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At its simplest, one would suspect that disease outcomes could be readily predicted if only the pattern of inheritance is known. However, things are rarely that simple when it comes to genetics. A few examples of genes, mutations, diseases, and modes of inheritance are in order to help explore the genetics of disease.

Fucosidosis is a rare but devastating lysosomal storage disease of English springer spaniels, transmitted as an autosomal recessive trait, meaning a dog would have to inherit an abnormal gene from each parent to be affected by the disease (see 11.3 Heritable Health Conditions – By Disease). Although the abnormal gene itself does not kill the dog, it codes for an absent or nonfunctional enzyme (alpha‐L‐fucosidase) that can result in clinical problems. If the enzyme is not capable of breaking down its substrate, complex sugars collect within cells and eventually kill them. When the cells are severely compromised by this accumulated substrate, they die, and then the organs (primarily the brain) begin to be compromised, and eventually the situation will lead to the dog's demise. Carriers, who only inherited an abnormal gene from one parent, appear outwardly normal. Their levels of fucosidase enzymes, however, may be lower than normal, but not necessarily low enough to cause expression of the disease.

The same is true for many of the bleeding disorders, in which a defective gene does not produce a fully functioning protein or an enzyme important to clotting. Although carriers appear normal, their level of clotting factor may be lower than normal, just not low enough to cause spontaneous bleeding problems.

Whether a trait/phene is truly recessive depends on how hard one looks at the phenotype (see 3.2 Modes of Inheritance). A dog with fucosidosis typically shows neurological signs early in life and has an abbreviated life span. The affected dog carries two copies of the abnormal gene. If a carrier, with one abnormal gene, has no outward problems as a pup but (theoretically) suffers from dementia and is euthanized at 7 years of age, can this truly be considered a completely recessive trait without influence? If the bitch that is the carrier of one hemophilia gene has no spontaneous bleeding episodes but has complications with bleeding during surgery or whelping, is she truly phenotypically “normal”? Young Labrador retrievers with late‐onset rod–cone degeneration may have measurably reduced retinal function.

In cutaneous asthenia, a disease of skin fragility, gene mutations that produce abnormal collagen are considered dominant because heterozygotes produce enough abnormal collagen to be evident, whereas procollagen‐processing mutations are considered recessive because heterozygotes (those with only one abnormal gene variant) usually have enough enzyme activity to convert procollagen to collagen. Thus, as more is learned about genetic conditions and testing improves, suppositions about dominant and recessive traits will have less impact on our genetic counseling skills.