What Genetic Disorders Are Produced In Breeding? Part I
by Kitty Angell - Originally published in Cats Magazine
"Good and bad may not be dissevered. There is, as there should be, a commingling."
- Euripedes, quoted in Plutarch's "Contentment"
What are the usual elements that breeders consider when breeding for type?
When breeders endeavor to "set a look" with a particular breed, they concentrate on working with cats that possess the desirable traits they want in their line. What some do not realize, however, is that by intensifying the good traits of a cat (big eyes, heavy top-of-the-head doming, extremely fine boning. etc.) they are also doubling the bad things present in that cat's genetic makeup. And once undesirable traits have been established in a line of cats, they can be more difficult to remove than the good ones.
It is wise to study a cat's pedigree before breeding. When correctly used, a pedigree is like a road map. pointing out the notable features, both good and bad, of a cat's background. Unfortunately, many breeders use pedigrees only as a shortcut to the winner's circle.
A number of genetic disorders plague the cat fancy; some occur most often in particular breeds while others can affect cats of any breed.
What is Cryptorchidism?
Bilateral cryptorchidism is the failure of both testes to descend properly. Unilateral cryptorchidism, more commonly known as monorchidism, is the failure of only one testicle to descend. Cryptorchid males are usually infertile and therefore useless to breeding programs; monorchid males are not sterile, but they should not be used for stud service because in most cases they can pass along the genetic defect.
The defect occurs when the testes fail to pass through the inguinal canal into the scrotum, either because the inguinal canal is incorrectly formed or is too small. A developmental maladjustment can also prevent the testes from descending at the appropriate time.
The defect is an example of sex-limited inheritance: Only males can have undescended testes; females may carry- the defect but obviously cannot show it. In this respect the carrier female is as big a menace as the monorchid male. Although outwardly normal, she can potentially transmit the condition to her sons, which is one of the main reasons the defect persists.
Selective culling should be carried out immediately upon discovery of this genetic disorder. Not only should the male and his parents be removed from the breeding program, but brothers and sisters of affected cats should be carefully scrutinized as well.
Can you describe the types of tail abnormalities?
A variety of tail abnormalities occur in cats. Tails can be nodulated, kinked or bent; loss of the terminal portion of the tail is also possible, Many of these defects can be traced to injury, possibly at birth or some later stage. However, a certain defect may appear in cats of one particular strain or bloodline Then, of course, the possibility of genetic influence should be considered.
It's difficult to determine whether kinks, nodules and fused vertebrae are all varying manifestations of the same genetic entity; difference in expression may be due to strain background. The advance of the deformity could be irregular, making it difficult to accurately determine the defect. It can also make simple culling of all affected animals a more difficult task. Breeders have noticed that tail defects often skip one generation, then turn up again to haunt the next. It is best not to use cats with tail abnormalities in a breeding program.
What is Hip Dysplasia and how does it occur?
Although hip dysplasia (HD) is seen most frequently in breeds known for their massive size (Maine Coons, for example), it can occur in cats of any breed, both short- and long-bodied, male and female. The pattern of inheritance suggests that it is polygenic, meaning that it is neither a simple dominant nor a simple recessive gene that can be easily bred out.
HD is a complex disorder in which the hip joint becomes damaged, inflamed and weakened. This causes the muscles to atrophy, which precipitates the onset of secondary osteoarthritis.
Dysplastic offspring may be born to parents with normal hips, but more commonly at least one parent has HD. Unfortunately. a breeder working toward certain desirable traits may also end up with a selection of cats that are susceptible to HD. Colleen Power, in her article "Hip Dysplasia in Cats" in the June 1992 issue of Persian News, says, "There is no single gene that is responsible for hip dysplasia. Instead there is a gradual piling up of genetic factors... The process is so dangerously gradual that, until the severe limp appears, the breeders' will have no idea that they have been gradually altering the skeleton and musculature with each generation.”
The disease is progressive and often crippling. Intense activity can irritate the condition, but also makes the symptoms more evident to veterinarians. HD may be diagnosed differently in cats than in dogs, due to the dissimilarity of the hip structure and movement of dogs and cats.
How is Hip Dysplasia diagnosed and what are the indications of severity?
HD is best diagnosed through radiography of the pelvic region, a tedious procedure that requires the cat to be anesthetized. Working with veterinarians skilled in reading the subtleties of X-ray images, the Hip Dysplasia Registry of the Orthopedic Foundation for Animals at the University of Missouri in Columbia, Missouri, assigns nine variations of congruity and fit between the femoral head (top of the thigh bone) and the acetabulum (hollow area in the pelvic bone into which the thigh bone fits):
1. Excellent conformation.
2. Normal conformation, for age and breed
3. Less than ideal. but within the normal limits
4. Near normal, minor hip joint abnormalities
5. Borderline minimal dvsplastic change
6. Grade 1 dysplasia, subluxated (partially dislocated) 25 percent
7. Grade 2 dysplasia, subluxated 50 percent
8. Grade 3 dysplasia, subluxated 75 percent
9. Grade 4 dysplasia, femoral head luxated (dislocated) out of acetabulum
Only cats with hips rated I through 3 should be used for breeding.
