Siamese cat
Appearance

The breed standard of the modern Siamese calls for an elongated, tubular, and muscular body and a triangular head, forming a perfect triangle from the tip of the nose to each tip of the ear. The eyes are almond-shaped and light blue, while the ears are large, wide-based, and positioned more towards the side of the head. The breed has a long neck, a slender tail, and fur that is short, glossy, fine and adheres to the body with no undercoat. Its pointed colour scheme and blue eyes distinguish it from the closely related Oriental Shorthair. The modern Siamese shares the pointed colour pattern with the Thai, or traditional Siamese, but they differ in head and body type.

The pointed pattern is a form of partial albinism, resulting from a mutation in tyrosinase, an enzyme involved in melanin production. The mutated tyrosinase enzyme is heat-sensitive; it fails to work at normal body temperatures but becomes active in cooler (< 33 °C) areas of the skin. This results in dark colouration in the coolest parts of the cat's body, including the extremities and the face, which is cooled by the passage of air through the sinuses. All Siamese kittens, although pure cream or white at birth, develop visible points in the first few months of life in colder parts of their body. By the time a kitten is four weeks old, the points should be sufficiently distinguishable to recognise which colour they are.

Siamese cats tend to darken with age, and generally, adult Siamese living in warm climates have lighter coats than those in cool climates. Originally the vast majority of Siamese had seal (extremely dark brown, almost black) points, but occasionally Siamese was born with "blue" (a cool grey) points, genetically a dilution of seal point; chocolate (lighter brown) points, a genetic variation of seal point; or lilac (pale warm grey) points, genetically a diluted chocolate. These colours were considered "inferior" seal points and were not qualified for showing or breeding. These shades were eventually accepted by the breed associations and became more common through breeding programmes specifically aimed at producing these colours. Later, outcrosses with other breeds developed Siamese-mix cats with points in other cat colours and patterns, including red and cream points, lynx (tabby) points, and tortoise-shell ("tortie") points.

In the United Kingdom, all pointed Siamese-style cats are considered part of the Siamese breed. The Cat Fanciers' Association, considers only the four original fur colours as Siamese:

Oriental Shorthair cats with colour points in colours or patterns aside from these four are considered Colourpoint Shorthair in that registry. The World Cat Federation has also adopted this classification, treating the Colourpoint Shorthair as a distinct breed.

Many Siamese cats from Thailand had a kink in their tails, but over the years, this trait has been considered a flaw. Breeders have largely eradicated it, but the kinked tail persists among street cats in Thailand.
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