I was taught that the sex of children is randomly determined. Since each sperm carries with it an X or a Y chromosome, whichever one happens to get to the egg first will determine whether the baby will be a boy or a girl. So each child has essentially a 50-50 chance either way.
But I've always thought that boys run in my family. I have more than twice as many boy cousins as girl cousins. I have one sister and four brothers, two nieces and six nephews. This never looked particularly random to me.
So this guy thinks he has proved that it's not random. In fact, it's an inherited trait passed down from father to son.
"The family tree study showed that whether you’re likely to have a boy or a girl is inherited. We now know that men are more likely to have sons if they have more brothers but are more likely to have daughters if they have more sisters . . . " Mr Gellatly explains.
The study suggests that an as-yet undiscovered gene controls whether a man’s sperm contains more X or more Y chromosomes, which affects the sex of his children. On a larger scale, the number of men with more X sperm compared to the number of men with more Y sperm affects the sex ratio of children born each year.
Interesting stuff. And it means that I was right all along. As usual.