BACK                                            MUNCHKIN FACTS AND DWARFISM EXPLAINED 

(Heed is listed on the world record listing below!)

 DWARF GENETICS EXPLAINED

Genetic dwarfism usually involves a bone disorder where the bones do not grow and develop normally. These are called skeletal dysplasias or chondrodystrophies. In humans, there are over 200 conditions causing stunted growth (possibly as many as 400 conditions) and over 100 known skeletal dysplasias; of which achondroplasia is the most commonly seen. Achondroplasia dwarfism is also found in cats. It is likely that other dysplasias also occur in cats, having subtly different dwarfing effects.

Achondroplasia Dwarfism (True Dwarfism, Genetic Dwarfism, Chondrodysplasia)

The most common (and easily recognisable) form is achondroplasia dwarfism which shortens the long bones of the limbs while leaving the trunk (body) and head unchanged.. Achondroplasia dwarfism is characterised by abnormal body proportions. Achondroplasia dwarfism is the result of a dominant genetic trait affecting the hormones which control bone growth. Typically, the growth of the limbs is stunted, while the size of the trunk and mental capacity are normal.

The condition causes abnormally short and deformed limbs; this is most noticeable in short-legged dog breeds where the limbs are bowed or twisted. It also typically produces a large head with undershot (bulldog) jaw and crowded, misaligned teeth. Other cranial problems may occur due to the abnormal head shape. The limbs are frequently bowed which may result in poorly articulating joints. The vertebrae may also be affected. Although most affected cats are mentally normal, their abnormal body proportions may result in slow development in early kittenhood. The large head may result in kittens being delivered by caesarian section as they are unable to pass through the birth canal. Stuck kittens are likely to die and would likely cause the mother to die.

In the Munchkin; these short limbs are the distinguishing feature of the breed and the other deformities associated with achondroplasia are avoided (as far as possible) by careful selective breeding, avoiding breeding those individuals which have spine or chest deformities. Although achondroplasia is typically associated with a large or abnormal head, Munchkins do not seem to suffer this effect and therefore may have a different condition with similar physical effects rather than true achondroplasia.

Achondroplasia also occurs at random in the feline population due to mutation hence the occasional report of short-legged cats (Kangaroo Cats and Squittens Revealed). It occurs in varying degrees, ranging from "nearly normal" to crippling with all legs severely deformed. The forelegs are usually more severely affected than the hind legs. In animals this form of dwarfism ranges from mildly disabling to crippling or lethal (stillborn). In Munchkins, homozygous embryos apparently die early in the pregnancy and are reabsorbed, decreasing the litter size compared to litter size in most other breeds.

Other Dysplasias

There are probably other dwarf conditions in cats, although they may not have not been identified as distinct from the Munchkin mutation. It is likely that many, or most, are reabsorbed during pregnancy, are stillborn or die in kittenhood before any dwarf mutation is detected. The following are examples of dysplasias seen in other species.

In Dexter cattle there is a form of dwarfism which causes short-legged individuals. Occasionally "bulldog calves" are born. Bulldog calves are usually stillborn or die soon after birth. They are dwarfed with large heads, cranial deformities and other skeletal abnormalities. There may be gross deformities of the skull.

A hereditary recessive form of achondroplasia is found in some sheep breeds. Known as "Spider Syndrome", it is related to abnormal transformation of cartilage to bone. Normally, as an animal grows, the cartilage at the end of the long bones grows and expands and is transformed into bone (calcified). In Spider Syndrome sheep, some cartilage is transformed to bone, but adjacent areas of cartilage are not. This causes skeletal abnormalities, particularly the splay-leggedness which gives the condition its name. The portion of the leg below the knee/hock protrudes outward at 30-45 degrees. There is usually S-shaped curvature to the spine, a pronounced humped back over the rib cage and the whole muzzle curves to one side. Spider Syndrome lambs often have long necks and legs and a shallow body.

