Have you ever noticed a family with mostly boys or girls? While it often seems coincidental, a Utah family study dating back to the 1700s suggests a biological reason—the “selfish” Y chromosome may reduce female births.

“This is a significant family,” says James Baldwin-Brown from the University of Utah. “Selfish genes have been identified in many organisms, but studying their effects in humans has been challenging.”

In mammals, male cells contain one X and one Y chromosome. As sperm develop, half carry X and half carry Y, leading to a balanced male-female offspring ratio. However, certain genetic variations can disturb this balance, producing more males or females. Some selfish chromosomes may even interfere with sperm navigation, while others eliminate non-carrier sperm, though the mechanisms are not yet clear. “This question has persisted for a century, and we still seek answers,” explains Nitin Phadnis, also from the University of Utah.

In various species, selfish X and Y chromosomes compete, attempting to tilt the sex ratio to their advantage. Evidence suggests that humans may harbor similar selfish genes, yet identifying active ones proves difficult. “It’s statistically probable to have five or six boys consecutively,” Baldwin-Brown notes.

To demonstrate that the observed sex ratio bias isn’t just chance, researchers must investigate multiple generations. Utilizing the Utah Population Database, which contains data on millions, this study focused on 76,000 individuals.

By applying two distinct statistical analyses, the researchers identified specific families as significant outliers. Over seven generations, 33 men passed down an identical Y chromosome. Of their 89 offspring, 60 were male, while only 29 were female.

The data, having been anonymized, restricts direct genetic testing. “It would be enlightening to de-anonymize these samples and request consent for sperm analysis,” Baldwin-Brown states. “However, the ethical considerations involve extensive paperwork and resources.”

Sarah Zanders from the Stowers Medical Research Institute in Missouri hopes her team has identified a selfish Y chromosome, though the sample size remains small. In her research on microbes, her team observed unexpected sex ratio biases, but these disappeared in larger sample sizes.

Infidelity also complicates findings, Zanders suggests. “While I lack expertise in human behavior, television has taught me that father misattribution could be more common than assumed.” Baldwin-Brown reassures, “We have substantial, reliable data.”

Understanding the selfish Y chromosome has broader implications beyond mere academic interest. According to Phadnis, these chromosomes might contribute to rising male infertility rates. A mechanism that eradicates half of all sperm could logically lead to reduced fertility. Studies suggest that selfish chromosomes can cause infertility in certain subjects.

The research team intends to analyze sperm samples for discrepancies in X and Y sperm proportions.

In this recent study, they focused specifically on the selfish Y chromosome for several reasons: tracing male lineage is simpler, and an increased female proportion may also stem from lethal mutations—not just a selfish X chromosome.

Notably, selfishness isn’t confined to X and Y chromosomes. Genes that enhance inheritance chances above 50% are referred to as gene drives, with various types identified in the animal kingdom. CRISPR technology allows for the creation of artificial gene drives, potentially aiding in malaria control and pest management.