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.
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.