O locus — orange (red)
Controls: Red/cream colouring, and tortoiseshell in females.
The orange gene sits on the X chromosome and replaces black pigment with red. Because females have two X chromosomes, a female carrying both red and a black-series colour shows both in patches — a tortoiseshell. Males have one X, so they are usually either red or non-red, not tortie.
Alleles
ORed (orange)RecessiveoNon-redRecessiveBreeding examples
Red father × Black mother
← All daughters tortoiseshell; all sons black. (Father's red X goes only to daughters.)
Black father × Red mother
← All daughters tortoiseshell; all sons red. (Sons take the mother's red X.)
Tortoiseshell mother × Black father
← Daughters: black or tortie. Sons: black or red. A tortie throws both.
Colours it produces
Frequently asked questions
Why are most tortoiseshell cats female?+
Showing red and a black-series colour together normally needs two X chromosomes, so torties are almost always female.
Can a male cat be tortoiseshell?+
Very rarely — only a male with an extra X chromosome (XXY) can be tortie, and such males are usually sterile. It is the exception that proves the rule.
Red father × black mother — what colours are the kittens?+
The father passes his red X only to daughters, so all daughters are tortoiseshell; sons take their single X from the black mother, so all sons are black. This is why the O gene is called sex-linked.
What is cream?+
Cream is diluted red — the D (dilution) locus softens red into a pale, buttery cream.
Related genes
See these colours on real cats.
Browse cat colours