A man with many brothers is more likely to have sons
If you want to know if you will have a son or a daughter just count your brothers and sisters. It’s the man’s genes that control the likelihood of whether it will be a boy or a girl. A man with many brothers is more likely to have sons, while a man with many sisters is more likely to have daughters.
https://www.sciencedaily.com/releases/2008/12/081211121835.htm