Midgetism (Pituitary Dwarfism, Hyposomatotrophism, Growth Hormone Deficiency; Panhypopituitarism)

Midgetism is quite different from achondroplasia dwarfism and it is caused by malfunctioning pituitary gland not producing enough growth hormone (hypopituitary dwarfism) before sexual maturity. In a few cases, the hormone is produced but the body fails to respond to it. This can have a knock-on effect on other glands which affect growth and development. The pituitary gland is the body's "master gland" and if it malfunctions, it can result in other metabolic disorders such as hypothyroidism, hypoadrenocorticism and diabetes.

Midget cats stop growing during kittenhood and, depending on which hormones are and are not being produced, may retain some juvenile characteristics such as soft (kitten-like) fur, kitten-like proportions (though the legs may become deformed due to abnormal bone growth) and a high-pitched mew. Midgetism produces a small individual with normal or near-normal proportions except for the head, which may appear relatively large when compared to the body. The relationship of head to body proportions approximates that of normal kittens. The lack of growth hormone during kittenhood means that bone sutures (the "cracks" which allow growth of some bones) fuse too early preventing further growth. Typically, a pituitary dwarf stops growing in kittenhood but retains normal body proportion and mental capacity.

According to some texts, pituitary dwarfs have normal sexual development and are capable of reproduction although they may be unable to carry a normal-sized foetus to full term. Other texts state that pituitary dwarfs are often sexually immature and cannot reproduce. Some texts contain contradictory information on the matter of sexual maturity and reproduction in midgets. Those which do not become sexually mature probably have panhypopituitarism, where not only growth hormone but also other pituitary hormones are deficient.

In humans, midgetism is caused by a dominant gene. In German Shepherd dogs it is a recessive trait. Although cats with features indicative of midgetism have been reported, little is known about the genetics. In addition, damage to the pituitary gland during early life may also cause non-hereditary midgetism.

While working at a cat shelter in Essex . England in the 1990s, I came across two dwarfed cats, probably pituitary dwarfs, where the cat was "compact" with only slightly shortened legs and a short body. The size was approximately that of a 4 month old kitten. Overall, both cats were thick-set and the head was slightly large and best described as "rounded". One cat was a pregnant farm cat,the other was a pet. Because of the risk of a large (i.e. normal sized) kitten becoming wedged in the birth canal the farm cat was aborted and spayed. The second such cat was a bad-tempered spayed female pet who was also tailless. Both cats were alert, active and mentally normal. The farm cat was extremely sweet-natured while the pet cat was a bad-tempered individual.

Cretinism (hypothyroid dwarfism)

Cretinism is a type of dwarfism accompanied by mental retardation and physical distortion and is caused by insufficient of thyroid hormone in early life. It is usually due to birth defect (missing, malformed on non-functioning thyroid gland) or by early damage to the thyroid gland. Cretinism seriously retards an affected kitten's physical and mental development, leading to stunted growth, poor skin tone, misshapen (flattened) muzzle, pot belly and general physical and mental lethargy. Cretinism is well documented in humans, and its treatment with thyroxine, is well-documented. It has been artificially induced in laboratory animals and may occur naturally although individuals are unlikely to survive.

Congenital hypothyroidism has been seen in Abyssinian cats; the kittens grew more slowly and remained small with kitten-like features and the typical goitre.

Miniaturisation

Small size can also be an inherited characteristic of a breed e.g. Singapura is not dwarfism or midgetism since the individuals in such groups are physiologically normal. In cat shelter work I have come across a number of adult cats which appear to be simply "small" and not actually dwarfs. Miniature cats are small, but normally proportioned, cats.

Miniaturisation by progressive downsizing is neither dwarfism nor midgetism, but is the result of always breeding from the smallest individuals in a population. In most animals there is a natural variation in size - a few unusually large individuals, a few undersized ones and the majority being "normal size". Some simply turn out smaller than other due to the interaction of various genes. By breeding only from the smallest individuals, it is possible to create a population of undersized animals. In nature, this occurs naturally where the environment is such that the smaller animals have an advantage over the larger ones e.g. not requiring as much food. Over many generations they become progressively smaller until eventually a minimum size is reached; any smaller and the offspring are too fragile to survive. Shetland ponies and Falabellas are examples of progressive miniaturisation.

