A UK researcher has a new explanation for how the human race manages to keep a fairly even balance of males and females, despite massive deaths of young males in war and selective abortion of female foetuses in certain parts of the world.
Corry Gellatly, a research scientist at Newcastle University, proposes that there's a gene that determines whether a man will father more sons, more daughters, or equal numbers of each. When females are in short supply, they have a better chance of snagging a mate, and are thus more likely to pass the gene for fathering daughters on to their offspring. And when men are scarce, they have a better chance of mating and passing along the gene for having sons.
A counter balancing mechanism
"It's kind of a counterbalancing mechanism," Gellatly explained in an interview. "You can't get a population that becomes too skewed toward males or too skewed toward females."
The ratio of male to female births jumped significantly at the end of each of the world wars in countries involved in the fighting. A number of hypotheses have been floated to explain why. One idea is that returning soldiers have extra-frequent sex with their partners, which could lead to fertilisation earlier in the menstrual cycle, possibly making male births more likely. Another hypothesis holds that larger males are more likely to survive wars and more likely to father boys.
After sorting through 927 family trees from North America and Europe, including 556 387 people in all, Gellatly proposes another explanation.
In an article published online in the journal Evolutionary Biology, the researcher suggests that men carry a gene that controls their ratio of X to Y sperm, and thus the likelihood of their fathering sons or daughters. Women carry the gene as well and pass it along, but do not express it.
Gellatly made a computer model simulating how the gene would act over 500 generations, and also examined whether offspring sex ratios in the real-life family trees supported his hypothesis. Both experiments bore out his idea of a gene for gender.
The gender gene appears to be very ancient, Gellatly said, and is possibly carried by any species - plant or animal - that reproduces sexually rather than asexually.
How it might work
Almost all of our genes come in pairs, with one being inherited from each parent. Gellatly hypothesizes that the gender-controlling gene comes in a "male" and "female" version, with three possible combinations of the two. A man could have a "male-male" gene, which would promote the formation of Y sperm; a "male-female" gene, which would cause him to produce about the same number of X and Y sperm; and a "female-female" gene, which would cause him to make more X sperm. "The structure of the proposed gene is essentially very basic, and its function is simply to say 'produce more boys' or 'produce more girls,'" Gellatly explains.
The gene makes fathering offspring of a particular gender more likely but not a certainty, he adds, and inheritance from father to son is diluted by the part of the gene that the mother contributes. "It's a fairly small effect. If it was a larger effect, it would have been noticed before."
Gellatly's theory can also explain why an increase in boy births may be seen after a war. Families with more sons will be more likely to have surviving male children, who can pass along their genes, while families with fewer male offspring are less likely to have surviving sons.
SOURCE: Evolutionary Biology, published online December 2008.
(Reuters Health, January 2009)