To date, there are at least ten known loci that control coat color in gerbils; each locus controls a different trait. Each of these loci has a dominant allele, that usually represents the form of the trait commonly found in the wild, and at least one recessive allele, representing a less common form of the trait. The eight loci discussed on this page are: A, C, D, E, P, Uw, Sp, and Re. I will also touch on Steel-factor, which is a different sort of locus. The loci not discussed include the Semi-Dominant Lethal Spotting and Wavy mutations, which are not known to be present in the USA at this point.
To understand how these loci work, it is necessary to know how color is structured on the hair.
The Agouti locus controls the variegation on the hair shaft, as described above. The gray-brown-black pattern is called "agouti", after the South American rodent of the same name. Wild gerbils have agouti-patterned fur on their back and flanks, with white undersides.
There are two known alleles at the agouti locus in gerbils. The dominant allele, A, creates the white-bellied agouti coat found in the wild. The recessive allele, a, represents the "nonagouti" mutation, which causes the hair shaft to be a single color from root to tip. Gerbils expressing the nonagouti mutation also lack the white belly that is characteristic of agouti-patterned gerbils.