The munchkin is a relatively new breed created by a mutation that causes achondroplasia, or possibly hypochondroplasia, resulting in cats with abnormally short legs.
Munchkin Breed Origins
The breed was brought to fruition in 1983 when Sandra Hochenedel found an extremely short-legged black cat living under a trailer in Louisiana. The cat, Blackberry, was pregnant and half of her kittens were born short-legged. One of Blackberry’s kittens, a tomcat named Tolouse, became the father of a breeding program and helped establish the breed in North America. The Munchkin breed is not recognised by all registering associations and is specifically banned by the Fédération Internationale Féline FIFe and other European registries, but it is accepted by The International Cat Association (TICA).
The munchkin gene is an autosomal dominant one. Thus far to date, there have been no viable kittens that are homozygous for the munchkin gene (that is, kittens with two copies of the munchkin gene, one from each parent), possibly because of gene lethality. Kittens that are heterozygous for the munchkin gene (that is, a munchkin gene from one parent and a normal gene from the other) will be ‘standard’ munchkins. Because only heterozygous munchkin cats survive to pass on the gene, all litters with at least one munchkin parent have the possibility of having all munchkin kittens, all normal kittens, or a combination of munchkins and normal kittens.
There is much controversy among breeders of pedigree cats as to what genetic mutations are abnormal and potentially disadvantageous to the cat. At one extreme, some governments consider the munchkin breed to be simply “malformed animals” and the deliberate breeding of them “unacceptable” because of the “genetic health problems associated with such breeding”. But keepers and breeders of munchkins declare them to be “a sound breed” that is “ideal” for small homes and not particularly susceptible to health problems.
A litter with two munchkin parents may be all munchkin kittens, all normal kittens, all non-viable kittens with two copies of the munchkin gene, or any combination of the three. At one time it was theorized that this short legged trait was due to the same locus of genes that cause achondroplasia in humans, however all attempts to prove this to date, have failed.
It is now believed to be the equivalent of hypochondroplasia which is much milder than achondroplasia. Achondroplasia affects more than the long bones of the legs. The munchkin cat is shorter than a standard domestic, but in all other respects it is identical, genetically and in size and overall appearance.
Punnett squares, in which the M represents the dominant munchkin gene and the m represents the recessive normal gene, may be used to illustrate the chances of a particular mating resulting in a munchkin cat.
Although the genetic abnormality causing the short-legged trait in munchkin cats is often called achondroplasia, it has not yet been demonstrated that the trait is due to a gene at the same locus as causing achondroplasia in humans. Furthermore, while achondroplasia is typically associated with an enlarged head as well as short legs, a combination of features not seen in munchkin cats, the condition has sometimes been referred to as hypochondroplasia instead.
As well as shorter limbs, munchkin cats are more prone to lordosis and pectus excavatum than other cats. Small litter sizes when two munchkin cats are crossed indicate that embryos that are homozygous for the munchkin gene are non-viable. The munchkin gene is an autosomal dominant one. Homozygous embryos for the munchkin gene are not viable due to gene lethality. Only kittens that are heterozygous for the munchkin gene develop into viable munchkin kittens.
Because only heterozygous munchkin cats are able to pass on the gene, all litters with at least one munchkin parent have the possibility of containing all munchkin kittens, all normal kittens, or a combination of munchkins and normal kittens. A litter with two munchkin parents may be all munchkin kittens, all normal kittens, all non-viable kittens with two copies of the munchkin gene, or any combination of the three.
Kittens bearing two copies of the munchkin gene (MM) will not survive. Kittens bearing one munchkin gene and one normal gene (Mm) will be munchkins. Kittens bearing two normal genes (mm) will be normal. Mm munchkin kittens will be able to pass on the munchkin gene to their own offspring. Normal mm kitten will not, as it does not have that gene.