LONDON--Mammalian parents are caught in a fierce conflict over developing embryos. A father wants bigger babies, because they are usually healthier and fitter. But a mother prefers them to be small, because a fetus that grows too large may drain her resources and jeopardize her ability to sustain additional pregnancies. Scientists have long suspected that special, chemically marked genes, called imprinted genes, are the weapons wielded in this parental warfare over size. Exactly how genes fight such a battle, though, has remained a mystery. Now biologists have discovered a clever strategy employed by the father¿s genes to gain the upper hand.

The father¿s genes tend to aim for large, nutrient-guzzling babies to maximize the chances of being passed on. Because few male mammals are monogamous, the father can be fairly sure that the next litter will not be his, and so his genes strive for copious nourishment to be pumped into his developing offspring. ¿Paternal genes are greedy and suck out precious nutrients from the mother,¿ explains Wolf Reik, a molecular biologist at the Babraham Institute near Cambridge, England. But the fetus ¿can kill the mother if [it grows] too big,¿ he says.

This article was originally published with the title Womb Wars.

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