Genetic variants that increase their chances of transmission are not new, but scientists in Utah consider this an exceptionally unlikely case to be explained by mere chance.
A family from Utah, USA, has recorded, over seven generations, many more births of boys than girls. It is a pattern that may have a rare biological explanation: a “selfish Y chromosome”capable of biasing the sex ratio of offspring.
The hypothesis is the result of one conducted by James Baldwin-Brown and Nitin Phadnis, both from the University of Utah, based on the Utah Population Database, a huge repository with genealogical and demographic information on millions of people. For the study, the team worked with data from around 76,000 individuals and applied two independent statistical methods, which identified the same family lineage as a exceptionally unlikely case to be explained by chance alone.
In mammals, males have one X and one Y chromosome, and females two X. During sperm formation, in principle half of the reproductive cells should carry the
However, remember, several species have already been described “selfish genes” — genetic variants that increase their chances of transmission, which goes against this rule. Some interfere with the ability of competing sperm to reach the egg; others appear to eliminate sperm that do not carry the variant, although the mechanism remains unclear.
In the case identified in Utah, 33 men inherited the same Y chromosome over seven generations. In total, they had 89 children: 60 boys and 29 girls. For researchers, this asymmetry suggests the action of a genetic element that favors the transmission of Y — and, therefore, the birth of more male descendants.
Still, the study is not 100% conclusive. Another researcher consulted by the magazine considers the hypothesis plausible, but emphasizes that the numbers remain relatively small and that apparent biases may disappear with larger samples. And the wrong attribution of paternity throughout the generations, something that the team itself admits, however, to having considered.
As the data is anonymized, it was not possible to perform direct genetic analysis. The next desirable step would be to sequence sperm samples to see if there is an abnormal proportion of sperm carrying X and Y and try to understand the biological mechanism at play.
The identification of “selfish” chromosomes could have implications beyond scientific curiosity, the team says. Mechanisms that eliminate a significant portion of sperm may contribute to high rates of male infertility, and animal studies have already linked these phenomena to infertility in some individuals.
