The rex mutation is a genetic variation in mammals that results in soft curly fur. These effects are due to changes in the structure of groups of hairs and cross-section of individual hairs. The rexed coats are unusual but occur (and have been preserved) in cats, rats, rabbits, horses, and dogs. The mutations, infrequent and spontaneous, occur in a variety of genes and genetic regulatory structures. The diversity of genetic factors results in variable coat thickness/density and fur length. A similar mutation can affect the feathers of birds.

Birds

There are several varieties of curly-feathered domesticated birds:

Cats

There are four main internationally recognised rex cat breeds: Cornish Rex, Devon Rex, LaPerm, and Selkirk Rex. Rex breeds are sometimes nicknamed "poodle cats" or "poodlecats", after the curly-coat dog breed group. There are also a number of rarer, less well known or developing rex-coated breeds, including the German Rex, Ural Rex, Tennessee Rex, Tasman Rex group, and Skookum. Spontaneous rexed variants have also been observed in breeds such as the Maine Coon and Persian as well as in random-bred cats such as Ohio Rex, Oregon Rex, and California Rex, however there has been no significant attempt to develop or seek recognition for these varieties and most have died out. The Tennessee Rex is a natural-mutation breed of cat dating back to 2004, discovered by Franklin Whittenburg. Rex breeds vary considerably in appearance, from the elegant slender build of the Cornish Rex to the larger, cobby and heavy-boned Selkirk Rex. Each results from a separate mutation rather than being bred from a common ancestor. Each mutation causes the hair follicles to be oval in shape, instead of the normal regular round form, which results in hairs curling round as they grow instead of growing straight. Many of the other spontaneously occurring rex mutations have occurred and some have proven to be the same gene as the main four breeds. Some of those mutations, such as the Dutch Rex, were found to have unmanageable or unattractive fur and were not pursued as breeds. The name rex was taken from the rabbit fancy because the first rex cat breed to be developed, the Cornish Rex, has a coat similar to that of a rex rabbit, in that it comprises only the undercoat with no topcoat or guard hairs.

Named mutations

Dogs

Horse

Pig

Rabbits

A great variety exists within rex rabbits. They vary from very dense and short-haired varieties (Standard Rex) to long-and-curly-haired breeds (Astrex). The rex rabbit was introduced in 1919 by Abbe Gillet from a spontaneous mutation. The next rex rabbit was discovered in the hutches of a breeder in Lübeck, Germany in 1926. This breed was originally called Deutsch-Kurzhaar and is due to the r2 (dek) gene. In 1927, yet another short-hair rabbit was discovered in the hutches of the French breeder Madame Du Bary, a fancier of Himalayan (ch-) rabbits. This breed was called Normannen-Kurzhaar and is due to the r3 (nok) gene. Of the three genetic sources of rex rabbits, the one due to the gene r1 is the most popular with fanciers and has the simple Mendelian inheritance pattern of autosomal recessive. It was not linked with any other genes known at the time although it was eventually found to be in the same linkage group as r2. The phenotype of r1 and r2 is a completely normal coat but r1 or r2 alone produces a short coat with curly whiskers. Although the phenotypes are identical for each gene, the linkage distance was eventually worked out. The gene r3 is independent of r1 and r2 in breeding tests so they fall on different chromosomes. The mutation associated with r1 was recently studied using modern genetic tools. A deletion of a single nucleotide was found in LIPH (lipase member H gene) of rex rabbits. In humans, this gene is associated with alopecia (hair loss phenotypes). This mutation results in a frameshift that causes the mRNA transcription machinery to sense the end of the gene prematurely. The stop codon generated by the deletion may be shortening the protein by 19 amino acids. The exact mechanism was not determined but the deletion is necessary and sufficient for the rex phenotype.

Rodents

Guinea pigs

Hamsters

Rex golden hamsters first appeared in the 1970s, characterised by curly whiskers and a curly coat; the curls in the coat are easily visible in short-coated hamsters but more difficult to see in long-coated ones. Rex in Campbell's dwarf hamsters is not recorded as to when it first appeared. Unlike with golden hamsters, the rex coat in Campbell's dwarfs is commonly very sparse and gives a bald appearance. The whiskers are curled.

Mice

Rat