I was complaining that I thought my kids would all be my height or a little taller because my ex was a little bit taller than me, so should they not be the average of our heights?
My kid who did genetics in school says no, the average result is add mom and dads heights, divide by 2 but then add 10cm for boys, subtract 10cm for girls. So if I wanted girls my height I would need a man taller than me. And boy children would likely end up taller than either of us.
Obviously it's not an unbreakable rule, but a likely result.