- There is an as yet unidentified gene that affects the number of X or Y chromosomes in sperm, and therefore the sex of future children.
- A man with more brothers will probably have more sons, and if he has more sisters, more daughters.
If you want to know the sex of a future child, consult the family tree of the father. A study by the University of Newcastle (UK) conducted among thousands of families suggests so. A total of 927 family trees were peeled. They include data from 556,387 people in North America and Europe, dating back to the year 1600. According to the study’s lead author, Corry Gellatly, men inherit the tendency to have more sons or daughters. Those with multiple brothers are more likely to have sons, while men with multiple sisters are more likely to have daughters. “In women, however, you can’t predict it,” he said. The results of this work have been published in the journal Evolutionary Biology.
The role of chromosomes
Until now, it was known that men determined the sex of their future babies based on the X or Y chromosome contained in their sperm. The combination of the paternal X chromosome with the mother’s X chromosome produces a girl (XX), while the father’s Y chromosome with the maternal X chromosome results in a boy (XY). For researchers at the University of Newcastle, there would be an as yet unidentified gene which would affect the number of X or Y chromosomes in the sperm, and therefore the sex of future children.
Several possible combinations
A gene is made up of two parts, called alleles. Each e allele is inherited from one of the two parents. In his work, Corry Gellatly hypothesized that males may carry two different types of alleles. This therefore implies that there would be three possible combinations for a gene. Males with the first combination (named mm) would produce more Y chromosomes and have more sons. Those with the second combination (mf) would spawn roughly the same number of sons and daughters. The last (ff), producing more X chromosomes, would therefore have more daughters.
The key to demographic balance?
“This gene passed down from both parents, which causes some men to have more sons and some more daughters, may explain why we see roughly equal numbers of males and females in the population. For example, if there are too many men in the population, women will find a mate more easily, so men who have more daughters will pass on more of their genes, what will produce more women in future generations“explains Corry Gellatly.
The case of wars
According to the authors of the study, this theory could explain the fact that there was a sudden increase in the number of boys in the countries which participated in the two world wars. In fact, the men with the most sons were more likely to have one return alive from the clashes. These survivors, who had their father’s genes, could therefore have more boys. Conversely, those who had more daughters were likely to lose their only son, and therefore also the gene to sire more daughters.