In humans, the melanocortin 1 receptor (MC1R) is a key protein involved in human skin and hair color. Several versions of the MC1R gene with changes in single nucleotides have been reported. One of these has the amino acid histidine where the amino acid aspartate normally is found at a certain location in the MC1R protein. Another version has a tryptophan where an arginine should be. Both of these versions of the gene are associated with red hair, whereas the wild-type version of this gene is associated with dark hair. What phenomenon is occurring with these genes?
1) Incomplete dominance
2) Codominance
3) Multiple alleles
4) Epistasis