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