Quoth Cecil: “Nöthen and company emphasize that this variant isn’t the sole cause of baldness–other recent research suggests that baldness on your father’s side of the family has something to do with it too. No doubt this helps explain cases of hairy maternal grandpa/bald dad/bald son.”
One doesn’t even need to go that far, really. The fact that male pattern baldness is X-chromosome-linked doesn’t let you directly predict phenotypes … Take me, for example. My mother’s father was bald, as are her three older brothers. Under the assumption that the X chromosome carries that gene, and it’s dominant, I’m doomed, right?
Nope, not necessarily. My maternal grandfather carried the baldness gene, obviously. And since my mother got one of her two X chromosomes from him, and he only had the one, she carries an X chromosome with the baldness gene. But–her other X chromosome she got from her mother, not her father-there’s a 50/50 chance that my X chromosome is that one! So, what are my odds?
Well, her brothers were also bald. They got their X chromosome from their mother, so at least one of her two X chromosomes also carry that gene. (They had to have gotten my grandfather’s Y, rather than his X, or they’d be my aunts instead of my uncles.) However, based on this research, my grandmother probably didn’t have this gene on both of her X chromosomes, or she’d have suffered hair loss. So, she most likely had one X with and one without, and it’s again 50/50 which she passed to my mother.
To summarize: there’s a 50% chance that my one X chromosome came from my maternal grandfather (in which case I definitely carry the baldness gene), and a 50% chance that it came from my maternal grandfather (in which case there’s a 50% chance that I got the version with the baldness gene and a 50% chance I didn’t). Net: 75% chance of greatly reduced haircut and shampoo costs in my future.
Obviously, this is considerably simplified from reality-there are more complex factors involved. But even in this completely stripped-down version, we can see that the state of my maternal grandfather’s and maternal uncles hair doesn’t let me predict with certainty my own genetic destiny, but only make a statistical estimate of my chances. While ‘not good’ sums them up succinctly, I can still hold onto hope; and if I live to old age with my naturally beautiful coiffure, that does not serve as a counterexample to the general theory …