By Sarah Hartwell
When cats are bred for appearance, disease-causing genes risk being overlooked until the gene becomes sufficiently widespread that numerous cases of the genetic ailment appear in that breed. There are a number of factors involved:
THE FOUNDER EFFECT: if one or more of the foundation cats has a genetic problem, this is inherited by a large proportion of descendents (unless breeders screen carriers to select against it). This has happened in the Singapura which has inherited pyruvate kinase deficiency from its Abyssinian ancestor.
GENE POOL SIZE: if the gene pool because highly inbred, problem genes can become widespread. Recessive (hidden) genes can double up in later generations and be expressed. New blood may invigorate the gene pool, but the recessive gene will still be there and may still manifest if closely related cats are bred together.
Some genes (or combination of genes) directly cause a condition while other genes give a breed a predisposition to developing a condition, sometimes as a side-effect of a different mutation. The Manx gene causes neural tube defects which affects the lower spine. The Devon Rex gene gives the breed a predisposition to contracting yeast infections of the skin. Genetic anomalies (i.e. anything that deviates from the normal wild-type) are divided into 3 main categories:
LETHAL GENES mean kittens that inherit the mutation die in utero or shortly after birth and often have major birth defects. A deferred lethal gene requires the kitten to inherit 2 copies of the gene for the effect to be lethal. The Manx mutation is a deferred lethal gene: kittens that inherit 1 copy of the gene are tailless or short-tailed, while those that inherit 2 copies die in the womb.
An IMPAIRING GENE causes a defect that requires long-term medication or surgery to control the ill-effects of the gene. Cleft palate is an example.
A COSMETIC MUTATION varies from having no medical or physical ill-effect to one that causes some lifestyle limitations e.g. hairlessness.
Genetic anomalies are noted more often in pedigree cats than random-bred cats for several reasons. Having a pedigree makes it easier to identify family links and inherited traits. Some breeds are relatively inbred so that recessive genes start to double up. When cats are bred for consistent appearance, any deviation is more easily noticed. Then there is the human factor: responsible pedigree breeders want to eliminate damaging genetic conditions and are more likely to take notice of multiple instances of a particular anomaly or condition.
Free-roaming random-bred cats aren’t free from inherited problems; but the wider gene pool means that they are at less risk of inheriting two copies of a mutated gene. An exception to this are feral colonies that become inbred.
Some breeds are based on mutations that have damaging side effects. The Manx gene is related to spinal nerve defects. The Scottish Fold gene has the side-effect of causing joint and cartilage problems (if 2 copies are inherited). The Ojos Azules gene disrupts embryo development. In Persian cats, there are mulitple genes that cause structural changes to the head. Over the decades, breeders have selected for a more extreme look, resulting in side effects such as non-draining tear-ducts, misaligned jaws, obstructed airways and a predisposition to cleft palate.
Using data from International Cat Care (Feline Advisory Bureau), Robinson’s Genetics and personal correspondence, I created a graph of breeds known to have one or more genetic issues. I’ve grouped closely related breeds together as they share many of the same genes. Details and descriptions of the conditions are listed alphabetically at www.messybeast.com/gene-anomalies.htm
Unsurprisingly, the cats that top the list are three of the oldest cat fancy breeds: the Siamese (including the Oriental Shorthair) with 42 genetic conditions/predispositions noted and the Persian (including the Colourpoint Persian/Himalayan) with 34. Both of these have been bred for extremes of conformation – extremely skinny and long-nosed for the Siamese, and extremely cobby and short-nosed for the Persian. Third on the list is the Abyssinian (including its longhaired version, the Somali) with 16 noted genetic conditions or predispositions. The Abyssinian group has only half of the conditions noted in the Siamese group; this reflects the genetic issues that can result when breeding cats for extreme conformation.
These three breeds were all developed from a limited number of imported cats. They have been “refined” based on their appearance since the late 19th century and it is only in recent decades that genetic health has become a major concern. The modern Siamese owes its extreme look to a cat called Wankee who appears in most pedigrees many generations back.
It’s very important to note that these breeds also have many healthy individuals. The statistics, based on data from around the world, reflect the overall health of the breed and not the health of any single individual. Many of the conditions can be detected using genetic screen so that cats carrying the gene aren’t bred from.
If those are (statistically speaking) the least genetically healthy breeds, what are the healthiest? A lot of breeds don’t appear on the graph because there isn’t enough data collected. Some breeds are too young to have generated any statistics. Some breeds don’t have a high number of ailments, but they show inbreeding depression (small litters, poor fertility, poor immune systems). The Singapura is so inbred that it risks becoming extinct unless it gets fresh genetic input.
Healthy breeds will be those derived from wide gene pools and where the foundation cats haven’t brought any nasty genetic baggage with them. They are likely to be closer to the “natural type” for the region where they have developed naturally before coming to the notice of cat fanciers and they won’t have been crossed to breeds on that graph. It’s impossible to give a definitive answer because lack of data doesn’t mean lack of genetic problems, but healthier breeds could well be the Siberian, European Shorthair and the genuine Turkish Angora*/Van (as they exist in Turkey today). The Sokoke is reputed to be genetically healthy, but this breed has a small gene pool and conditions caused by recessive genes may appear in later generations.
*the American Angora was recreated using Persians and may have predispositions to conditions found in American lines of Persian.
About the author: Sarah is the creator of Messybeast.com. She kindly agreed to write this article for PoC on the basis that I (MIchael) made a donation to Cats Protection Chelmsford.
Associated post on cat genetic diseases (by Michael)