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 cats 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
Jb Japanese Bobtail gene (autosomal dominant). Cats homozygous and heterozygous for this gene display shortened and kinked tails. Cats homozygous for the gene tend to have shorter, more kinked tails. This can be distinguished phenotypically from the Manx cat mutation by the presence of kinking in the tail, often forming what looks like a knot at the distal end of the tail. Unlike the Manx tailless gene, there are no associated skeletal disorders and the gene is not associated with lethality. M Manx tailless gene (dominant with high penetrance). Cats with the homozygous genotype (MM) die before birth, and stillborn kittens show gross abnormalities of the central nervous system. Cats with the heterozygous genotype (Mm) show severely shortened tail length, ranging from taillessness to a partial, stumpy tail. Some Manx cats die before 12 months old and exhibit skeletal and organ defects. Because it was discovered in naturally occurring populations of cats, the Manx gene could confer some kind of selective advantage to the cats, or it may simply be an example of the founder effect. The trait also occurred and died out in Cornwall (peninsular England), but persisted in the Isle of Man population where outbreeding was not frequent due to isolation. Shortened tails, most of which are indistinguishable from the Japanese Bobtail or the variably expressed Manx mutation, may occur sporadically in any cat breed or mixed-breed population. However, some may be novel mutations that have not been investigated. There are multiple types of curly-tailed cats whose tails loop over the back or form tight corkscrews. One such mutation has been developed into the American Ringtail, but others have been regarded as curiosities and not perpetuated. The gene(s) responsible have not been fully investigated. However, a research study is under way at UC Davis under the guidance of Leslie A. Lyons, currently at the University of Missouri.
Dwarfism
Mk Munchkin or dwarf cat gene (dominant). Cats heterozygous for this gene (Mkmk) have shortened and thickened legs, because this gene causes dwarfism in cats. Cats with this mutation have lived naturally in feral populations worldwide. However, cats with this gene are also the basis for several intentionally selected breeds of dwarf cat. The Munchkin is the first created dwarf cat breed, which was used as a foundation stock for other “spin-off” breeds, such as the Minskin, Minuet or Napoleon, Bambino, Lambkin Dwarf, Kinkalow, Skookum, Genetta, and Scottish Kilt. The homozygous form (MkMk) may be lethal as litter sizes are smaller than average. Unlike undersized cats of normal proportions (such as Toy and Teacup Persians), dwarf cats suffer from a genetic mutation called pseudoachondroplasia (dwarfism without enlarged heads), a type of osteochondrodysplasia (bone and cartilage disorder). These genetic disorders of bone and cartilage are typically manifested as abnormal bone shape, poor growth or lack of growth, bowing of the limbs to the side – front legs are more likely to be affected, and spinal malformations. The disorder leaves the cat with a full-size body, but disproportionately short and thick legs. Dwarf cats often suffer from spinal disorders, such as lordosis (excessive curvature of the spine) and pectus excavatum (hollowed chest). As with human dwarfism, there are still a lot of unknown mysteries in the science behind feline dwarfism. It is found that achondroplasia in humans is the result of the FGFR3 gene.