Genetic defects appear in all living creatures. Some genetic defects are very harmless while others are life threatening. Genetic or hereditary defects are not the same as congenital defects. Hereditary defects are passed on by the parents to the offspring. Congenital defects can be genetic, but they can also be caused by external influences. If the mother becomes ill during the pregnancy, or if she is exposed to toxic substances during the pregnancy, this can influence the growth and development of the embryo.
If a litter of kittens is born and some of the kittens have abnormalities, you can often tell by the abnormalities if it is congenital or hereditary. When the affected kittens all have the same abnormality (for instance kinked tails) then it is very likely that this is hereditary. However, if the litter has several kittens with abnormalities, but the abnormalities are all different (for instance one with a kinked tail, one with a cleft palate, and one with spina bifida) then these abnormalities are probably caused by external influences. The kittens are not at the same stage of development at the same time in the uterus. So when a pregnant female comes in contact with a toxic substance, the poison will affect every kitten in another place. This might cause different effects for each kitten.
Hereditary traits are divided in dominant and recessive genes. This means that if you receive one dominant and one recessive gene for one trait, the dominant gene will be visible and the recessive gene will be suppressed. This makes you a carrier of the trait that lies on the recessive gene.
There are sex-linked defects, and defects that are not sex-linked. The non sex-linked defects are called "autosomal", the sex-linked defects are called "X-linked". This is because sex-linked defects are located on the X-chromosome, females have two X-chromosomes while males only have one X-chromosome. When a defect is recessive and sex-linked, then males have a bigger chance as females to get the disease. Females have to have the recessive gene on both X-chromosomes to develop the defect, while males only have to have the recessive gene on one X-chromosome.
Some hereditary diseases are breed specific, others occur in more then one breed and even in more then one species. Although it is said that some breeds have a certain hereditary defect very often, this is not always true. It is possible that the disease is indeed only occurring in one or two (related) breeds, but it can also be that breeders of breed A do test for a certain disease, and breeders of breed B do not test their animals. This automatically means that affected animals are found more often in breed A as in breed B. But this does not mean that in breed B this defect does not occur, and that breed B is healthier than breed A. The breeders of breed B can never be certain whether or not their breed has the defect. After all they do not test for this defect.
Testing for defects
The testing for defects should always be performed by an experienced vet or a specialist! If you are just out for a quick test and you visit the "vet on the corner", you might as well flush your money down the toilet. These days many vets specialize in different subjects, so try to find a vet that is specialized in the defect that you want your cat to be tested for, and preferably one who has a lot of experience in this area.
When you consider buying a kitten from a breeder, it is always wise to ask for the parents' health. Did the breeder test the animals on the defects that occur in the breed? If not, why not? If yes, ask for copies of the test results! Asking for proof may sound very suspicious, but unfortunately some breeders will not shrink back from lying about testing, or about the test results. A breeder who has nothing to hide will be happy to show you his test results.
In the future more and more genetic test will be available, the genetic marker of some defects/illnesses has been found already, for others defects scientists are still searching for the marker.