What this means

MC1R controls the balance between brown/black eumelanin and yellow/red pheomelanin in skin and hair. R151C (the C→T change at rs1805007) is one of three classic "R" alleles — variants that shift pigment production toward pheomelanin. Red hair is a recessive trait, so it usually requires two MC1R R alleles, but a single copy still often shows up as fair skin, freckling, or auburn highlights. About 1–2% of the global population has red hair, concentrated in northern and western Europe.

MC1R is the gene that decides whether your hair and skin lean toward brown/black pigment or red/yellow pigment. R151C is one of three classic "red hair" versions of this gene — it shifts production toward the red side. Red hair usually needs two of these versions showing up together to be obvious, but even one copy often gives you fairer skin, freckles, or auburn highlights. Only about 1–2% of people worldwide have red hair, mostly in northern and western Europe.

Caveats

• Hair colour can change over a lifetime — childhood red may darken to auburn or brown.
• There are three classic MC1R R alleles; combinations across them matter more than any single one.
• Red hair is a polygenic outcome — MC1R is the dominant factor but not the only gene.
• Most published work is on European-ancestry populations.

References

• Valverde et al. — Variants of the melanocyte-stimulating hormone receptor gene are associated with red hair and fair skin in humans (Nature Genetics, 1995)
• Sulem et al. — Genetic determinants of hair, eye and skin pigmentation in Europeans (Nature Genetics, 2007)