Why Some Men Father Boys, Others Father Girls

It's a good bet a new dad will be yelling "It's a boy!" if boys run in his family, and "It's a girl!" if he's got a lot of sisters. 

It's all in the father's genes, according to scientists at Newcastle University in England.  They say a still undiscovered gene causes some men to father mostly sons, while others father mostly girls.

Doctors have long known that sperm carrying an X chromosome will result in a girl, while sperm with a Y chromosome results in a boy.  Now they've found that the X or Y trait is hereditary, but they have yet to narrow down the specific gene. 

They say it also explains why more sons are born after a war.  That's because it's more likely that big families with a lot of brothers are more likely to have at least one son survive a war, and then father another son, than it is for families with more daughters.  In that case, the odds for the single son in that family to survive a war are less, so he wouldn't be around later to father a girl.

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