C-Locus (Siamese, Burmese, Mink, Albinism c, Albinism c2)

C-Locus (Colourpoint) – Siamese, Burmese, Mink, Albinism c and Albinism c2

The Colour Locus (C-Locus), associated with the gene for tyrosinase (TYR), plays a crucial role in determining coat colour in cats. Various recessive mutations in the TYR gene can result in different forms and degrees of Type 1 Oculocutaneous Albinism (OCA1), characterized by the inability to produce pigment. This condition manifests in a silvery-white coat and light-coloured eyes, with a spectrum ranging from distinct point coloration seen in Siamese cats to full albinism.

Some of these mutations cause the affected tyrosinase to become temperature-sensitive, functioning normally at lower temperatures but failing to produce pigment at higher temperatures. Consequently, cats with colour point phenotypes exhibit darker extremities due to the cooler temperature in these areas compared to the main part of the body. They are typically born entirely white and may develop darker coats when residing in cooler environments.

Inheritance

The wild-type allele (C) is dominant and results in the absence of point coloration; the cat will simply have its normal base coat, as determined by its other coat colour genes.

The Siamese Point mutation (c^s) causes the classic Siamese colourpoint coloration, characterized by a light body with noticeably darker extremities (paws, ears, muzzle, tail) in colours such as black (Seal Point), Red (or Flame Point), or tabby (Lynx Point). In some breeds, such as the Bengal, the colouration resulting from the c^s mutation is known as Snow seal lynx instead.

Sepia, also known as Solid or Burmese colourpoint, is the result of the mutation c^b. This is the colour pattern distinctive of the Burmese cat. It is similar in pattern to point, but darker and with less contrast. In Bengal cat this coloration is also known as Snow seal mink.

Cats carrying both the c^s and c^b mutations exhibit the “Mink” or “Tonkinese” phenotype, showcasing an intermediary coloration lighter than Sepia but darker than Point. In Bengal cats this phenotype is also called Snow seal mink.

Mutations c and c^2 lead to complete albinism, indicating a total absence of coat pigmentation. These mutations are entirely recessive to c^b and c^s mutations.

Understanding the dynamics of these mutations at the C-Locus sheds light on the vast array of coat colour variations observed in feline populations.

Relevant tests

• K314
• K337
• K338
• K405