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The sex-linked orange gene, O, determines if there will be orange fur. This gene is located on the X chromosome. In cats with orange fur, phaeomelanin (orange pigment) completely replaces eumelanin (black or brown pigment).
Males have only one X chromosome, so only have one allele of this gene. O results in orange fur, and o results in non-orange fur.
Since females have two X chromosomes, they have two alleles of this gene. OO results in orange fur, oo results in non-orange fur, and Oo results in a tortoiseshell cat, in which some parts of the fur are orange and others areas non-orange. A cat with Oo and white spotting genes is commonly called a calico. The reason for the patchwork effect in female cats heterozygous for the O gene (Oo) is X-inactivation – one or the other X chromosome in every cell in the embryo is randomly inactivated (see Barr body), and the gene in the other X chromosome is expressed.
