Genealogy isn’t just a great way to learn about your personal history; it may also hold the key to longstanding mysteries in biological science.
For decades, scientists and philosophers around the world have wondered what causes the “returning soldier effect” — a boom in baby boys born after a society’s soldiers return from war. Theories ranged from the divine (a heaven-sent replenishment of lost sons) to hormonal (surviving soldiers were more ardent and likely to fertilize their wives when sons were most likely).
But a biologist says he’s found the real answer by combing through 973 family trees, containing information on 556,387 people going back to 1600. If a man produced more sons than daughters, those sons were more likely to have more sons as well. Newcastle University’s Corry Gellatly, who conducted the research, believes a gene that’s expressed in men may be the cause.
Gellatly’s research leads him to believe that the gene causes some men’s sperm to produce X chromosomes, while the same gene in other men causes them to produce more Y chromosomes. Chromosomes from one lucky sperm combine with an egg to produce a baby: XY chromosomal pairs produce boys and XX pairs produce girls.
The way this phenomenon plays out over a few generations helps explain how the population may stay fairly balanced between males and females after a war. Let’s say two patriarchs, Harry and Sam, live in a post-war nation where women outnumber men due to war casualties. Before the war, Henry had three boys and one girl, and Sam had one boy and three girls. With three boys, Henry stands a better chance than Sam of having a son return from war to start his own family. And when Henry’s sons do start having kids, Henry’s grandchildren are more likely to be boys, thereby helping to replenish the stock of available men in a subsequent generation.
“The family tree study showed that whether you’re likely to have a boy or a girl is inherited,” Gellatly told Popular Science. “We now know that men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters.” Women, however, did not have the same tendency.”
For now, Gellatly’s theory remains just that; no one’s identified the gene that makes men more likely to send forth X versus Y emissaries. But luckily, anyone can replicate Gellatly’s research — if on a smaller scale — by investigating their own family tree and tallying the number of Jacks versus Jills and from generation to generation.
—Sandie Angulo Chen
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