Genetic mutation may sway gender balance in human offspring

Although the number of boys and girls born globally is nearly equal, some families show a clear predominance of one gender among children. Scientists decided to investigate this phenomenon to uncover its genetic determinants.

Jianzhi Zhang, an evolutionary geneticist from the University of Michigan, has been analysing this issue for years. He noted that previous studies have not conclusively confirmed the existence of genetic variability affecting the sex ratio among humans.

"Scientists have been pondering and researching a genetic basis for sex ratio for decades, yet no unambiguous evidence for a genetic variation that alters the human sex ratio from an approximately 50:50 ratio has been found," explained Zhang.

He added that almost all human traits result from genetic variability, which is why the lack of impact of mutations on the sex ratio seems unlikely.

One of the main problems with previous studies was the difficulty in precisely measuring the sex ratio of a person's children, which complicated the detection of genetic influence. To address this problem, Zhang and his doctoral student, Siliang Song, analysed data from the UK Biobank, a vast database covering about 500,000 participants from the United Kingdom.

Through this study, they identified a rare mutation, labelled rs144724107, which increases the chance of having a daughter by about 10 percent. However, this mutation is present in only 0.5 percent of those studied.

This mutation is located near the ADAMTS14 gene, which plays a role in spermatogenesis and fertilisation, potentially explaining its impact on the sex ratio. However, scientists emphasise that this finding requires further research to confirm its validity in other population samples.

Further analysis also led to the identification of two other genes, RLF and KIF20B, which may play a similar role in regulating the sex ratio. These findings align with the so-called Fisher's principle, an evolutionary theory that natural selection favours genetic variants that increase the numbers of the rarer sex in the population, ultimately leading to balance.

Professor Zhang also pointed to the potential applications of this research in agriculture. In many branches of livestock farming, such as egg or milk production, one gender, usually females, is preferred. Genetic modifications aimed at increasing the number of animals of this gender could bring significant economic benefits while reducing the number of lower-value animals, such as young males, often culled shortly after birth.

Although the study's results are promising, scientists know the challenges they may face in verifying them. The rarity of the mutation and the need to analyse large study groups pose a challenge for further research.