Binding of the MC1R cognate ligand, α-melanocyte stimulating hormone (α-MSH), induces a melanogenic cascade resulting in the production of dark eumelanin. MC1R is a G-protein-coupled receptor, expressed on the surface of skin and hair melanocytes. Other genetic factors must be interacting with MC1R to modify the penetrance of these variants. Less well known is the observation that most of these variants are only partially penetrant, and some of them have very low penetrance indeed 6. Red hair is well established as being associated with coding variation in the MC1R gene 4, 5. Several studies have examined the genetic basis of hair colour variation. Hair colour variation is partially correlated with skin and eye colour variation, reflecting differences in cellular interaction in different tissues 2, 3.
It is thus an excellent model system to explore genetic and cellular interactions in development and homoeostasis.
Furthermore, hair colour is largely determined by only a few well-characterised cell types: the melanocytes where the melanin pigment is made, the keratinocytes of the hair to which the pigment is transferred and fibroblasts of the dermal papilla, which signal to and regulate the melanocytes. Natural hair colour within European populations is strikingly variable and is a complex genetic trait that is impacted relatively little by known, non-genetic, factors 1.
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