But isn't it true that more boys than girls are born (because boys die younger so evolution tries to balance it out a bit)?
Does anyone know if certain fathers can only produce one sex of child? If so then having one girl would increase the chances of having another girl slightly.
There is a slight difference in the swimming speed of XX vs XY sperm, which accounts for the difference. Also I believe females have a slightly better survival ratio, the reasons are complicated, but include the fact that all fetuses start as female, and then are modified by testosterone to be male, i.e. female is the default. Plus females have XX so some genes are doubled which helps them.
It's pretty much impossible for a father to make just one or the other because of that way it's produced.
The father has XY cells, which split in half to make sperm, one half become a male sperm the other female. So sperm is always made in pairs.
There can be differences in the mother that affect one or the other differently though (PH for example, and the sperm are not the same size).
Your explanation of whether certain couples can only produce males or females was pretty interesting though, thanks.
Although women may live longer they lose their ability to reproduce earlier so I would expect more girls than boys.