Relationship of genotype and degree of chimerism in coat color to sex ratios and gametogenesis in chimeric mice

Abstract Genotype and the degree of chimerism expressed in the coat color of chimeric (allophenic) mice were found to be related to the frequency of multicolored chimeras, the sex ratio, and the production of gametes. The sex ratios of 310 chimeric mice of various combinations of genotypes were studied. The male: female ratios varied from 1:1 to ratios approaching 3:1. Multicolored chimeras in which one genotype predominated (“unbalanced” genotypes) had normal sex ratios of 1:1 (19 ♂ ♂, 21 ♀ ♀ ) suggesting that sex was determined by the predominating component. Multicolored chimeras in which neither component predominated (“balanced” genotypes) had sex ratios approaching 3:1 (70 ♂ ♂, 31 ♀ ♀ ) suggesting that most sex chimeras (XX ⟷ XY) develop as phenotypic males. Regardless of genotype, solid colored chimeras had normal sex ratios. Only two hermaphrodites (0.65%) were observed. A high incidence of gonadal mosaics which produced progeny with both parental genotypes and a relatively low incidence of possible sex chimeras which produced progeny with only one genotype occurred among multicolored females. A very low incidence of gonadal mosaics and a very high incidence of possible sex chimeras was found among males. These observations would be expected if most sex chimeras developed as phenotypic males.