Having found a dramatic shift in sex ratio at birth in one species of mammal, I have for decades kept an eye on discussions as to whether the human sex ratio at birth differs in some circumstances from 1 male: 1 female. Mammals in theory have two ways of controlling the sex ratio at birth: (i) at conception, or (ii) later by selectively reducing the number of young in utero. We found the latter mechanism at work in the guinea-pig. Since human litter size usually equals the number of eggs and rarely exceeds 1 and since gestation is relatively long, it always appeared that if there were to be any maternal or paternal genetic control of the sex of the offspring it would have to be at the time of conception, rather than by selective death and reabsorption of embryo or fetus.
In the 1980s I once did the experiment of asking biologists from different disciplines why they thought the human sex ratio is 1:1. The reproductive biologists replied that it was just the result of random segregation of the sex chromosomes. The sex of a human offspring depends on whether it inherits and X or a Y chromosome from its father. Random segregation will, on average, result in a 1:1 ratio. By contrast, the evolutionary biologists said that the sex ratio is explained by Sir Ronald Fisher’s Principle: with the sex of a subject to genetic variation, the the sex ratio will always stabilise at 1:1. I will not repeat the simple explanation that can be found here.
Shifts of the of sex ratio at birth in various animals has been explained in terms of the Trivers-Willard hypothesis: parents which possess a heritable trait that benefits the lifetime reproductive success of one sex will bias the sex ratio towards that sex. There have been various claims in studies of human populations along these lines; for example, male-biased sex ratios in taller, wealthier, high status parents with the offspring likely to be more successful in competition for a mate. However, such claims have been controversial because of the statistical methods used and the results have often not borne out when larger samples were taken from the population.
Both the Fisher Principle and Trivers-Willard rely on there being genetic variation in the sex ratio, in other words that a bias towards one sex or the other is heritable. Thus a key test is to look for heritability in a very large human population. On standard scale of 0 to 1, ‘0’ denotes that a trait is not heritable while ‘1’ all differences in a trait can be explained entirely by genetic variation. Many traits fall somewhere between those two extremes.
But what does determine human sex ratio? It would be predicted if sex ratio is a heritable trait then Fisher’s Principle would apply. By contrast, if it is not heritable then simple Mendelian segregation of the sex chromosomes would suffice as an explanation.
Over the years there have been all sorts of suggestions and claims that the tendency in a family to produce offspring completely or partially biased to one sex is hereditary, and that particular genes could be involved. However, these conclusions have been criticised because the sample sizes were small and the statistical inferences drawn were invalid.
A recent, important paper has tackled the problem by using data from the entire population born in Sweden in and after 1932. That was 3,543,243 individuals and their 4,753,269 children. The results of the analysis were clear. There was no evidence of heritability at all. The calculated heritability was 0. In other words, there was no need to invoke Fisher’s Principle since with no heritability there can be no Fisher.
The authors summed up their results:
In sum, all of our results are consistent with the simple explanation that variation in offspring sex ratio in humans is due to unbiased Mendelian segregation of sex chromosomes during spermatogenesis and unbiased fertilization. The slight excess of male births is likely to be due to a general difference in survival of male and female embryos in the womb, the reasons for which are not yet understood.
Looks like my reproductive biology colleagues were right. Pity I can’t tell most of them; they are long dead.
The question now, of course, is whether the same conclusion, that the human sex ratio at conception is simply the outcome of random segregation of the sex chromosomes, applies to other or to all mammals? And are the statistically-robust demonstrated shifts in sex ratios at birth in some species and in certain environmental conditions all the result of differential loss of embryos and fetuses in utero? I shall permit myself to guess that the answers are ‘Yes’ and ‘Yes’ even though, as with previous human studies, there have been claims that the answer to the first question is ‘No’.
Zietsch BP, Walum H, Lichtenstein P, Verweij KJH, Kuja-Halkola R. 2020 No genetic contribution to variation in human offspring sex ratio: a total population study of 4.7 million births. Proceedings of the Royal Society B 287: 20192849. http://dx.doi.org/10.1098/rspb.2019.2849
Peaker M, Taylor E. 1996. Sex ratio and litter size in the guinea-pig. Journal of Reproduction and Fertility 108, 63-67.
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