Sex determined 'by men's genes'

The secret to whether a baby will be a boy or a girl is carried in the father's genes, according to a new study. Research conducted by the University of Newcastle studied hundreds of years of family trees. They found that men who had more brothers in their own family were more likely to have sons, while those with more sisters were prone to have girl babies. The study, which examined 927 family trees, with details on 556,387 people from North America and Europe, going back to 1600, did not establish the same link between sibling sex and offspring sex for women.

Published in Evolutionary Biology, the study could help to clear up the mystery of why there was a flood of boy babies after World War I.

Dr Corry Gellatly, who led the study, explained that families with multiple sons would have been more likely to see at least one survive and consequently those men would be more likely to go on and father boys.

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

"If there are too many males in the population, for example, females will more easily find a mate, so men who have more daughters will pass on more of their genes, causing more females to be born in later generations."