A research team analyzing a vast, anonymized genealogy archive has identified a remarkable multigenerational pattern in a Utah family: for seven consecutive generations, the family appears to have produced roughly twice as many boys as girls. The researchers argue that this pattern may represent the first clear evidence in humans of so-called “selfish genes” capable of skewing the natural sex ratio at birth away from the expected 50:50 balance. Their findings were recently shared in a preprint on bioRxiv and have not yet undergone peer review.
In laboratory settings, sex-ratio distorters have long been documented in model organisms such as mice and fruit flies. Through controlled breeding experiments, scientists have repeatedly observed genetic elements that bias inheritance in their own favor. According to Nitin Phadnis, an evolutionary geneticist at the University of Utah who co-led the new study, these distorters are often discovered when researchers look carefully enough. In animal systems, they are not rare anomalies but recurring features of genetic conflict.
Evolutionary theory has predicted for decades that similar distorters could exist in humans. Such genes would increase their own transmission to the next generation even if they do not enhance — and may even reduce — the overall biological fitness of the individual. However, studying these elements in people presents serious challenges. Human generation times are long, birth rates are relatively low compared to laboratory organisms, and ethical considerations prevent experimental breeding approaches. As a result, if sex-distorting genes do operate in humans, they would be difficult to detect through conventional methods.
To address this challenge, Phadnis and his collaborators turned to the Utah Population Database, a uniquely detailed genealogical resource that integrates family histories, health records, and demographic data from the late eighteenth century to the present day. This extensive archive allowed the team to analyze reproductive patterns across thousands of paternal lineages without identifying specific individuals.
In humans, biological sex is determined by the sex chromosome contributed by the father. Each sperm cell carries either an X chromosome or a Y chromosome, while the mother’s egg cell typically carries a single X chromosome. When fertilization occurs, sperm carrying a Y chromosome result in offspring with one X and one Y chromosome, who are biologically male. Sperm carrying an X chromosome produce offspring with two X chromosomes, who are biologically female. Under ordinary circumstances, the proportion of X-bearing and Y-bearing sperm is approximately equal, leading to a near 1:1 sex ratio at birth.
The researchers focused specifically on paternal lineages showing an unusually high number of male births. A distortion linked to the Y chromosome would be easier to trace in genealogical data because it would pass strictly from father to son. By contrast, certain X-linked mutations can produce female-biased patterns, but these are driven by different biological mechanisms.
The team examined records for 76,445 individuals and applied two independent statistical tests to determine whether any family’s male-heavy birth pattern was unlikely to have occurred by chance alone. Out of more than 26,000 paternal lineages analyzed, only one family satisfied both statistical criteria. Over seven generations, this lineage produced 60 documented male births and 29 female births, yielding a ratio exceeding two boys for every girl.
The researchers considered alternative explanations. Historically, extreme events such as war or famine can influence birth ratios, and in some societies, female infanticide has produced male-skewed populations. However, the investigators found no historical evidence of such pressures in the Utah context that would explain this family’s pattern. They also evaluated the possibility of misattributed paternity, including cases arising from extramarital relationships or the practice of polygamy, which was historically present in Utah communities. According to the study authors, the statistical signal in this family is strong enough that these factors are unlikely to account for the observed imbalance.
External experts have offered both support and caution. Polly Campbell, an evolutionary biologist at the University of California, Riverside, noted that similar genetic phenomena are well established in other animal systems. From her perspective, finding a comparable mechanism in humans is not inherently surprising. What is unusual, she suggests, is observing such a clear signature in a natural human population rather than under laboratory conditions.
Other scientists urge restraint. Wynn Meyer, a population geneticist at Lehigh University, points out that modern reproductive technologies can allow parents to influence the sex of their children, particularly through preimplantation genetic testing in the context of in vitro fertilization. Cultural practices may also play a role in shaping family composition in certain settings. Although there is no evidence that these factors affected the Utah lineage in question, they remain possible confounders in any contemporary analysis of sex ratios.
Jianzhi Zhang, a geneticist at the University of Michigan, raises additional questions. He notes that underreporting of female births, whether intentional or accidental, could theoretically produce an apparent male bias in genealogical records. One way to test this hypothesis would be to examine whether female members of the same extended family — who would not carry a Y-linked distorter — had balanced sex ratios among their own children. That information was not included in the preprint.
Zhang also expresses skepticism about the biological mechanism itself. The human Y chromosome contains relatively few functional genes compared to other chromosomes. Identifying a gene on the Y chromosome capable of driving a significant distortion would therefore be both surprising and scientifically intriguing. Pinpointing the precise molecular basis of any such distorter, he argues, should be a priority for future research.
Phadnis and his team are already exploring candidate genes that might operate similarly to a well-characterized sex distorter found on the mouse Y chromosome. Although individuals in the Utah database remain anonymous, it may be possible to identify related lineages or similar patterns in other populations through large-scale genomic studies.
In many animal species, sex-ratio distorters act by altering sperm performance — for example, by impairing the motility or viability of sperm carrying one type of sex chromosome. To investigate whether a comparable mechanism could be operating in humans, the research team is developing high-throughput tools to study sperm behavior. Among these innovations is a device informally described as a “sperm racetrack,” designed to measure and compare the movement characteristics of large numbers of sperm cells under controlled conditions.
If confirmed, the discovery of a sex-distorting gene in humans would reshape long-standing assumptions about reproductive biology. The widely accepted 50:50 sex ratio at birth has been considered one of the most stable features of human populations. Evidence that genetic elements can subtly bias this balance — even in rare lineages — would highlight the ongoing evolutionary tug-of-war occurring within our own genomes.
For now, the findings remain preliminary. The study has yet to undergo peer review, and alternative explanations have not been fully excluded. Nonetheless, the Utah family’s seven-generation pattern provides a compelling case study and opens the door to deeper investigation into the hidden genetic conflicts that may influence human reproduction.
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