Miniaturisation can also occur spontaneously in a single generation by mutation of genes which govern size. The gene most often implicated in normally formed, but miniature, cats is one which affects the asp count on the asp gene. Such a mutation could halve or double the body mass of an animal by changing the asp count by one unit. It does not appear to affect body proportions or general health. The brains and other organs of asp-miniature cats contain approximately half the number of cells compared to the same organs in normal sized cats because in a normal sized cat the asp count is one unit larger i.e. there has been one additional cellular division cycle. One would expect asp-miniatures to weigh half as much as a normal sized cat, however natural variation (governed by numerous other genes [polygenes, modifiers] and also by environment) means they are likely to range from one-third to two-thirds the size of a normal cat. The cases of "Treker" (Persian) and "Cotie" and "Scamp" are consistent with this type of mutation.

Stunted Growth

Stunted growth can also be caused by non-inherited factors such as environmental factors such as poor nutrition during the main growth stage of the animal. Although the animal carries all the genes for normal size, the environment prevents it from reaching that size. Metabolic disorders and some congenital defects (birth defects) can cause the individual to simply stop growing. Chronic illness or acute illness in kittenhood can both result in stunted growth. Another stunting condition is porto-systemic shunt where an aberrant arrangement of blood vessels causes the liver to be bypassed. In some cases this is operable, but in others it is managed using medication (though life expectancy is reduced). The smallest kitten in a large litter has to compete for food against stronger siblings and may not get enough food to grow fully.

Other Forms of Dwarfism

The appendix at the end of this article lists a number of conditions known to cause dwarfism in humans and which are potential causes of dwarfism in cats.

MINIATURE AND MIDGET CATS RECORDED IN HISTORY

In 1830, the German explorer Rengger described a South American Cat which was "smaller by a fourth" than the common domestic cat. This indicates a cat 75% of usual size - not particularly small. In 1890, Charles Darwin gave a second hand account of the "Paraguay Cat" described by Rengger: "The domestic cat, which has been bred for 300 years in Paraguay , presents a striking difference from the European cat; it is smaller by a fourth."

In 1896, Richard Lydekker wrote of a midget cat found in South America which was "one fourth the size of the ordinary domestic breed and is characterised by its elongated body, and its covering of short, shiny and close-lying hair, more especially on the tail." He called it the Paraguay Cat and described it in his book "Handbook of the Carnivora". Although many dismiss this as a figment of Lydekker's imagination (or poor copying of "smaller by a fourth" into "one fourth the size"), the modern occurrence of miniature cats down to only 3 lbs mature weight, suggests that there may indeed have been a localised mutation of domestic cats.

In Frances Simpson's "The Book of the Cat" (1903), contributor H C Brooke wrote "We have seen specimens of a very tiny domestic cat, full-grown individuals of which weigh only about three pounds. Those we saw came from South America ." In 1973, the Cats Protection League's Haslemere Shelter reported the case of "Little Purrer" a dwarf kitten that had come into their care and was doing well. Little Purrer was reported in the Branch news of "The Cat" but there are no details of his final size or weight.

In 1955, Albert C Jude reported the occurrence of miniature cats in a breeding line of British Shorthairs. While his book "Cat Genetics" was in preparation, an interesting case of dwarf or miniature cats came to his attention though there was not enough time to establish any genetic facts. He added that some combined problems, which he did not elaborate upon, were involved. A sadly indistinct photographic illustration showed the relative size of one of the miniatures as being two thirds the size of the well-known black Shorthair Ch. "Bourneside Inky Bits." The miniatures were being bred by Mrs B Farquhar of Coventry , England . A somewhat smaller than normal Black Shorthair mated with an unknown male resulting in 2 Black Shorthair kittens: "Peterpan Black Jade," and "Peterpan Black Angus Gohome.". As adults, the two offspring were also smaller than normal, but not miniatures. Jade and Angus were mated together 3 times (brother-sister inbreeding) and each succeeding litter contained kittens smaller than those of the previous litter. One of the offspring was mated back to its sire (father-daughter inbreeding) resulting in smaller kittens. Such inbreeding brings recessive genes to the fore. Jude omitted to note that there is no genetic reason for each succeeding litter to contain smaller kittens.

