Segregation Analysis of a Sex Ratio Distortion Locus in Congenic Mice

The congenic HG.CAST-(D17Mit196-D17Mit190) (HQ17(hg/hg)) mouse strain showed a significant departure on the expected 50%/50% offspring sex ratio in more than 2400 progeny (55.7% females). The entire pedigree file included data from 13 nonoverlapping purebred generations and an F(2) cross with the C57BL/6J inbred strain. Offspring sex ratio data were analyzed on the basis of 40 purebred HQ17(hg)(/hg) sires and 29 F(1) HQ17(hg)(/hg) x B6 sires under a Bayesian Binomial segregation model accounting for 4 different autosomal inheritance models of gene action (i.e., additive, dominance, recessive, and overdominance) and X-linked and Y-linked loci. For each model, the segregation effect was evaluated as a single regression coefficient for all sires or assuming 2 independent regression coefficients accounting for offspring sex ratio departures in purebred and F(1) sires, respectively. The deviance information criterion clearly favored the autosomal dominance model with different regression coefficients for the 2 groups of sires. Under this model, the dominance effect increased the percentage of female offspring by 4.3% (HQ17(hg)(/hg) purebred sires) and 8.2% (F(1) sires) with the highest posterior density regions ranging from 0.5% to 10.6% and from 1.3% to 14.4%, respectively. This article provides significant evidence of genetic determinism for sex ratio distortion in the HQ17(hg)(/hg) strain and develops new analytical tools to perform segregation studies on dichotomous traits.