Half-chromatid Mutation as a Possible Cause of Mosaic Males and Females in Hymenoptera and Rare Fertile Male Tortoiseshell Cats.

Half-chromatid mutations occur when a single base change in a gamete is transmitted to the zygote which, after DNA replication and cleavage, will result in a mosaic individual. These mutations will be passed on through the germ plasm and also may be expressed somatically. Half-chromatid mutation has been suggested to account for the observed lower frequency of males than expected for lethal X-linked recessive disorders in humans, such as Lesch-Nyhan syndrome, incontinentia pigmenti and Duchene muscular dystrophy. Although attention has been paid to half-chromatid mutation in humans, it otherwise has been ignored. Here I show that half-chromatid mutation in haplodiploid organisms, such as Hymenoptera, has some interesting and important consequences: (1) since all genes follow the X-linked pattern of inheritance half-chromatid mutations should be relatively easier to detect, (2) recessive mutations of all viabilities may be expected, (3) mosaics of both sexes are expected in haplodiploids with half-chromatid mutation, (4) gynandromorphs could result from half-chromatid mutation at the sex-determination locus, in species with single-locus complementary sex-determination. Finally, half-chromatid mutation can account for the rare fertile male tortoiseshell phenotype observed in the domestic cat, Felis catus, and which still has not been fully accounted for by other mechanisms.