Asked by: Phil Aspinall, Darlington
The effect is more obvious in men. In mammals, sperm carrying an X chromosome produce girls whereas sperm carrying a Y produce boys. So fathers with genetic defects on the X or Y tend to produce the opposite sex.
Studies reveal that in the US white fathers produce about 105 sons per 100 daughters, African American and American Indian fathers produce about 103, while older fathers produce more daughters. Other effects exist too, for example fathers with Hepatitis B have more boys.
In a sense nearly all women are predisposed to have more boys – the average sex ratio of 105 boys to 100 girls is influenced by partner choice, which will have a genetic component. So we might expect genetic effects in women too, albeit weak ones.