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Cat fur colors and patterns: Basic genetics, part 2

In part 1 of this article we learned about the genetics of the black-based fur colors: black, chocolate, and cinnamon. In this part we explore the genetics of red/orange fur, and how it affects expression of black-based colors.

Little Milly has beautiful orange tabby fur. Her genotype is either OO or Oo.
Little Milly has beautiful orange tabby fur. Her genotype is either OO or Oo. Courtesy of Getty Images

The Red (Orange) Gene

The Red (Orange) gene controls production of the pigment phaeomelanin. It reflects light in the red, yellow, and orange part of the spectrum, so fur that is pigmented with phaeomelanin appears to be a shade of orange. We commonly refer to this color fur as orange or marmalade, but the official term in cat fancier circles is red fur.

You may see this gene referred to as the Red gene, the Orange gene, or simply the O gene. The O gene has two alleles: O causes the production of the red pigment instead of the black pigment, while o does not. But the O allele is not dominant over the o allele. A cat with the Oo combination will actually have some red fur and some black fur (or modified versions of both depending on other genes).

For females, the possible genotypes (allele combinations) and their corresponding phenotypes (physical traits) are as follows:

OO – Red
Oo – Red & black (tortoiseshell)
oo – Black

Why is this the case only for female cats? The O gene is X-linked, meaning it is located on the X chromosome. Females get two copies of this chromosome (XX), but males get only one (XY). So males either get an O allele or an o allele, but not both. The exception is a male with an extra X chromosome (XXY). Such a cat could have the Oo genotype, and thus be a tortoiseshell, but this is very rare.

The possible genotypes and phenotypes for male cats are:

O – Red
o – Black
Oo – Tortoiseshell (only for XXY males, which are rare and usually sterile)

In an OO (female) or O (male) cat, the B gene becomes irrelevant. Regardless of whether the cat carries the alleles for black, chocolate, or cinnamon, the black-based color will not be expressed. The O allele blocks the action of the B gene, and the cat’s fur will be red unless it is modified by another gene. If a cat has fur in the black color spectrum (including dilute and tabby versions), it must have the oo genotype.

A cat with the Oo genotype will almost always be female and will have both red and black fur. This occurs because some cells express the red fur that the O allele produces, while some express the black fur coded in the o allele. A tortoiseshell cat has only these two colors, while a calico cat has white fur as well.

In part 3 of this article, we will explore the effects of a third gene, which can dilute either the black or red colors.

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