They looked at 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.

But the Evolutionary Biology study could clear up a long-standing mystery - a flood of boy babies after World War I.

While a woman will always pass a female "X" chromosome via her egg to her child, 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.
   
The family tree study showed that whether you're likely to have a boy or a girl is inherited
Dr Corry Gellatly
Newcastle University

While the birthrate is almost 50/50, suggesting that overall men will deliver equal amounts of "X" sperm and "Y" sperm, scientists have suspected that in some individual couples the balance is shifted in favour of either boys or girls.

Various explanations have been put forward for this, ranging from differences in the time in the woman's monthly cycle that sex happens, to the amount of time that sperm spend waiting in the testicles.

The Newcastle study, by Dr Corry Gellatly, is strong evidence that there is a genetic component.

He 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.

War babies

Dr Gellatly said it was likely that a genetic difference affected the relative numbers of "X" and "Y" sperm within those produced by the man.

This gene, while only active in the man, could be carried by men and women.

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