Babies tend to have the same probability to be born biologically male or or female — but family genetics could skew those odds in either direction. Credit: Waltraud Grubitzsch/dpa via Alamy
By sifting through an anonymized genealogy database, researchers have discovered a Utah family that has been having twice as many boys as girls for seven generations. It is the first clear evidence that humans might have ‘selfish genes’ that distort the sex ratio of offspring from roughly 50:50, the researchers argue in a preprint posted on bioRxiv earlier this month1. The findings have not been peer-reviewed.
Such sex ‘distorters’ have been discovered — and studied in great depth — in laboratory animals such as mice and flies, in which their effects can be detected through selective breeding. “If you look, more often than not, you find them,” says Nitin Phadnis, an evolutionary geneticist at the University of Utah in Salt Lake City, who co-led the study.
Theoretical predictions suggest that sex distorters probably do exist in people as well, and that they could produce excesses of biological boys or girls at birth. But humans’ long generation times and low birth rates as well as ethical issues have made such genes — and other ‘selfish’ genetic elements , meaning that they bias their own transmission to future generations whether or not they improve an individual’s biological fitness — difficult to spot.
To overcome such issues, Phadnis and his colleagues looked to the Utah Population Database, which contains genealogical, health and other data for people from the late eighteenth century through to the present.
In humans, biological sex is determined by the sex chromosome that fathers pass onto their offspring: each sperm cell typically carries either a Y or an X chromosome, but not both. The mother’s egg cell, by contrast, usually carries a single X. Therefore, when sperm cells fertilize an egg cell, those with a Y chromosome give rise to biological male offspring (people who have both an X and a Y) and those with an X chromosome create biological female offspring (people with two Xs).
Boys boys boys
The researchers focused their search on families with a bias towards male offspring that could carry a sex distorter on the Y chromosome, because they are easier to detect in genealogical data. Other factors, such as deadly recessive mutations on the X chromosome, can lead to female-heavy families.
Using data on the recorded sex of 76,445 individuals in the Utah database, the researchers applied two statistical tests to identify families that were unlikely to be male-biased just by chance. Of more than 26,000 paternal lineages, just one family passed both tests. Across seven generations, the database documented 60 male offspring and 29 female — a ratio of more than 2:1.
War, famine and other extreme hardships can lead to male-skewed families, as can female infanticide. But there is no historical record of such drivers in Utah, says Phadnis. Misattributed births, for instance due to extramarital affairs, or polygamy — which was once common in Utah — are also unlikely to have affected the results, says co-author James Baldwin-Brown, also an evolutionary geneticist at the University of Utah. “The signal in this family is very strong.”
Polly Campbell, an evolutionary biologist at the University of California, Riverside, says that the researchers make a good case that the Utah family’s pattern of births is caused by a sex-distortion gene. But she isn’t surprised to see one found in humans, because similar genes have been found in other well-studied animals. “It’s really cool to see it,” she says. “We don’t usually expect sex-ratio distortion to be visible in natural populations.”
Cultural relics
However, Wynn Meyer, a population geneticist at Lehigh University in Bethlehem, Pennsylvania, isn’t convinced that other explanations have been ruled out. Humans can, for instance, influence the sex of offspring by using preimplantation genetic testing on embryos conceived through in vitro fertilization or through culturally transmitted practices, Meyer adds.
