Family members have different genetics. That your mom or dad had thick and/or abundant hair is not a guarantee you will inherit that trait. It could go either way. Your brother's full head of hair is also not a guarantee.
I'm bald and my son still has a pretty full head of hair. He's about to turn 50. If he had inherited male pattern baldness, it would have shown up by now.
It's statistics of course. You can have the lucky draw.
If your son hasn't started getting bald at 50, he won the genetic lottery in this case !
My own father is 88 and not bald at all, his father died at 95 with a full head of white hair.
I am in my 40s and haven't started getting bald at all.
The Force is strong in the Dr Dutch heritage
