O-Locus (Orange)

O-Locus (Orange)

In cats, the Orange locus (O-locus) is a major genetic factor determining orange coat colour. This locus is located on the X chromosome, meaning its inheritance depends on the sex of the cat. Recent studies have identified the gene ARHGAP36 as underlying the Orange locus, with regulatory variation affecting pigment production pathways. The O-locus influences the type of pigment produced in the coat by shifting the balance between eumelanin (black/brown pigment) and phaeomelanin (yellow/red pigment).

The dominant allele ‘O’ shifts pigment production from eumelanin to phaeomelanin, resulting in an orange coat colour. Cats carrying this allele express orange pigmentation instead of black or brown in the affected areas of the coat.

Inheritance

Because the O-locus is X-linked, its expression differs between males and females. Male cats (XY) have only one X chromosome and will therefore be either fully orange (O) or non-orange (o). Female cats (XX), however, can be homozygous (OO = fully orange, oo = non-orange) or heterozygous (Oo). In heterozygous females, X-inactivation leads to a mosaic expression of the O-locus, resulting in tortoiseshell or calico coat patterns.

The wild-type allele is designated as ‘o’ (non-orange), which allows normal production of eumelanin. The final coat colour in non-orange cats may still be influenced by other loci, such as the B-locus and D-locus, which affect pigment type and dilution.

Relevant test

• K510: Coat colour O-locus (Orange)