Is male pattern baldness inherited? Who's to blame?
I've always heard that baldness in males is inherited from their mother's father. However, I've seen plenty of families where the father is bald, the mother's father has plenty of hair, and the kid is bald. So, what's the scoop on inheriting baldness?
Your note arrived more than a year ago, Debbie, and I considered answering it promptly, which I realize is a departure from usual Straight Dope practice. However, I didn't. Most of what Little Ed turned up on an initial reconnaissance of the Internet was expressed in terms so brisk and confident you knew these guys had no idea, leading me to think I'd better bide my time. Good thing. A newly published paper suggests that much of the conventional scientific wisdom about inheriting baldness (specifically male pattern baldness, also known as androgenetic alopecia, the most common type in men) is out the window. Here's the scoop as of now.
As you note, popular belief has long held that male baldness is inherited through the mother, and that any lad curious about what's in store for him hairwise should check out his mom's father and her paternal uncles. For just as long health care types have been patiently explaining that this belief was groundless--that instead, as one doctor's Web site (still) puts it, "a single dominant autosomal gene controls male pattern baldness." In other words, baldness isn't a sex-linked characteristic, meaning it's not transmitted via the X or Y chromosomes that determine whether you're male or female. Rather it's passed down via one of the non-sex-linked chromosomes, or autosomes, which are randomly contributed by both parents. So, according to the old theory, you were equally likely to get baldness from mom or dad.
It turns out, however, that the old theory was based primarily on one study conducted in, get this, 1916. Doctors were still prescribing mustard plasters in 1916. Clearly the time had come for a fresh look, particularly in light of our expanding knowledge of the human genome. Preliminary investigations hinted that baldness had something to do with the sex chromosomes--to be precise, the X chromosome, which a man inherits from his mother. (His father contributes the Y chromosome that makes him male.) Now comes a German study (Hillmer et al, American Journal of Human Genetics, July 2005) stating flatly that the major determinant of early baldness is a gene men get from their moms.
The researchers, led by Markus Nöthen of the University of Bonn, rounded up several hundred balding and nonbalding men and compared their androgen receptor (AR) genes, which are located on the X chromosome. Androgens (male sex hormones) play a key role in balding--castrated men don't go bald--and the AR gene helps them do their work. The researchers found the balding men were much more likely to have a particular version of the AR gene than the nonbalding men, which they took to mean they'd found a genetic variant that triggers baldness. 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. But Nöthen's group thinks the mother's contribution is the principal factor.
In retrospect it seems obvious baldness is at least partly an X-linked trait. (Nöthen gently reminds me that another, frankly more important reason is that men have an abundance of androgens and women generally don't.) About 1,000 genes are X-linked, among them those encoding red-green color blindness, one type of muscular dystrophy, and hemophilia, conditions predominantly afflicting males. Men are more prone than women to X-linked abnormalities because they receive an XY chromosome combination while women get XX. In women, a recessive abnormality on one X chromosome will likely be masked by a dominant normal gene on the other. Men, however, have only one X chromosome--their other sex chromosome, remember, is a Y. That means an X-linked abnormality, e.g., the aforementioned baldogenic variation in the AR gene, won't be masked and has a greater chance of being expressed, one reason you see a lot fewer balding women than balding men.
In short, after 89 years geneticists are finally getting straight on the hereditary basis of hair loss. But let's not be too quick to criticize--theirs is a confusing business. A previous demonstration of this fact, if you'll excuse my digressing a bit, is the seemingly simple matter of ascertaining how many chromosomes we humans have. By the early 1920s researchers were in wide agreement that there were 48, and for decades it was stated in textbooks as incontestable fact, leading at least one investigator to halt a research project when she kept coming up with a different figure. Not until more than 30 years later, in 1956, did Joe Hin Tjio count the chromosomes again and find, admittedly with the aid of an improved microscope slide preparation technique, that there were (ahem) only 46.