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Man's genes play a role in baby's sex

11th December 2008

A study of hundreds of years of family trees has concluded that a man’s genetic make-up may determine whether he has a son or a daughter.

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Work carried out by a team of researchers at Newcastle University found that if a man had more brothers, he was more likely to have sons, and vice versa if he had more sisters.

However, in the Evolutionary Biology study of 927 family trees, with details on 556,387 people from North America and Europe going back to 1600, the same link between sibling sex and offspring sex was not found for women.

The precise way that genes can influence baby sex remains unproven.

A woman always passes a female "X" chromosome via her egg to her child, while the father effectively "decides" the sex of the child by passing on either another "X" in his sperm, making a girl, or a "Y" chromosome, making a boy.

While the birthrate is 50/50, scientists suspect that balance is shifted in some couples with the Newcastle study indicating there may be a genetic component.

Dr Corry Gellatly found that within families, boys with lots of brothers were more likely to have a higher number of sons themselves and those with lots of sisters were more likely to have lots of daughters.

He said: "The family tree study showed that whether you're likely to have a boy or a girl is inherited."

Dr Gellatly added said that the effect was to actually balance out the proportion of men and women in the population.

 

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