Читать книгу Pet-Specific Care for the Veterinary Team - Группа авторов - Страница 229

In a simple world, as described earlier, one might consider the genome as a passenger train, with the genes as railway cars coupled to one another. In reality, it is not that orderly. This unsophisticated look is what provides an opportunity to learn a lot from the genome. If the genome were arranged like a train, researchers would have to isolate each gene (passenger car) to learn anything. Fortunately, however, nature has provided spacers, known as microsatellites, that can act as markers for traits. They might be considered as dining cars spaced between groups of passenger cars.

As more and more useful microsatellites are identified, a higher resolution map of the genome will continue to be developed of which markers are inherited along with specific genes. Then, by measuring these markers, we can tell with some accuracy an individual's genetic make‐up for that specific trait without having to necessarily identify the actual gene. This can be very helpful when trying to predict some disorders which are unlikely to be completely controlled by individual genes alone (e.g., hip dysplasia, atopic dermatitis, etc.).

To use the train example once more, when train cars are unhitched and transferred to other trains, which passenger cars always seem to go along with the same dining car? If you identify a specific dining car, you know which passenger cars are also likely to be found there, too. Obviously, the closer the gene (passenger car) is to the marker (dining car), the greater is the probability that the two will be transferred together. A gene that is farther away from the marker may not be transferred 100% of the time, which would affect a test's specificity and predictive ability. Thus, some microsatellite markers may be associated with increased risk of a condition, and others with decreased risk, and when considered in aggregate, they could provide an impression of the risk for such traits in individual animals.

Microsatellites have more to offer than just their role as signposts. They have considerable variability (which molecular geneticists call polymorphism), making them unique identifiers of individuals and their offspring. Variations in a DNA sequence are called SNPs, some of which have no impact on health while others may predispose an individual to disease or signal responsiveness to medical therapies. Microsatellites are one way to identify an individual, as well as its progeny, and have become a voluntary part of the DNA identification for the different purebred registries. If a sire has six repetitive sequences relative to a specific marker and the dam has eight, the offspring will have one copy from each parent. By comparing findings at multiple markers, the accuracy of predicting parentage becomes a virtual certainty.

Short interspersed elements are often inserted within or near genes and may regulate their expression. The pet population has thousands of short interspersed element differences, which are associated with breed differences related to behavioral and physical characteristics and also act as signposts for specific genes linked to disease susceptibility or traits.