Читать книгу Pet-Specific Care for the Veterinary Team - Группа авторов - Страница 241

Now let's take a look at dominance. A trait that is completely dominant is easy to spot because the parent is affected and so are all or most offspring. In the case of a dog that is homozygous for a dominant trait, all pups should be affected. In the case of a dog that is heterozygous for a dominant trait, that dog and 50% of its offspring should be affected. What happens in many instances is that there is incomplete dominance or expression of a dominant trait with variable expressivity. This basically means the trait has dominant features with a spectrum of possibilities in the offspring. If penetrance of a dominant gene is complete, all progeny receiving the allele will express the trait. If penetrance is 50%, only half will express the trait. With variable expressivity, some genes may produce different degrees of expression of a phenotype, ranging from severe expression to absence of the trait.

For example, dermatomyositis in collies was originally believed to be an autosomal dominant trait, but it is now believed that, although the trait has dominant features, environmental components (likely viral infection) are required for full manifestation of the disorder. Other traits may be co‐dominant, each contributing to the phenotype. For dermatomyositis, the current DNA testing situation is that there appear to be at least three loci associated with an increased risk for developing the disease (PAN2 gene on chromosome 10, MAP3K7CL gene on chromosome 31, DLA‐DRB1 on chromosome 12) and the three can be considered in aggregate when trying to determine relative risk. This can help identify pets that might be a low, moderate, or high risk for developing the condition. Other loci could eventually also prove to be associated with the condition.

An example of a dominant trait is merling of coat color. The normal, recessive genotype is the homozygote, mm. The heterozygous (Mm) animal has the characteristic merle coat coloring. The homozygous dominant (MM) animal is nearly all white, has blue eyes, and has a higher incidence of deafness. Although merle is a desirable feature in some dog breeds, the deafness and white coat color are completely undesirable. Breeders wishing to perpetuate merle in their lines will breed normal (mm) to merle (Mm) to achieve half typically colored offspring and half merle offspring. Fortunately, genetic testing for the merle trait is now available to facilitate that process, and avoid inadvertently creating homozygous dominant animals (MM) with health issues.

In most cases, recessive disorders are often attributable to enzyme deficiencies because heterozygotes with only 50% as much enzyme likely still have enough to perform needed functions. In contrast, dominant traits may be caused by defects in structural or substrate proteins, such that heterozygotes will be expected to be affected because these polypeptides are required in relatively large quantities. As more and more research accumulates, however, these suppositions seem to consist of more generalization than fact, and each trait is best considered individually.

A lot has been learned since Mendel began playing with peas, and new genetic tests that actually identify genotype deserve much of the credit (see 3.6 Genetic Testing). Some of the old rules just do not apply, though. For example, mitochondrial myopathy in Clumber and Sussex spaniels is believed to be a sex‐linked but not an X‐linked trait. The trait is passed from the mitochondrial DNA of the maternal line to both sexes. Therefore, hemophilia A is X linked and is transmitted principally from mother to son, whereas mitochondrial myopathy is believed to be passed from mother to both sons and daughters.