Sub Link: Human sex ratio

The two sex chromosomes determine the biological sex of the person. People with two X chromosomes are biologically female, while people with one X and one Y chromosome are biologically male. 

Men have both X and Y chromosome-bearing sperm, and can therefore pass down either one to a given child. If the Y sperm fertilizes an egg, the child will be male (XY), and if the X sperm fertilizes an egg, the child will be female (XX). Therefore, sperm are responsible for determining the sex of a child.

In theory, there is an equal opportunity for either type of sperm to fertilize an egg. However, as we have illustrated above, there are many reports that suggest that a given individual does not produce an equal number of X sperm and Y sperm.

Human Sex Ratio

Worldwide, the human sex ratio at birth is fairly constant at 51.4% males (James, 1996). This supports the hypothesis that the average X:Y ratio in humans is roughly 1:1. Yet at the individual level, this ratio can vary widely.

The assumption has long been that the percentages of X- and Y-bearing sperm in a given sample would be equal. But a deeper review of the existing literature reveals wide variation in percentages among individuals. Similar observations have been reported in farm animals. 

References

• Bibbins, P. E. Jr., Lipshultz, L. I., Ward, J. B. Jr., and Legator, M. S. (1988). Fluorescent body distribution in spermatozoa in the male with exclusively female offspring. Fertil. Steril. 49, 670–675. doi: 10.1016/s0015-0282(16) 59838-0

• Chandler JE, Steinholt-Chenevert HC, Adkinson RW, Moser EB. 1998. Sex ratio variation between ejaculates within sire evaluated by polymerase chain reaction, calving and farrowing records. J Dairy Sci. 81:1855–1867. doi:10.3168/jds.S0022-0302(98)75756-X.

• Chaudhary I, Jain M, Halder A. 2014. Sperm sex ratio (X: y ratio) and its variations. Austin J Reprod Med Infertil. 1(1):7.

• Checa ML, Dunner S, Cañón J. 2002. Prediction of X- and Y-chromosome content in bovine sperm by using DNA pools through capillary electrophoresis. Theriogenology. 58:1579–1586. doi:10.1016/S0093-691X(02)01077-4.

• Han TL, Ford JH, Webb GC, Flaherty SP, Correll A, Matthews CD. 1993. Simultaneous detection of X- and Y-bearing human sperm by double fluorescence in situ hybridization. Mol Reprod Dev. 34:308–313. doi:10.1002/ mrd.1080340311.

• Landrum, B., and Shettles, L. B. (1960). Nuclear structure of human spermatozoa. Nature 188, 916–918. doi: 10.1038/188916a0

• Martin, R. H., Balkan, W., Burns, K., Rademaker, A. W., Lin, C. C., and Rudd, N. L. (1983). The chromosome constitution of 1000 human spermatozoa. Hum. Genet. 63, 305–309. doi: 10.1007/bf00274750

• Quinlivan, W. L., and Sullivan, H. (1974). The ratios and separation of X and Y spermatozoa in human semen. Fertil. Steril. 25, 315–318. doi: 10.1016/s0015-0282(16)40330-4

• Rorie RW, Delgado P, Lester T. 2014. Variation among beef bulls in the ratio of X- to Y-chromosome bearing spermatozoa. Adv Reprod Sci. 2:69–75. doi:10.4236/arsci.2014.24008

• Shettles, L. B. (1960). Nuclear morphology of human spermatozoa. Nature 186, 648–649. doi: 10.1038/186648a0

• Checa ML, Dunner S, Cañón J. 2002. Prediction of X- and Y-chromosome content in bovine sperm by using DNA pools through capillary electrophoresis. Theriogenology. 58:1579–1586. doi:10.1016/S0093-691X(02)01077-4.

• James, W. H. (1996). Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels at the time of conception. The Journal of Theoretical Biology, 180(4), 271–286.

• Rorie RW, Delgado P, Lester T. 2014. Variation among beef bulls in the ratio of X- to Y-chromosome bearing spermatozoa. Adv Reprod Sci. 2:69–75. doi:10.4236/ arsci.2014.24008.

• Umehara T, Tsujita N, Shimada M, Yamashita YM. 2019. Activation of Toll-like receptor 7/8 encoded by the X chromosome alters sperm motility and provides a novel simple technology for sexing sperm. PLoS Biol. 17(8):e3000398. Published 2019 Aug 13. doi:10.1371/jour- nal.pbio.3000398