Jude wrote of the kittens, litter sizes and maturation rates: "The smallest kittens so far are about equal in size to an average six-months-old Siamese. These miniatures are slow to mature and are not ready for breeding until about two years old. Litter number is small, which is in accordance with the general expectation for smaller breeds of particular animals. For instance, in dogs the average litter size for bloodhounds is 10.06, whereas for the toy breeds it is 4.24 (Heape 1908). It is interesting to note that "Angus" mated with normal-sized queens, has produced only normal-sized kittens, This may be significant. A further matter of interest is that in this household a Siamese queen contracted show fever and was nearly lost. She passed this on to all other cats on the premises in varying degrees, but none of the "miniature" breed was affected, although these were in contact throughout."

The following recent cases were all loosely termed "dwarf" cats when reported. The descriptions generally suggest pituitary dwarfism (midgetism) or miniaturisation not achondroplasia dwarfism so I have loosely classified them as midgets.

Some genuine feline dwarfs or midgets have weighed under 4 lb (under 2 kg) when fully-grown. A tiny female shorthair called "Kitu" owned by Wanda Mitka of Laverton , Victoria , Australia was examined at the University of Melbourne School of Veterinary Science in 1979. Kitu was 7.87 in (20 cm) tall at shoulder, but perfectly formed and in perfect health. Her diminutive stature was due either to a genetic mutation or a congenital problem (birth defect) causing her to simply stop growing.

Even smaller than Kitu, was a male Siamese/Manx cross called "Ebony-Eb-Honey-Cat" owned by Angelina Johnston of Boise , Idaho , USA . In 1984, aged 4, he weighed only 1 lb 12 oz (792 g) but ate as much as a 12 lb (approx 5.5 kg) cat in the same household.

The smallest dwarf cats on recent record were "Pete" (b. 1973) and an un-named female (report dated 1983). Pete was described as a "boisterous" dwarf male cat (b 1973) owned by Jackie Reed of Bognor Regis , Sussex , England weighed only 12 oz (339 g) at the age of 3 months compared to 3.4 lb (1.6 kg) for his normal-sized littermates. Pete's weight never exceeded 2 lb (907 g) when he was full grown. A 2 year old dwarf female owned by Mrs M Wildgoose of Gillingham , Kent , England weighed only 2 lb 15 oz (1.3 kg) in 1983.

More recently, the world's smallest cat was reported to be a male blue point Himalayan (Colourpoint Persian) named "Tinker Toy" owned by Katrina and Scott Forbes of Illinois . Tinker Toy stood 7 cm (2.75 inches) at the shoulder and 19 cm (7.25 inches) long - presumably excluding tail. He weighed 1 lb 8 oz, lighter than Pete at maturity.

"Smallest Living Domestic Cat" World Records  

One contender (claimed by some cat magazines to be the actual record holder) was Mr Peebles, from Pekin, Illinois, USA, weighs only 3.3 lbs (1.5 kg) at 2 years old (normal weight for a 2 year old cat is 8 - 10 lbs/3.6 - 4.5 kg). Mr Peebles is now full grown and his small size is due to a genetic mutation. He eats 4 meals a day to sustain his weight, squeaks rather than meows and, because of his small size, he feels the cold. He is co-owned by Robin Svendson and Dr Donna Sassman of the Good Shepherd Veterinary Clinic in Pekin . He was found when Sassman did a house call to a farm in Tazewell County ; his mother and several siblings were all of normal stature. Sassman was told she could have the runt of the barn cat litter if she could catch him! During 2004, clinic manager Cathy Smith, researched the Guinness World Record website for records of small cats. After providing photographic evidence and independently verified measurements, Mr Peebles' vital statistics were confirmed as 6.1 inches (15.5 cm) tall and 19.2 inches (48.76 cm) from his head to the tip of his tail (his tail is just over 7 inches /17.8 cm). His head and eyes are disproportionately large; a spinal defect means he cannot arch his back like normal cats (his back is very stiff, but apparently not fused though there is a risk of arthritis) and his spinal problems make him waddle like an alligator when walking, but he is otherwise healthy. 

