In the first two parts of this article, we learned about the genes that code for black, chocolate, cinnamon, and red (orange) fur. Now let’s take a look at the gene that can convert these colors to their dilute counterparts.
The dense/dilute gene
The D gene codes for either dense coloring or dilute coloring, based on the density of the pigment in the hair shaft. There are only two alleles, and the D allele shows full dominance over the d allele. So of the three genes we’ve looked at thus far, this one is the simplest.
If a cat has a D allele, whether via the DD or the Dd genotype, it will have dense fur. This means that a cat coded for black, chocolate, cinnamon or red fur will express that color. A cat with two d alleles (dd genotype) will express the dilute version of the base color: blue, lilac, fawn or cream.
Calico and tortoiseshell cats
Recall that a female cat gets two copies of the O gene because she has two X chromosomes. The O allele produces red (orange) fur, while the o allele preserves the black-based color (black, chocolate or cinnamon). The alleles are co-dominant, so a female cat can express both a black color and red.
If such a cat has the D allele, she would have black and red fur, making her a tortoiseshell. With white added, she becomes a calico (also known as tortoiseshell and white) cat. (For simplicity, let’s assume she has the BB genotype, so we do not concern ourselves with the chocolate or cinnamon variations.)
If instead of the dominant allele she has two copies of the d allele, both the black and red are expressed in their dilute versions: blue and cream. We call this girl’s coloring dilute tortoiseshell, or dilute calico (a.k.a. dilute tortie with white) if she also has white fur.