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Changing sperm speed can influence offspring’s sex, mouse study suggests

If you want a baby girl, don’t have sex too close to ovulation. So goes a common belief, which asserts that sperm with Y chromosomes—those make male babies—swim faster, so you have a better chance of having a daughter if the sperm has to travel a long way to the egg. The idea has no scientific merit, but researchers have now found a way to make it true in mice, sort of.

X and Y chromosomes aren’t just different sizes—the X is about three times longer than the Y—they also contain different genes. In the new study, scientists used these genetic differences to sort “male” mouse sperm from “female” sperm.

They zeroed in on a specific gene, called Toll-like receptor 7/8, which is expressed only in X-chromosome sperm and codes for two receptors on the sperm cell’s tail and midpiece. The scientists incubated mouse sperm in a mixture containing molecules that would bind to the receptors and activate them. The molecules slowed energy production in X-chromosome sperm while not affecting the Y-chromosome sperm at all, the team reports today in PLOS Biology.

To confirm their findings, the researchers staged sperm races. Many of the modified X sperm swam at less than half the speed of the Y sperm. And when researchers used only the faster Y sperm for in vitro fertilization, 90% of the mouse pups were born male. Separating out the slowest sperm produced litters that were 81% female.

Other methods for sex selection, such as staining sperm with a dye and sorting by length of the sex chromosome, are expensive and can sometimes harm the sperm cells or their DNA. The new method appears safe and is relatively cheap, the team asserts. If confirmed in other species, the strategy could be used for breeding livestock such as dairy cows, where female animals are preferred. Don’t expect to choose the sex of your baby anytime soon with this strategy, though; ethical concerns aside, there’s no evidence the approach works in humans.