A mother’s chance of conceiving a baby boy or a girl is as random as a coin toss. In the case of one Utah family, however, ‘selfish genes’ might have tipped the balance. By looking through a genealogy database, scientists have discovered that this family has had twice as many boys as girls over the last seven generations. It is considered the first clear evidence of the presence of ‘selfish genes’ in humans that distort the sex ratio. The findings have not yet been peer-reviewed.
Sex ‘distorters’ like these genes are not uncommon in the animal kingdom and have been discovered and studied in lab animals like mice and flies, where the effects of distortion are observed through selective breeding. Theoretical views suggest that sex distorters perhaps exist in humans, too, and favor the birth of biological boys over girls. Low birth rates and long generation time, paired with ethical boundaries, have made ‘selfish genes’ difficult to spot. These genes are called selfish as they bias their own transmission to future generations, irrespective of their effect on an individual’s biological fitness.
In humans, biological sex depends on the combination of sex chromosomes. A father’s sperm cell carries either a Y or an X chromosome, but not both. When sperm cells fertilize an egg cell, which usually carries a single X chromosome, the resulting combination can be XY, which gives rise to a biological male, or XX, which gives rise to a biological female.
The researchers focused their search on families with a bias toward male offspring who might carry a sex distorter on the Y chromosome, as these are easier to detect in the genealogical database. Other factors lead to female-heavy families, such as deadly recessive mutations on the X chromosome. Using data on the recorded sex of 76,445 individuals in the Utah database, the scientists implemented two statistical methods to identify families that were unlikely to be male-biased just by chance. Of the over 26,000 paternal lineages, only one family showed positive results for both methods. Across seven generations, the database documented 60 male children to 29 female, a ratio exceeding 2:1.
Scientists took note of some of the factors that lead to male-skewed families, such as war, famine, or female infanticide, and found no historical record of such events in Utah. Once common in Utah, polygamy and misattributed births are also unlikely to affect the results. The lack of human factors that could bias the demography points toward a good case that the Utah family’s pattern of births is the result of a sex-distortion gene.
Despite a lot of strong data, Wynn Meyer, a population geneticist at Lehigh University in Bethlehem, is far from convinced. Humans can influence the sex of offspring through IVF or through practices that have been passed across the culture. Unreported female births could also account for the pattern, notes Jianzhi Zhang, a geneticist at the University of Michigan in Ann Arbor. This factor, however, could be ruled out if a female member of the family not carrying the sex distorter had roughly equal numbers of boys and girls. The study did not cover that data.
Zhang also finds the presence of a sex distorter on the Y chromosome, which contains few functional genes, quite surprising. “In the future, it would be very interesting to identify the molecular nature of the distorter and the distortion mechanism involved,” he adds.
Nitin Phadnis, an evolutionary geneticist at the University of Utah in Salt Lake City, who co-led the study, and his colleagues are trying to pin down the genes that might work ‘selfishly’ as a sex distorter. Identifying the genes in living participants in the Utah database is not feasible, but their relatives or people carrying other such sex-distortion genes could show up in other studies.
Sex distorters in other animals have a profound effect on sperm motility, so Phadnis’ team is developing tools to study human sperm at scale. One such device, which they call a sperm racetrack, can allow them to study the biological mechanisms behind the sex distortion and even uncover new ones in the process.
Copyright @smorescience. All rights reserved. Do not copy, cite, publish, or distribute this content without permission.
