted, but it is pointed out that since three triple gene characters have been discovered, the allopolyploid nature of the New World cottons is not alone sufficient to account for the multiple-factor segregations observed. No evidence was discovered of divergence in function between duplicate loci, but it is suggested that a search among the allopolyploids for homologues of genes not markedly deleterious in the diploids, might reveal its existence. All deleterious genes recorded in Old World cottons, with the exception of crumpled, are completely recessive. The deleterious characters in the New World cottons are very often not completely recessive, easily distinguished heterozygotes having been recorded in five characters, and incomplete dominance in four others. It is suggested that this low dominance level may be a consequence of the rapid evolution of the New World cottons, which has been such that a high degree of internal stability has not yet been achieved.