Study Finds Fathers Play a Role in Determining the Sex of Their Children

A father has more to do with the sex of his offspring than you might think. In a recent study by researchers from Spain, the UK and the U.S., the belief that only mammal mothers influenced the sex of offspring was refuted.

While studying wild mice, the researchers found a possible indication that the father does have a role in determining sex after all. Aurelio Malo, lead researcher and associate biology lecturer at Oxford University’s Department of Zoology, said the study challenges previous assumptions about how an offspring’s sex is determined.

“In mammals, theory predicts that offspring sex ratios can only be determined by the mother, as fathers have always have been thought to inseminate an equal proportion of X and Y sperm, having a random effect on offspring sex that they could not shift from equality, or 50:50,” Malo said in a press release.

The mother does have a bigger role in determining the sex of her offspring, and she sacrifices the most physically. Fathers have control over sperm only, Malo said.

The study found a link between the father’s own genetic quality and the number of sons and daughters he produces. The relationship depends on the size of the nuclei within their sperm, which directly correlates to how many X and Y sperm the father carries. Those who have sperm with smaller nuclei, or a higher proportion of Y sperm, are most likely to father more sons than daughters, Malo said.

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“The implications are important, as we now have the proof that fathers matter independently of any maternal effects,” he said. “Scientists can now improve their predictive models of sex ratios at birth, including not only mothers but also fathers.”

The study said the results reveal a plausible mechanism that drives unexplored male-driven sex-ratio biases. Using wild mice in the study allows for some wiggle room to make inferences of other species, Malo said.

“These findings are potentially applicable to any other mammalian species, including our own. However, the extent to which we find the effects uncovered here depends very much on the mating systems,” he said. “For instance, in more monogamous species the expectation that fathers would evolve an ability to manipulate sex ratios in their own interests is less clear.”

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The research refutes any idea that the father’s contribution to sex ratio variation should be ignored in vertebrates, and it could inspire research into evolutionary hindrances to sex ratios. Malo said the expectation for fathers to produce the same amount of X and Y sperm has stopped any further research into paternal effects in mammals.

“By showing that fathers can adjust sex ratios by varying sperm types, we help open the gates of a new research area of paternal effects on sex ratios,” he said. “For example, do mothers and fathers have the same or opposing sex allocation interests? Does this vary across species and contexts? In a nutshell, we now know that dads, as well as mums, can alter the sex of their offspring, and that the ability to do so might have evolved through natural selection.”