Colorpoint (TYR gene, Siamese)

Specific alterations in the TYR gene, which regulates melanin production, can cause reduced pigmentation in areas of the body. These changes are also associated with oculocutaneous albinism type I.

Definition

The colorpoint trait in the Siamese breed is related to the TYR gene, which encodes the tyrosinase enzyme, essential for melanin production. In Siamese cats, tyrosinase is temperature-sensitive, functioning better in the cooler parts of the body and producing more melanin in these areas (ears, face, paws, and tail). In warmer areas, the enzyme activity decreases, resulting in a lighter coat.

Genetic basis

The C locus in the TYR gene determines coat coloration. At this locus there is the following allelic series: C > c^b = c^s > c, c^2. The C allele corresponds to the wild-type allele that produces melanin. In this case, we analyzed the c^s allele which causes a characteristic "dotted" coloration pattern due to the c. 904G>A substitution in the TYR gene. This trait is inherited in an autosomal recessive manner, meaning that it requires the presence of two copies of the genetic variant for expression of the phenotype. Cats that have only one copy of this variant will not show the trait, but can transmit the allele to their offspring with a 50% probability.

Other relevant information

Other alleles of the same C locus are associated with other characteristic coats: the C allele allows normal melanin production, the c allele causes complete absence of coloration, while the c^b (Burmese cat) and c^s (Siamese cats) alleles reduce melanin production, creating specific color patterns. Although the TYR gene is related to oculocutaneous albinism, in this case this mutation does not cause complete albinism but hypopigmentation.

Bibliography

Lyons LA, Imes DL, Rah HC, et al. Tyrosinase mutations associated with Siamese and Burmese patterns in the domestic cat (Felis catus). Anim Genet. 2005 Apr;36(2):119-26.