Population genetics researchers have long suspected that selfish chromosomes distort sex ratios in humans, just as they do across much of the animal kingdom. A study drawing on one of the largest genealogical databases in the world has now produced the most compelling human evidence yet.
The analysis focuses on a single family in Utah, traced across seven generations back to the 1700s. According to the report, 33 men who shared the same Y chromosome fathered 89 children — 60 boys and just 29 girls. That ratio is far from the roughly equal split that basic biology predicts. Under normal conditions, when stem cells in the testes divide to produce sperm, half should carry a Y chromosome and half an X, giving any offspring an approximately even chance of being male or female.
The researchers used the Utah Population Database, which holds records on millions of individuals. For this study, they examined 76,000 of them. Two separate statistical methods both identified the same family as a significant outlier — a finding that James Baldwin-Brown at the University of Utah describes as notable precisely because selfish genes of this kind have been well-documented in other organisms but have proven difficult to pin down in people.
How a Selfish Chromosome Works
The proposed mechanism involves genetic variants on a chromosome that allow it to skew reproduction in its own favor. Some selfish chromosomes interfere with competing sperm’s ability to navigate toward an egg. Others, according to the report, kill off any sperm that don’t carry them — though the precise mechanism behind that remains unresolved. “That’s a 100-year-old question that we’re still working on today,” says Nitin Phadnis, also at the University of Utah.
The statistical challenge is significant. Even a run of five, six, or seven boys in a row carries odds that aren’t astronomically low. Only by examining multiple generations can researchers accumulate enough data to rule out chance with confidence — which is exactly what the Utah database made possible here.
Skepticism and What Comes Next
Sarah Zanders at the Stowers Institute for Medical Research in Missouri welcomes the work but urges caution. She notes that in her own research on microbes, apparent sex-ratio biases frequently turned out to be statistical noise that disappeared with larger sample sizes. She also raises the question of mis-assigned paternity across seven generations as a variable the data cannot fully account for. Baldwin-Brown says his team has considered this and believes the core data remains reliable.
Because the database records are anonymised, no genetic sequencing of the family’s sperm has been possible. Baldwin-Brown describes de-anonymising the samples as “a pretty big hurdle” requiring extensive ethical approval and funding.
The stakes extend beyond academic interest. Phadnis points out that a mechanism capable of eliminating half of all sperm would directly reduce male fertility — and animal studies have already linked selfish chromosomes to infertility in specific individuals. The team’s stated next step is to analyse sperm samples directly, looking for skewed sex ratios at the cellular level.
Photo by National Institute of Allergy and Infectious Diseases on Unsplash
This article is a curated summary based on third-party sources. Source: Read the original article
