Health. The sex of siblings: a genetic lottery not so risky?

According to a United Nations report, slightly more boys than girls are born on Earth, 106 boys for every 100 girls.

Yet we all know siblings made up of only girls or only boys. So why don't Barry, Robin, and Maurice Gibb, the Bee Gees, have sisters? Just like the three Hemthworth brothers (Liam, Chris, and Luke)?

Why don't these families respect the global, almost equitable proportions? Could a child's biological sex not be as random as previously thought?

This is what a study from Harvard University (United States) suggests, published on July 18 in the journal Science Advances.

Never three without four?

The researchers examined more than 146,000 pregnancies of 58,000 women, all nurses, between 1956 and 2015 using eight characteristics (height, BMI, hair color, blood type, age at birth of first child, etc.)

They found that some families were more likely to have children of the same sex. And mothers with three or more children were more likely to have all boys or all girls.

"If you've had two or three girls and you're trying for a boy, know that your chances are not equal. You're more likely to have another girl," said Jorge Chavarro, a professor of nutrition and epidemiology and lead author of the study, as quoted by the Washington Post.

According to the researchers' calculations, families with three boys have a 61% higher chance of having a fourth boy.

In those with three girls, they statistically present a 58% probability that the fourth child will be female.

Are age-related biological changes to blame?

According to the study, maternal age may also play a role in a child's sex. Women who gave birth to their first child after age 28 were slightly more likely to have only boys or only girls.

According to Jorge Chavarro, this could be linked to age-related biological changes in women (shorter follicular phase, vaginal acidity, etc.).

A shorter follicular phase would tend to favor the survival of the Y chromosome, while a more acidic vaginal environment would favor the survival of the X chromosome.

And every woman may have a predisposition to one or other of these factors.

This work also highlighted two genes that could influence whether people have only girls or only boys.

"We don't know why these genes would be associated with sex at birth, but they are, which opens up new questions," Jorge Chavarro told the Washington Post.

However, the scientist recommends that future studies examine lifestyle, diet, and chemical exposure. Can these factors influence a child's biological sex?

This study has several limitations, firstly the fact that no information was collected on the biological fathers.

The researchers also acknowledged the lack of diversity within the sample. "The study population is composed predominantly of white women (95%) residing in the United States," they specify.

And the population studied, all nurses, could be predisposed to a single-sex sibling group through occupational exposures.

Sources: The Washington Post, Science Advances, School of Public Health of Harvard