Three genes (six alleles) contribute to skin colour. The three genes are A, B, and C and their alleles are A and a, B and b, and C and c, where A, B, and C contribute to dark skin colour and a, b, and c contribute to light skin colour. For human skin colour, the alleles code for the production of the skin pigment melanin, and their effect is cumulative. This implies that a person with genotype AABBCC has six alleles that code for high production. of melanin, thus indicating that their skin would be very dark. On the other hand, a person with genotype aabbcc has six alleles that code for low production of melanin, and thus their skin would be very light.