rs1805007: T or C
TT tend to have red hair
TC less likely to have red hair
CC least likely to have red hair
This polymorphism is in the MC1R gene, which encodes a protein involved in producing melanin, a pigment in your hair and skin. The T allele encode an Arginine at position 151 whereas the C allele encodes a Cysteine at this position. The MC1R gene is a receptor on the surface of melanocytes, which are the cells that produce melanin.
Hair color is determined by the amount of eumelanin (which is dark brown) and pheomelanin (which is reddish). The amount of eumelanin ranges continuously from very little, producing light-blonde hair, to large amounts, producing black hair. People with large amounts of pheomelanin have red hair, which can range from pale red ("strawberry blond") to bright red to reddish brown.
Extensive and interesting information from John McDonald, U. Delaware.
More info at 23andme.
A nice link on hair color from The Tech museum: Understanding Genetics.