Trends in Population Sex Ratios May be Explained by Changes in the Frequencies of Polymorphic Alleles of a Sex Ratio Gene
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.
KeywordsSex ratio Heritable variation Human genetics Polymorphism Mortality War
- Ancestry.com. (2008). British Army WWI Service Records, 1914–1920 [online database]. The Generations Network, Inc., Provo, UT, USA. http://www.ancestry.co.uk. Accessed 29 Oct 2008.
- Beladjal, L., Vandekerckhove, T. T. M., Muyssen, B., Heyrman, J., de Caesemaeker, J., & Mertens, J. (2002). B-chromosomes and male-biased sex ratio with paternal inheritance in the fairy shrimp Branchipus schaefferi (Crustacea, Anostraca). Heredity, 88, 356–360. doi:10.1038/sj.hdy.6800061.PubMedCrossRefGoogle Scholar
- Charnov, E. L. (1982). The theory of sex allocation. Princeton, NJ: Princeton University Press.Google Scholar
- Darwin, C. 1871. The Descent of Man. London: Murray.Google Scholar
- Fisher, R. A. (1930). The genetical theory of natural selection. Oxford: Oxford University Press.Google Scholar
- GenealogyForum.com. (2008). Genealogy Forum GEDCOM Library. Golden Gate Services, Inc., Armada, Michigan. http://www.genealogyforum.com/gedcom. Accessed 29 Oct 2008.
- Gini, C. (1908). Il Sesso dal Punto di Vista Statistico. Milan: Sandron.Google Scholar
- Gini, C. (1955). Sulla probabilita che x termini di una serie erratica sieno tutti crescenti (o non decrescenti) ovvero tutti decrescenti (o non crescenti) con applicazioni ai rapporti dei sessi nelle nascite umane in intervalli successivi e alle disposizioni sessi nelle fratellanze umane. Metron, 17(3–4), 1–41.Google Scholar
- Graffelman, J., & Hoekstra, R. F. (2000). A statistical analysis of the effect of warfare on the human secondary sex ratio. Human Biology, 72(3), 433–445.Google Scholar
- Gray, R. H., Simpson, J. L., Bitto, A. C., Queenan, J. T., Li, C. J., Kambic, R. T., et al. (1998). Sex ratio associated with timing of insemination and length of the follicular phase in planned and unplanned pregnancies during use of natural family planning. Human Reproduction (Oxford, England), 13(5), 1397–1400. doi:10.1093/humrep/13.5.1397.CrossRefGoogle Scholar
- Seger, J., & Stubblefield, J. W. (2002). Models of sex ratio evolution. In I. C. W. Hardy (Ed.), Sex ratios concepts and research methods (pp. 2–25). Cambridge, MA: Cambridge University Press.Google Scholar
- Trichopoulos, D. (1967). Evidence of genetic variation in the human sex ratio. Human Biology, 39(2), 170–175.Google Scholar
- Wilcox, A. J., Weinberg, C. R., & Baird, D. D. (1995). Timing of sexual intercourse in relation to ovulation––effects on the probability of conception, survival of the pregnancy, and sex of the baby. The New England Journal of Medicine, 333(23), 1517–1521. doi:10.1056/NEJM199512073332301.PubMedCrossRefGoogle Scholar