From 2008: Is it a boy or a girl? The father’s family might provide a clue

From 2008: Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene. Corry Gellatly. Evolutionary Biology volume 36, pages190–200, December 10 2008. https://link.springer.com/article/10.1007%2Fs11692-008-9046-3

Abstract: A test for heritability of the sex ratio in human genealogical data is reported here, with the finding that there is significant heritability of the parental sex ratio by male, but not female offspring. A population genetic model was used to examine the hypothesis that this is the result of an autosomal gene with polymorphic alleles, which affects the sex ratio of offspring through the male reproductive system. The model simulations show that an equilibrium sex ratio may be maintained by frequency dependent selection acting on the heritable variation provided by the gene. It is also shown that increased mortality of pre-reproductive males causes an increase in male births in following generations, which explains why increases in the sex ratio have been seen after wars, also why higher infant and juvenile mortality of males may be the cause of the male-bias typically seen in the human primary sex ratio. It is concluded that various trends seen in population sex ratios are the result of changes in the relative frequencies of the polymorphic alleles of the proposed gene. It is argued that this occurs by common inheritance and that parental resource expenditure per sex of offspring is not a factor in the heritability of sex ratio variation.

Popular writing: Is it a boy or a girl? The father’s family might provide a clue. Robyn Horsager-Boehrer. The University of Texas Southwestern Medical Center, June 25, 2019. https://utswmed.org/medblog/it-boy-or-girl-fathers-family-might-provide-clue/

Researchers in England set out to determine whether this is true. They downloaded family trees from the Genealogy Forum, then eliminated data they felt weren’t accurate – for instance, people reported as having more than two parents or a discrepancy in an individual's sex. This left researchers with 927 family trees that had at least three generations and included over half a million individuals dating back to 1600.

Their findings were telling. In the computer models, when researchers removed men from the population data before they had a chance to start families, there was an increase in the number of male babies born in the next generation. The researchers also found that the sex ratio for families followed the father's side, not the mother's side. For example, if a man had more brothers, his own children were more likely to be male; if he had more sisters, he was more likely to have daughters. This was not found to be the case for women.

According to this study, the explanation might be due to a gene that controls the balance of X- and Y-carrying sperm. Men carrying a gene that leads to their sperm having more Y chromosomes have more sons. During times of war and large casualties of male soldiers, those families are more likely to have more surviving sons. And when those men have children, they, like their fathers, might be more likely to have baby boys. This could account for the temporary increase in the sex ratio for that time period.