Hair loss is a condition where hair, usually on the top of the head, starts to fall out and ceases to grow back fully. The most common type is referred to as male pattern baldness, or more technically as androgenic alopecia, where hair thins until it is eventually gone from the head. Although it was long believed that this condition was inherited from the mother's side of the family, researchers now think that it is likely that any genetic factors can come from either parent.
Hair grows on the head from a number of different hair follicles. The average person has about 100,000 follicles, each of which can grow somewhere around 20 distinct hairs over the lifetime of the person. When these follicles stop producing hair, the most common types of hair loss occur.
For years it has been thought that hair loss was inherited from the mother’s side of the family. Some studies have suggested that it is caused by the presence of excess androgen receptors on the scalp. This was seen as the result of a genetic difference, which caused either more androgen receptors to form or for those that formed to be more stable and less susceptible to breaking down.
The androgen receptor gene resides on the X chromosome, which men inherit from their mother. As a result, it was thought that a tendency to lose hair in certain ways was passed down through the mother, and could usually be traced to the maternal grandfather. If the maternal grandfather lost his hair, common wisdom holds, the grandson will too.
The truth, however, seems to be that hair loss is inherited no more often from the mother than from the father. The amount of data available on the issue is surprisingly sparse, with most evidence for inheritance through the mother’s side coming from a 1916 study with fairly shaky methodology. The actual genetics are not well understood. Androgen receptor genes appear to be a somewhat simplistic explanation, and don’t necessarily hold up to large sample studies.
The data now seems to show that whatever the cause or causes of hair loss in men, they have a few characteristics that people can be fairly certain of. First, because of the range of people who lose their hair and the correlations between both father-son and maternal-grandfather-grandson baldness, it appears the genes responsible are actually autosomal, residing on neither the X nor the Y chromosome. It also appears that the genes have variable penetrance, since full siblings don’t necessarily have the same frequency of hair loss. The genes responsible seem to be dominant as well, and not recessive as was once thought.
It is surprising that an issue discussed so frequently has so little data available. In the past few years, however, especially since the mapping of the genome, more large-scale studies have begun on hair loss in men. Although as of yet inconclusive, it is likely that within the next few years the genes responsible will finally be pinned down, and researchers may be able to say once and for all whether the mother or the father is more responsible for passing the gene along.