The E gene
The extension gene is a modifier of the A gene. This gene is either on or off. If the dominate allele is present Ed then the animal is black. If the animal is homozygous recessive E+ then the extension gene lets the Agouti gene express the pattern.
It has been theorized that there are additional alleles at the extension locus. These being Ebl, Ebr, and Ey. In this scenario the alleles show incomplete dominance. Meaning that a heterozygote is a mixture of the two.
- Ebl Ebl= black or blackish brown
- Ebl Ebr= black or blackish brown
- Ebl Ey= dark brown, brown, or dark red
- Ebr Ebr= dark brown, brown, or dark red
- Ebr Ey=red, light red, or tan
- Ey Ey= red, light red, or tan
This is only theory some other color geneticists say that the E+E+ alleles are present and that ranges of tan are due to modifications of the White Tan allele at the Agouti locus. Alleles of the Brown gene could also be involved.