Hair color

Hair pigmentation, similar to skin pigmentation, is an aspect determined mainly by the amount and distribution of melanin. The proportion and amount of melanin is specific to each individual, resulting in a wide variety of hair colorations.

The hair is the continuation of the scalp, formed mainly by keratin. Melanin is the chemical substance that, together with keratin, gives hair its color. There are two types of melanin, eumelanin and pheomelanin, and their quantity and proportion is what determines the final natural color. Specifically, people with darker shades have a higher amount of eumelanin, while those with lighter or reddish shades have a higher proportion of pheomelanin. In this hair coloring process, a few cell types are involved: the melanocytes where the pigment is manufactured, the keratinocytes in the hair to which the pigment is transferred and the fibroblasts in the dermis that regulate the melanocytes.

There are many possible hair shades (black, brown, blond, red, gray or white), with dark colors being the most common. The greatest heterogeneity is found in Europe, where in the north blond hair is the most common color. It is also common for hair color to be associated with skin and eye color, although not always.

Hair color can also vary in different cases. Age is the most important factor that can cause a change in hair color. As we age, melanin production progressively decreases, so that the hair takes on a grayish hue. The age at which this begins is mainly determined by genetics. Other conditions that can affect hair color are albinism, a genetic anomaly in which the ability to produce melanin is lost, vitiligo, which can cause loss of pigment in skin and hair, and other conditions such as malnutrition.

Natural hair coloration is a trait with a complex genetic architecture. In previous work, variants in the MC1R gene have been shown to be the main genetic cause of red hair color, albeit with variable penetrance. To date, multiple studies have examined the genetic basis of hair color. Among the association studies carried out so far, a GWAS study with more than 320000 individuals has identified more than 120 associated loci that explain 90% of the heritability associated with red hair color, 73% with blond hair color and 47% with brown hair color. Among the genes identified, most of them are involved in the regulation of hair growth or texture, such as IRF4 or KITLG, which highlights the importance of cellular connections between keratinocytes and melanocytes in determining hair color.

Number of observed variants

13.5 million variants

Number of loci analyzed in the study

121 loci

Bibliography

Morgan MD, Pairo-Castineira E, Rawlik K, et al. Genome-wide study of hair colour in UK Biobank explains most of the SNP heritability. Nature Communications. 2018 Dec;9(1):5271.

National Library of Medicine (NIH). Is hair color determined by genetics? [Jun 2020]

The DNA test you were looking for