Why are humans born with almost equal numbers of boys and girls? This question is at the heart of evolutionary biology and has traditionally been explained in terms of Fisher’s principle. The theory suggests that natural selection favors a balance between male and female reproduction because it maximizes reproductive success. However, the exact genetic factors behind this balance are difficult to determine, leading scientists to wonder whether Fisher’s principle fully explains why this balance occurs in human populations.
University of Michigan researchers Siliang Song and Professor Jianzhi Zhang have taken an important step toward understanding this issue. Their research, published in Proceedings of the Royal Society B, explores the genetic reasons behind sex ratio at birth in humans. They explore why this ratio – roughly equal numbers of boys and girls – seems to persist. By using a large amount of data from the UK Biobank, they aimed to discover specific genes that may play a role in determining a baby’s sex.
As the study explains, one of the challenges of this research is that it is difficult to accurately measure sex ratios within households. As Professor Zhang explains, “Human families tend to have very few children, so it is difficult to estimate the true proportion of boys and girls in each family, which makes it difficult to detect any genetic influences.” This difficulty has led past research to suggest , how many boys or girls a person might have with little or no genetic variation, calling into question whether Fisher’s principle applies to humans.
By analyzing a large data set, Song and Zhang were able to find a genetic variant that appeared to affect the likelihood of having a boy or girl. The variant, found near the ADAMTS14 gene, is associated with a significantly lower chance of having a boy. While the discovery is groundbreaking, the researchers cautioned that the variant is rare and requires further confirmation. “Because this genetic variant is very rare, we need more research to verify whether it really affects the sex ratio of other ethnic groups,” Professor Zhang said.
Even with this finding, the researchers noted that the overall heritability (the degree to which a trait can be passed on through genes) of sex ratio appears to be extremely low. This is not surprising given the difficulty of accurately measuring sex ratios. The team’s computer modeling shows that even if the genetic basis of sex ratio is as strong as human height (heritability of height), it would still be difficult to detect due to inherent measurement problems.
Nonetheless, this study provides new evidence that Fisher’s principle still applies to humans. Using computer simulations, the researchers showed that human sex ratios evolved in a way consistent with Fisher’s predictions. Fisher’s principle states that if one sex becomes more common, the rarer sex will have a higher reproductive advantage, pushing the population back toward equilibrium. “Our results suggest that there may be multiple genetic factors that influence the ratio of boys to girls in humans, but their effects may be small and difficult to detect,” commented Professor Zhang.
The study also identified two other genes, RLF and KIF20B, that may be related to sex ratio. KIF20B, in particular, plays an important role in the development of sperm and egg cells, which may affect the success of fertilization and influence whether the baby is a boy or a girl. However, the exact role these genes play remains unclear. More research is needed to figure out how they are involved in determining the sex of offspring.
The research has broader implications beyond evolutionary theory. Understanding the genetic factors that determine the ratio of boys to girls could have practical applications in areas such as fertility treatments and even animal breeding. Although the genetic control of sex ratio in humans is not fully understood, the discovery of these genetic variants opens up new possibilities for research.
Going forward, the researchers are calling for larger, more comprehensive studies to confirm their findings and reveal other genetic factors that may influence whether a child is born male or female. Because the identified genetic variants are so rare, larger data sets and more precise methods are needed to truly understand how these factors play out. Only through such efforts can scientists fully reveal the genetic complexity behind nearly equal human sex ratios.
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Journal reference
Song S, Zhang J. 2024. “Searching for genetic variation in human birth sex ratio: Was Fisher wrong about the evolution of sex ratio?” Proc. R. Suker. B 291:20241876.
About the author
Zhang Jianzhi is a professor in the Department of Ecology and Evolutionary Biology at the University of Michigan. As a leading expert in molecular evolution, Jianzhi’s research focuses on understanding the evolutionary mechanisms of genetic variation and adaptation in humans and other species. He has made significant contributions in the fields of population genetics, molecular biology and evolution and has more than 20 years of research experience. His work often explores how evolutionary pressures shape genetic traits, with a particular focus on how mutations and natural selection affect populations. Jianzhi has authored many influential studies and continues to push the boundaries of evolutionary biology through innovative methods.
Song Siliang is an evolutionary biology researcher focusing on genetics and population biology. He currently works at the University of Michigan, working on understanding the genetic factors that influence traits in humans and animals. His recent work focuses on uncovering the genetic basis behind human birth sex ratio, a key 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, evolution, and bioinformatics, making him a key figure in using modern tools to solve complex biological problems.
