Scientists have discovered a gene variant that could make some parents more likely to have girls than boys.
In the new study, researchers analyzed the genomes of around 450,000 males and females in the U.K. who contributed genetic and health data to the U.K. Biobank — a large, publicly available database.
The scientists found that one particular version of a gene — named rs144724107 — boosted a person's likelihood of having a daughter by 10%. That means, in a given pregnancy, a person with this variant has a 60% chance of having a daughter rather than the expected 50% chance. It's unclear whether the rs144724107 variant needs to be carried by mom, dad or both to influence this offspring-sex ratio.
It's also not yet known exactly how this gene variant raises the chances that someone has female offspring. But researchers do know that, within DNA molecules, rs144724107 is located near a member of a gene family called ADAMTS. This family has been linked to both sperm production and fertilization, the process by which sperm and eggs fuse to form embryos. Given this proximity, rs144724107 could exert its effects on sex by affecting how this gene gets switched on and off, the researchers told Live Science.
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The findings, published Oct. 16 in the journal Proceedings of the Royal Society B: Biological Sciences, may bolster the idea that a classic evolutionary biology theory applies to people.
The theory, known as Fisher's principle, suggests that, most of the time, sexually reproducing organisms produce equal numbers of male and female offspring.
The theory goes that if a population has more males than females, evolutionary pressures would favor gene variants that boost the number of females, thus balancing the ratio, said study co-author Jianzhi Zhang, a professor of ecology and evolutionary biology at the University of Michigan. The same thing would also happen the other way around, if there are more females than males.
To prove this theory, researchers need to identify gene variants that influence the number of male and female offspring that an individual has. In humans, such variants have been difficult to detect, in part because people generally bear relatively few offspring, Zhang said.
However, the enormous U.K. Biobank dataset enabled the team to detect the rs144724107 variant, Zhang said. The database doesn't record the sex of its participants' offspring, but it does include information on the sex of participants' siblings. So instead of linking the gene variant in a parent to the sex of their offspring, the team worked in reverse, checking the sex of the siblings and then tracing this back to the gene variants carried by their parents.
The researchers found that only 0.5% of the participants carried rs144724107. But they suspect there are other variants that similarly influence offspring sex. Some people might happen to have variants that increase the sex ratio in favor of males or females, while others' genes will decrease the sex ratio, Zhang said.
"But overall, at the population level, the sex ratio is about 0.5," he said.
The team's findings will need to be confirmed in other groups of people. In a separate analysis in the same study, the team couldn't find the rs144724107 variant in the genomes of 14,500 postmenopausal women in the U.S. from the Women's Health Initiative (WHI). Unlike the U.K. Biobank, the WHI includes data on the sex of participants' offspring.
The team will also need to see if similar variants exist in the genomes of more diverse populations. The scientists exclusively included people of European ancestry in this study.
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