Cats, like all living organisms, occasionally have mutations that affect their body type. Sometimes, these mutations are striking enough that humans select for and perpetuate them. However, in relatively small or isolated feral cat populations the mutations can also spread without human intervention, for example on islands. Cat breeders exploit the naturally occurring mutations by selectively breeding them in a small gene pool, resulting in the creation of new cat breeds with unusual physical characteristics. The term designer cat is often used to refer to these cat breeds. This is not always in the best interests of the cat, as many of these mutations are harmful; some are even lethal in their homozygous form. To protect the animal’s welfare it is illegal in several countries or states to breed with parent cats that bear certain of these hypertype mutations. This article gives a selection of cat body type mutant alleles and the associated mutations with a brief description.

Tail types

Dwarfism

Paws

Ear types

Hairlessness

Miniature-sized

A germ-cell mutation occurred in a male Persian cat called Treker in 1995, resulting in diminutive, but healthy and normally-proportioned, offspring. Treker and the females with which he was mated were normally sized, but 75% of the kittens sired by Treker inherited diminutive stature, but of normal proportions. The gene was found to be dominant and the diminutive offspring were sold as teacup or toy Persians. Toy and Teacup Persians are a separate breed and not all cats advertised under those names result from Treker's dominant mutation. Most teacup and toy size Persian kittens raised now are descendants from Silver and Golden color division to reduce cat size and are in no way related to Treker.

Breeding ethics and ban of designer cats

Many of the major cat registries do not allow the registry and breeding of cat breeds which bear certain of these genetic mutations, nor recognise them, due to concerns about the animal’s welfare. Breeding with parent animals that bear hypertype mutations is effectively banned in Europe under the European Convention for the Protection of Pet Animals: "Any person who selects a pet animal for breeding shall be responsible for having regard to the anatomical, physiological and behavioural characteristics which are likely to put at risk the health and welfare of either the offspring or the female parent." Since 2014, it is illegal in the Netherlands to breed with parent cats which bear genetic mutations or other physical abnormalities that cause health or welfare issues in their offspring. Examples of these genetic mutations and physical abnormalities are dwarfism, the lack of protecting fur in hairless cats, and fold-eared cats. Furthermore, several countries or states have prohibited breeding with certain of the genetic mutations in cats, including Scotland, Victoria and the Australian Capital Territory (Australia), Austria, and Flanders (Belgium).