In March 2006, a 9-week-old kitten from Potrero, California was expected to take the record for the smallest cat in the world. At 16 weeks, "Heed" was 3-inches tall and weighed 2 pounds - . Heed will not reach adult size until 6-8 months, but was expected to take the record for smallest cat in the world. His owner FLYING PAINT RANCH has already contacted the Guinness Book Of World Records.

MODERN DWARF BREEDS

The Munchkin's short legs are due to dominant gene achondroplasia (or, more likely, one of the mimic conditions pseudochodroplasia or hypochondroplasia) which stunts the growth of the long bones and causes them to bow slightly; they are therefore normal-sized cats with short legs. There are two abnormalities associated with the condition. One is lordosis which causes a dip in the spine behind the shoulders and compression of the chest, the other is pectus which affects the chest. There is disagreement over how prevalent these traits are within the breed. Munchkin litters are often smaller than average cat litters, suggesting that some embryos die early and are reabsorbed by the mother (probably where the embryo inherits two copies of the gene). In Munchkins, the skull, spine and pelvis do not seem to be affected and, providing breeders are careful to avoid breeding from cats with lordosis or pectus, hence this form of dwarfism is considered cosmetic.

Once the gene for short legs has appeared, it is possible to introduce it into other breeding programs, for example the Minskin which combines the Munchkin's short legs with the Canadian Sphynx's hairlessness. The Minskin is neither a short-legged Sphynx nor a hairless Munchkin, but has its own unique look. Other breeds derived from Munchkin crosses include the curly-coated Skookum (Munchkin/LaPerm) and the Napoleon (Munchkin/Persian). For more information on the Munchkin, Minskin and other short-legged breeds please see Short-Legged Cats.

The condition found in the Munchkin has been reported in various locations throughout history and in slightly varying forms. In some, only the forelegs are affected (foreleg micromelia) while modern Munchkins have forelegs and hindlegs equally affected (foreleg and hindleg micromelia). Cases of Munchkin-type cats have been reported in England in the 1930s and again in England in 1944, this time with 4 generations of cats being documented with foreleg micromelia. A short-legged condition was described by Schwangart and Grau (1931) and noted to be hereditary. Another was reported in Stalingrad , Russia in 1953. The modern Munchkin, with all four legs shortened, was discovered in Louisiana , USA in 1983. Since then, awareness of the condition has grown and similar, unrelated cats have been reported throughout the USA and other countries, occurring as spontaneous mutations. In 1993, I encountered shortened forelegs in a Mexican stray cat imported into Britain by a couple who took pity on his deformity. See Kangaroo Cats and Squittens Revealed for more detailed information. 

Miniaturisation is self limiting as there is a threshold weight below which offspring are unlikely to survive (as seen in the miniature Siamese) and, in some lines at least, there may also be problems with miscarriage and ability to produce milk.

Breeders can be disappointed to find that some apparently miniature cats simply do not produce miniature offspring. This may be because genetic dwarfs also carry the gene for normal size and all the offspring inherit those genes. Where the cats consistently produce normal-size offspring, it is most likely because the small stature is due to non-hereditary causes such as environmental effects or a birth defect. In other cases, the miniature stature is not always apparent from birth - normal size kittens may grow into full size cats or may grow at a reduced rate and become miniature adults.

Why is all this important?

Breeders must know what sort of mutation they are dealing with. To perpetuate any breed, female cats are required and dwarf females can have problems giving birth to normal sized kittens. If the condition is due to recessive genes then two related dwarf cats will breed true for dwarfism. If it is due to a dominant gene, then a dwarf mother cat can have normal sized kittens and this can be dangerous for her. If two unrelated dwarf cats are mated, it may turn out that they have different genes causing the dwarfism.

When a dwarf female has normal sized kittens, those kittens may be too large to pass through her birth canal. Her body may be too small to accommodate non-dwarf kittens so they may be miscarried before they are able to survive. A dwarf mother may be unable to produce enough milk to feed rapidly growing non-dwarf kittens. If the kittens are born small, they will be more fragile than normal sized kittens and their survival chances may be reduced. In addition to hazards associated with being small, some genes also cause variable side-effects which be disabling or deadly.

