The genetics of human sex ratio evolution

Abstract

This study examined the hypothesis that natural selection exerts control of the human sex ratio via allelic variation in an autosomal gene that is phenotypically expressed in the male reproductive system. The hypothesis was supported by results from an analysis of a large genealogical dataset, in which inheritance of sex ratio variation by male but not female offspring was found. A series of simulations with a population genetic model showed that equality of the sex ratio may be maintained in a dynamic equilibrium by frequency dependent selection acting on such a gene. These simulations also suggest that long-term oscillations and autocorrelation between years in annual human sex ratio data may be explained by the hypothesis. A further set of simulations showed that an episode of increased male mortality - in a population with a sex ratio determined by the proposed gene - may result in a sudden increase in male births, provided the mortality is limited to a narrow cohort of males and that families with a greater tendency to have male offspring tend to be larger than those with a tendency to produce equal male and female offspring. To explore whether this could provide an explanation for significant increases in male births observed during periods of war, military service records and genealogical data were examined to determine the age structure of recruits to the British Army in the First World War and the typical age of fatherhood at the time. It was found that the cohort of men lost to the war were younger than men who typically became fathers. It was also found that families with offspring of a single sex tend to be larger than those with both sexes. As such, this work supports the hypothesis that the loss of young men in war results in a relative increase in male births, due to increased fatherhood by men from families with more male offspring (i.e. men with more brothers than sisters), because these men are most likely to have inherited a greater tendency to produce male offspring.