Why do humans tend to have almost equal numbers of boys and girls at birth? This question, at the heart of evolutionary biology, has traditionally been explained by Fisher’s principle. This theory suggests that natural selection favors a balance between male and female births because it maximizes reproductive success. However, the exact genetic factors behind this balance have been hard to pin down, leading scientists to wonder whether Fisher’s principle fully explains why this balance happens in human populations.
Researchers Siliang Song and Professor Jianzhi Zhang from the University of Michigan have made an important step in understanding this question. Their study, published in Proceedings of the Royal Society B, looks at the genetic reasons behind the human sex ratio at birth. They explored why this ratio—roughly equal numbers of boys and girls—seems to persist. By using a huge amount of data from the UK Biobank, they aimed to uncover specific genes that might play a role in deciding the sex of a baby.
One of the challenges in this research, as the study explains, is that it’s difficult to measure the sex ratio accurately within families. As Professor Zhang explained, “Human families tend to have very few children, so it’s hard to estimate the true ratio of boys to girls for each individual family, which makes it tricky to detect any genetic influence.” This difficulty has led to past studies showing little or no genetic variation in how many boys or girls a person might have, making some question whether Fisher’s principle applies to humans at all.
By analyzing a large dataset, Song and Zhang were able to find a genetic variant that seems to influence the likelihood of having boys or girls. This variant, found near a gene called ADAMTS14, was linked to a noticeable reduction in the chance of having male children. Although this discovery is groundbreaking, the researchers caution that this variant is rare and needs further confirmation. “Because this gene variation is so uncommon, we need more studies to verify whether it really affects the sex ratio in other groups of people,” said Professor Zhang.
Even with this discovery, the researchers noted that the overall heritability—how much of a trait can be passed down genetically—of the sex ratio seems to be extremely low. This isn’t surprising, given how difficult it is to measure the sex ratio precisely. The team’s computer models showed that even if the genetic basis of the sex ratio were as strong as something like human height (which is highly inherited), it would still be very hard to detect due to the inherent measurement issues.
Nevertheless, the study provides new evidence that Fisher’s principle still applies to humans. Through their computer simulations, the researchers showed that the human sex ratio evolves in a way that aligns with Fisher’s predictions. Fisher’s principle suggests that if one sex becomes more common, the rarer sex will have a higher reproductive advantage, pushing the population back toward balance. “Our findings suggest that there could be multiple genetic factors that influence the ratio of boys to girls in humans, but their effects are probably small and difficult to detect,” Professor Zhang commented.
The study also identified two other genes, RLF and KIF20B, that could be linked to the sex ratio. KIF20B, in particular, plays a role in how sperm and egg cells develop, which could affect the success of fertilization and influence whether a baby is a boy or a girl. However, the exact role these genes play is still unclear. More research is needed to figure out how they might be involved in determining the sex of offspring.
This research has broader implications beyond just evolutionary theory. Understanding the genetic factors that determine the ratio of boys to girls could have practical applications in areas like fertility treatments and even animal breeding. While the genetic control of sex ratios in humans is still not fully understood, discovering these genetic variations opens up new possibilities for research.
Looking ahead, the researchers are calling for even larger, more comprehensive studies to confirm their findings and uncover additional genetic factors that might influence whether a child is born male or female. Because the identified genetic variants are rare, much larger datasets and more precise methods will be necessary to truly understand how these factors work. Only with such efforts will scientists be able to fully unravel the genetic complexities behind the nearly equal sex ratio seen in human populations.
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Journal Reference
Song S, Zhang J. 2024. “In search of the genetic variants of human sex ratio at birth: was Fisher wrong about sex ratio evolution?” Proc. R. Soc. B 291: 20241876. DOI: https://doi.org/10.1098/rspb.2024.1876
About the Author
Jianzhi Zhang is a professor in the Department of Ecology and Evolutionary Biology at the University of Michigan. A leading expert in molecular evolution, Jianzhi’s research focuses on understanding the evolutionary mechanisms underlying genetic variation and adaptation in humans and other species. He has contributed significantly to the fields of population genetics, molecular biology, and evolutionary theory, with over two decades of research experience. His work often explores how evolutionary pressures shape genetic traits, with a special interest in how mutations and natural selection affect populations. Jianzhi has authored numerous influential studies and continues to push the boundaries of evolutionary biology through innovative approaches.
Siliang Song is a researcher in evolutionary biology with a focus on genetics and population biology. He is currently based at the University of Michigan, where he works on understanding the genetic factors that influence human and animal traits. His recent work has been dedicated to unraveling the genetic basis behind the sex ratio at birth in humans, a critical question in evolutionary biology. Siliang is passionate about using large-scale genetic data to uncover hidden patterns in human evolution and reproduction. His research spans multiple fields, including genomics, evolutionary theory, and bioinformatics, making him a key figure in addressing complex biological questions with modern tools.