In owned cats, a vet can deliver kittens by caesarian and an owner can help feed them using kitten formula milk. In stray or feral cats there is a high probability that the mother, or the kittens, or both will die - this goes some way to explain the rarity of dwarfism. If the mother produces normal sized kittens, a breeder must be very careful about mating her to other cats in order to avoid this hazard. In the wild, dwarf cats may be at a disadvantage when competing against larger, more robust, cats for food or mates so the trait might not get passed along.

If the dwarfism is due to a dominant gene, then it is possible to breed from dwarf males, but only using normal sized females. This way, only some of the kittens will inherit the dwarfism trait (unless the male has two copies of the dominant gene in which case all of his offspring will be dwarfs). This will mean that the breed increases slowly because only some of the offspring can be used in breeding programs. At least it would not expose dwarf female cats to unnecessary risk; they could be spayed as pets. There will always be the danger that the new owner doesn't understand the risks and will try to breed from the female with possibly deadly results for her.

CONCLUSION

In general, there is inadequate research into feline dwarfism. There are probably many more forms of dwarfism than are formally recognized, visual impression alone being insufficient to identify which form the cat has. As far as vets are concerned, many dwarf cats present with no health problems while others have problems ranging from mild to severe. At present, there is a great deal of guesswork involved in dealing with dwarf cats and each must be investigated as an individual. Only when the different dysplasias can be distinguished will it be possible to say which conditions (affecting skeleton and/or internal organs) are definitively linked to the cat's dwarfism.

When dwarf cats are encountered, X-rays, body scans and blood tests can be used to look for skeletal effects, internal abnormalities and abnormal hormone levels respectively. This can identify why a cat is undersized. Identification of the underlying cause is not always possible since some causes do not leave easily detectable physical traces; some individuals are small due to natural variation and there are also a multitude of genetic mutations which have yet to be encountered or identified!

APPENDIX 1: SIMPLE MENDELIAN INHERITANCE

How is dwarfism inherited? In the simple Mendelian inheritance formulae below D = dominant Dwarf and d = recessive normal size. Each cat carries 2 genes and it is assumed that the trait is not linked to gender.

DD x DD = DD (all offspring are dwarf)
Dd x DD = DD and Dd mix (all are dwarf, but some have a hidden gene for normal size)
Dd x Dd = DD, Dd and dd (DD are dwarf, Dd are dwarf but carrying a recessive gene, dd are normal size)
dd x dd = dd (all normal size)
dd x Dd = Dd and dd (Dd are dwarf but carrying a recessive gene, dd are normal size)
dd x DD = Dd (all are dwarf, all have a hidden gene for normal size)

To see how recessive dwarfism/miniatures happen, simply reverse the meanings of D and d:

DD x DD = DD (all offspring are normal size)
Dd x DD = DD and Dd mix (all are normal size, but some have a hidden gene for dwarf)
Dd x Dd = DD, Dd and dd (DD are normal size, Dd are normal size but carrying a recessive gene, dd are dwarf)
dd x dd = dd (all dwarf)
dd x Dd = Dd and dd (Dd are normal-size but carrying a recessive gene, dd are dwarf)
dd x DD = Dd (all are normal size, all have a hidden gene for dwarf)

The tables below show the likely ratios of dwarf/normal-size/carrier.

APPENDIX 2: OTHER DWARFING CONDITIONS

Many conditions have multiple names while some genetically different conditions have a similar appearance and may be mistaken for each other. Some disorders have a range of effects and are not primarily associated with dwarfism. This list is based mainly on human data and only a subset of these conditions have been identified in cats. There is the possibility that others do occur but are not identified because the affected kittens may be reabsorbed during pregnancy, be stillborn or die early (and the bodies may be eaten by the mother). Such conditions are generally only noted in pedigree and pure-bred cats and for two reasons: firstly inbreeding causes normally hidden recessive genes to be exhibited and secondly (and perhaps most important) breeders are far more likely to investigate abnormal kittens and to ask for an autopsy on kittens which die early.

