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An Urban Experience
The expression of some genetic disorders cannot be altered. If a genetic test is available, it should be utilized pre-purchase so that owners are not burdened with predictable genetic disease. However, for owned animals it is a personal decision whether the owner wants to know if their pet has liability of developing a non-treatable genetic condition later in life. These include polycystic kidney disease (PKD) in Persian, Himalayan and related cats, hypertrophic cardiomyopathy (HCM) in Maine
Coon and Ragdoll cats, dilated cardiomyopathy (DCM) in Doberman Pinschers, and the (poorly penetrant) liability gene for degenerative myelopathy (DM). These tests could also be used to rule out diagnoses in clinical patients with suspected genetic disease.
Dietary recommendations should be offered for identified genetic predispositions such as; FUS/inflammatory cystitis in cats, dogs and cats with non-struvite bladder stones, and obese “pre-diabetic” cats. Behavioral counseling and early training recommendations should be offered with breeds or individuals demonstrating aberrant or pathological behaviors.
Breeding Recommendations
Based on different modes of inheritance, there are guidelines to preserve breeding lines and genetic diversity while reducing the risk of producing carrier or affected individuals.
In the case of a simple autosomal recessive disorder for which a direct genetic test for carriers is available, the recommendation is to test breeding- quality stock, and breed normals to normals, or quality carriers to normal-testing individuals. This prevents affected offspring from being produced. Breeders should be counseled to replace carriers in a breeding program with a quality normal-testing offspring. This will maintain breed quality and diversity.
If a breeder finds that a quality individual tests carrier, many are inclined to remove it from their breeding consideration. This is the wrong decision for the breeder and the breed. The individual dog or cat was already determined to have qualities acceptable for breeding. Genetic testing should be used to increase a breeder’s choices, not limit them. Eliminating all carriers of testable disease-causing genes significantly restricts breed genetic diversity. Any quality individual that would have been bred if it had tested normal should
still be bred if it tests as a carrier. A genetic test for a simple autosomal recessive disorder should not change who gets bred, only who they get bred to. As each breeder tests and replaces carriers with normal- testing offspring, the problem for the breed as a whole diminishes.
A simple autosomal recessive disorder for which no carrier test exists allows the propagation and dissemination of unapparent carriers in the gene pool.
Carrier risk must be determined based on the knowledge of affected or carrier relatives. These can be visualized through vertical pedigrees on the OFA website, or other health databases. Quality carriers should be replaced with non-affected relatives, and bred to individuals with low carrier risk (depth and breadth of pedigree normalcy). High carrier risk individuals should be bred to low risk individuals. The high risk parent should be replaced in breeding with a lower risk quality offspring. To further limit the spread of the defective gene, the offspring should
be used in only a limited number of carefully planned matings, and then should also be replaced with one or two representative offspring. The rest of the litter should be placed in non-breeding (pet) homes. With this mating scheme, you are maintaining the good genes of the
line, reducing the carrier risk with each generation, and replacing, not adding to the overall carrier risk in the breeding population.
Autosomal dominant genetic disorders are
usually easy to manage. As affected individuals have approximately 50% affected offspring, they should be replaced for breeding with normal relatives. Issues with some autosomal dominant disorders include incomplete penetrance. With these disorders the presence of the defective gene still confers risk of producing affected offspring and should be selected against.
For sex-linked (also known as x-linked) recessive defective genes, selecting a normal male for breeding loses the defective gene in one generation. High carrier- risk females should not be used, as carrier females produce 50% affected sons. Rare sex-linked dominant disorders are managed the same way as autosomal dominant disorders.
Most complex/polygenic disorders, and those with an undetermined mode of inheritance have no tests for carriers, but they do have phenotypic tests that can identify affected individuals. Controlling polygenically inherited disorders involves; 1) identifying traits for selection that more closely represent the expression
of disease causing genes, 2) the standardization of nuisance factors (such as environment) that can limit your selective pressure against the genes and 3) selecting
for breadth of pedigree as well as depth of pedigree as demonstrated by vertical pedigrees.
With polygenic disorders, a number of liability genes must combine to cross a threshold and produce an affected individual. A clinically normal individual from
a litter that had one or no individuals affected with a complexly inherited disorder is expected to carry a lower amount of liability genes than an individual with a greater number of affected littermates. This is why it is important to screen both pet and breeding dogs and cats for polygenic disorders. Information on the siblings of the parents of potential breeding individuals provides additional data on which to base breeding decisions.

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