I have vaguely heard of Patellar Luxation, but what is this condition?
Patellar luxation is a dislocation of the kneecap that manifests itself in one of two ways, both of which affect a cat's gait. Medial luxations usually cause a bowlegged appearance while lateral luxations result in a knock-kneed stance. Luxation may be intermittent and the effect on gait may not be apparent with every stride. The condition can become particularly severe by the age of 1 to 2 years.
The occurrence of patellar luxation in certain breeds suggests a genetic influence, but the mode of inheritance is unknown; there is a possibility the defect is polygenic. The Devon Rex was one of the first breeds to be diagnosed with this problem. Abyssinians and Chartreux have also had increasing incidence of dislocated kneecaps.
Clinical signs are the best indicators for diagnosing patellar luxation. The kneecap can also be palpitated or X-rayed. Radiography and arthroscopy help evaluate osteochondritic lesions and deformities of the trochlear (pulley-like) ridges.
Treatment for cats with serious congenital deformities of the trochlear ridges can be difficult, especially if the patella does not stay within the trochlear groove for a reasonable length of time following manual replacement Cats diagnosed with patellar luxation should not be used in a breeding program.
What can you tell me about Progressive Retinal Atrophy?
Progressive retinal atrophy (PRA) is often quite advanced before outward signs are apparent. Affected cats display cautious behavior and stumble into things as a result of diminished vision.
Clincal signs of PRA include bilateral dilation of the pupils, jerking movement of the eye (nystagmus). hyperreflectivity of the capetum lucidum (reflective tissue layer) and progressive diminishing of the retinal blood vessels The photoreceptor layer of the retina (containing the rods and cones) also undergoes steady degeneration.
It has been noted that some Siamese display a familial tendency for this disorder.
At present there are three forms of PRA: rt, Rdy and rdg. They are differentiated from each other according to age of onset and mode of inheritance.
Monogenic recessive (rt) PRA is clearly inherited in some Persian cats. The condition becomes apparent at around 12 to 15 weeks of age. Abnormal pupil dilation (mydriasis) is a clinical sign. Examination reveals only remnants of rods and cones and thinning of the retina’s outer layers.
PRA expressed through the dominant gene designated Rdy has been diagnosed in Abyssinians at 4 to 5 weeks. By 12 weeks the degeneration is quite advanced. In one study, a stock of affected Abyssinian cats was found to have descended from a single male. The disorder resembles the effects of taurine deficiency, although no problems with taurine homeostasis have been noted.
Some Abyssinians were also found to have a third type of retinal degeneration, expressed in the recessive gene rdg. This third form begins later in life and progresses slower than the other two forms of PRA. Most cats with rdg PRA are diagnosed between 18 and 24 months of age. The advanced stages of the disorder are not reached until affected cats are about 4 years old.
What is Osteodystrophy and how does it occur?
Osteodystrophy results in a skeleton that is seriously and cripplingly deficient in calcium. Radiography reveals distinctly rarefied (less dense) bones. Osteodystrophic cats usually resent being handled, especially when gently manipulated in the lumbar region. Affected cats become lame and, in some instances, are unable to even stand. In severe cases the hindlimbs may become paralyzed
While the disease may be congenital, there has been some discussion as to whether or not other genetic factors are involved; some researchers believe the condition may be linked to recessive genes
In 1974, Dr. Oliphant F Jackson of the Royal Free Hospital School of Medicine in London, England, conducted a study of congenital bone lesions in cats with folded ears. He warned that heritable osteodystrophy in cats was associated with the fold-ear breed and suggested their breeding be limited to only fold-car/straight-ear matings.
Patrida Turner, an English breeder, was one of the first to study heritable osteodystrophy in Scottish Folds. She felt the condition might be inherited independently, but given expression by the presence of the fold gene. Unfortunately, no further studies have been done to support her theory.
This disease is apparently the result of a diet low in calcium. Diets consisting almost exclusively of meat are particularly conducive to the onset of osteodystrophy. Familial incidence could be simulated if the same deficient diet is fed to successive litters or generations. Whether or not this is a factor is best determined by the response of the skeletal dystrophy to corrective feeding.
Genetic influence cannot be totally ruled out because certain strains or breeds have particularly high calcium requirements. This may explain the regular recurrence of the disease in the Burmese and Siamese breeds.