Achondroplasia: This is the classical form of dwarfism and, in humans, the most common form. A number of other genetic mutations produce almost identical effects e.g. hypochondroplasia, pseudoachondroplasia. True achondroplasia results in an enlarged head; pseudoachondroplasia does not. Although the Munchkin's condition is termed achondroplasia, the head size is normal and it may ultimately prove to be a different mutation. The occurrence dwarf cats with cranial abnormalities also suggest that different genes are at work. Dwarfism is not common enough in cats for a distinction to be made between these conditions.

Acrodysplasia with exostoses (acrodysplasia type V, trichorhinophalangeal dysplasia type II, trichorhinophalangeal syndrome type II, Ttichorhinophalangeal syndrome with exostoses, Langer-Giedion syndrome, Giedion-Langer syndrome): Genetic condition characterised by distinctive facial and cranial features. In humans these include mild/moderate dwarfism, large protruding ears, a bulbous nose, sparse hair, mild microcephaly (small-headedness) and often with mental retardation.

Acromesomelia (acromesomelic dysplasia): Characterized by short arms, legs, and fingers, and a slightly enlarged head.

Bulldog Syndrome: Noted in Dexter Cattle and always lethal. Affected offspring have gross cranial deformities and other skeletal abnormalities. Individuals are either stillborn or die soon after birth.

Chondroectodermal dysplasia (Mesoectodermal dysplasia, Ellis-van Creveld syndrome): A short-limb dwarfism associated with polydactyly, abnormal development of nails and often with congenital heart defects. Probably recessive as it is prevalent in inbred populations.

Congenital adrenal hyperplasia: A wide-ranging endocrine condition with serious medical problems. Afflicted individuals may grow faster initially, but will become smaller adults.

DeMorsier's syndrome (septo optic dysplasia, optic-nerve hypoplasia): A dwarfing condition and pituitary-gland disorder that also affects vision and brain function.

Diastrophic dysplasia (DD): This was once thought to be a form of achondroplasia. It is a form of short-limbed dwarfism accompanied by serious orthopaedic problems. In humans it may be associated with cleft palate, clubfeet, deformed hands, cauliflower ears and respiratory problems in early life. Kittens with cleft palates are unable to suckle and generally die, so this condition is unlikely to be diagnosed in cats.

Flat-Chested Kitten Syndrome: This syndrome is typified by a flat chest or compressed chest which, in severe cases, causes breathing difficulties and displacement of internal organs, ultimately leading to poor growth and death. Mildly afflicted individuals recover fully.

Focal mucopolysaccharidosis (acromicric dysplasia, geleophysic dysplasia, geleophysic dwarfism): Characterised by short limbs, small hands and feet and (in humans) a genial facial appearance. Associated with heart/breathing problems, overall delayed development and ear infections. Two types of mucopolysaccharidosis are seen in cats - these are described later.

Hypochondrogenesis: Characterised by short body, short limbs, large head where the jaw may be small and the forehead enlarged (in humans the face appears oval or egg-shaped). Associated with cleft palate and severe respiratory distress due to the small rib cage. Most affected individuals die soon after birth; survivors are generally found to have spondyloepiphyseal dysplasia congenita which has a similar appearance. In kittens, cleft palate would prevent suckling and lead to death, so this condition is unlikely to be diagnosed.

Hypochondroplasia: Visually this looks like milder form of achondroplasia, but genetically it is different. The effects are less pronounced than achondroplasia and there are fewer associated health problems. In species where dwarfism is poorly documented, it may be misclassified as achondroplasia.

Hypothyroid: A lack of thyroid hormone results in poor growth and dwarfed cats which retain kitten-like features.

Jeune's syndrome (asphyxiating thoracic dystrophy): Potentially lethal form of dwarfism characterised by respiratory and kidney complications. In kittens this would probably prove lethal and therefore not be identified.

McCune-Albright syndrome: Precocious-puberty condition (affects mostly females) typified by bone, endocrine, and skin abnormalities. Varies from lethal, through severe to benign and almost entirely asymptomatic.

