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The finding that some chemicals slow down sperm that carry the X chromosome could lead to gels for home use that make a couple less likely to conceive a girl, scientists have warned.

“I am concerned about the social impact of this,” says Alireza Fazeli of Tartu University in Estonia. “It’s so simple. You could start to do it in your bedroom. Nobody would be able to stop you from doing it.”

It was thought that the sperm of mammals that lead to male and female offspring are identical except for the DNA they carry. But Masayuki Shimada of Hiroshima University in Japan and his colleagues have found that 500 genes are active in sperm that carry the X chromosome, which give rise to female offspring, that aren’t active in sperm that carry the Y chromosome, which lead to male offspring.

Of these genes, 18 code for proteins that stick out from the sperm cell’s surface. The team has found that chemicals that bind to two of these proteins can slow down the movement of X-carrying sperm without affecting the Y-carrying ones.

This discovery makes it simple to separate sperm according to the sex of the offspring they could produce. When the researchers used this method on mouse sperm, they found that selecting the fastest swimmers for conception led to 90 per cent of the resulting pups being male. When they used slowed-down sperm, the pups were 81 per cent female.

The researchers focus on livestock, and they have found that the technique works in cattle and pigs (see “Why sort sperm?”, below). They haven’t tried it on human sperm, but Shimada says he thinks it would work.

“It’s fairly convincing,” says George Seidel of Colorado State University. It could take up to a decade to turn this into a commercial method for sorting human sperm, he says.

However, Fazeli thinks it may not be necessary to select the fastest or slowest swimmers before insemination or IVF to influence the sex of an embryo. He says that, if the chemicals were added to a gel or foam applied inside the vagina before sex, this could be enough to greatly increase a couple’s chances of conceiving a boy. Seidel agrees.

There is likely to be an appetite for such products, especially in countries where sex ratios have already been distorted in favour of boys. “In countries where there is already a skewed sex ratio, it is clear that if there are more easy, cheap, accessible technologies they will be used,” says bioethicist Wybo Dondorp of Maastricht University in the Netherlands.

Several methods for sex determination already exist. The most reliable is to analyse the chromosomes of IVF embryos prior to implantation. This pre-implantation genetic diagnosis (PGD) method is almost 100 per cent effective, but it is expensive.

Another method, which Seidel helped develop, is to add a fluorescent dye to semen that binds to DNA. As Y-carrying sperm have less DNA, these will be less bright and can be separated one at a time. This “flow cytometry” gives an 82 per cent chance of having a boy if desired, or a 93 per cent chance of a girl. Both IVF PGD and flow cytometry require specialised equipment and expertise.

Sex selection is banned for non-medical uses in the UK, Australia, Canada, China and India, but is legal in most other countries, including the US, where a small number of people pay to use it for “family balancing”.

Shimada’s work could make sperm sorting much simpler and more widely available – and more prone to misuse. “The sexing is much easier than the conventional technique,” he says. “Therefore, I am worried about the problem.”