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 of hair loss is referred to as male pattern baldness, or more technically as androgenic alopecia, where hair thins until it is eventually gone from the head.

Hair grows on the head from a number of different hair follicles. The average person has about 100,000 follicles on their head, 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 hair loss 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 which 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 the cause of hair loss was passed down through the mother, and could usually be traced to the maternal grandfather. If the maternal grandfather was prone to hair loss, common wisdom holds, the grandson will also be susceptible.

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.

Many people now believe that hair loss is inherited from both the mother and the father, and that the actual mechanism by which it is inherited is not 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 more likely causes, of hair loss in men, they have a few characteristics we can be fairly certain of. Firstly, because of the range of people having hair loss, 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 responsible for hair loss have variable penetrance, since full siblings don’t necessarily have the same frequency of hair loss. Lastly, it appears that the genes responsible are in fact dominant, and not recessive as was at one point 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 for hair loss will finally be pinned down, and we can say once and for all whether the mother or the father is more responsible for passing the gene along.