Metaphyseal chondrodysplasia, McKusick-type (cartilage-hair hypoplasia, CHH): A form of short-limb dwarfism associated with fine, sparse hair, impaired immunity and anaemia.

Metatropic dysplasia: Similar to, and often confused with, Kniest syndrome.

Metatropic dysplasia type II (Kniest syndrome, Kniest dysplasia): Similar to spondyloepiphyseal dysplasia congenita (SEDc) and spondyloepimetaphyseal dysplasia, Strudwick (SEMD).

Morquio-Brailsford syndrome (Morquio syndrome, MPS-IV, mucopolysaccharidosis): A mucopolysaccharidosis (MPS) disease, caused by the body's inability to produce certain enzymes. In cats, MPS-I and MPS-6 have been recorded, both causing facial deformities (short broad nose with depressed nasal bridge) and spinal deformities

Multiple epiphyseal dysplasia (MED): Not usually noticed at birth, but in juveniles it causes unexplained pain in the hips, knees, and/or ankles and mild dwarfism.

Neuroaxonal dystrophy: A lethal disorder which causes degeneration of main neurons in the brain and progressive incoordination; it also reduces the body size but the kittens do not survive into adulthood.

Osteodysplastic primordial dwarfism (primordial dwarfism): Causes extremely short stature and, in humans, bird-like facial features.

Osteogenesis imperfecta (OI): A brittle bone disease. In humans, there are four types of OI; Type III being a dwarfing condition and also associated with hearing loss.

Pituitary dwarfism (growth-hormone deficiency, hypopituitary dwarfism, hypopituitarism, pituitary dwarfism, panhypopituitarism): Body proportions are preserved resulting in a midget individual. Panhypopituitarism means absence of all pituitary hormones; in this case, sexual development does not occur, juvenile features are retained and life expectancy is reduced. Deficiency of only the growth hormone prevents growth, but the affected individual matures sexually as normal.

Precocious puberty: Early growth is often faster than normal, sexual development occurs early, after which growth slows so that the adult size may be small.

Progeria (Huntington-Gilford progeria syndrome (Huntington-Gilford syndrome): A rare genetic disease that causes ageing at several times the normal rate. One of its effects is profound short stature.

Pseudoachondroplasia: Mimics achondroplasia in some ways, but is a different condition entirely. A short-limb dwarfism, but in this case, the head size and facial features are unaffected. It is associated with osteoarthritis and other orthopaedic problems.

Rhizomelic chondrodysplasia punctata (RCP): A very severe form of dwarfism with profound delays in developmental and physical growth.

Russell-Silver syndrome (Silver-Russell syndrome, Russell syndrome, Silver syndrome): Affected individuals are of low birth-weight and may have a poor appetite. The condition causes small stature and (in humans) a small, triangular face, low-set ears and incurved fifth fingers. Low birth-weight kittens with poor appetite would be unlikely to survive so the condition might go unnoticed should it occur in cats.

Spondyloepimetaphyseal dysplasia, Strudwick (SEMD , SMD): This is genetically different from spondyloepiphyseal dysplasia congenita (below) but has indistinguishable characteristics.

Spondyloepiphyseal dysplasia congenita (SEDc, SED): This causes in moderate to severe small stature and is associated with clubfeet, a cleft palate, barrel-chest and other orthopaedic problems. It is considered one of the most common forms of dwarfism in humans. Cleft palate would prevent kittens from suckling.

Spondyloepiphyseal dysplasia tarda (SEDT, X-linked spondyloepiphyseal dysplasia tarda, X-SEDT): A sex-linked variant of SED carried on the X chromosome, which means it affects males (who only have one X chromosome) more often than it affects females. Females must inherit two copies of the gene in order to be affected. Mild dwarfism.

Spider Syndrome: Causes irregular calcification of cartilage so that legs become distorted at the knee/hock joints and the affected individual develops a splay-legged stance. In addition the spine is affected, causing hunchback and a long neck. Noted in some sheep breeds.

Thanatophoric dwarfism: A lethal form of dwarfism that results in death at birth or shortly thereafter..