Читать книгу Pet-Specific Care for the Veterinary Team - Группа авторов - Страница 261

Although establishing harsh criteria to rid breeds of genetic disorders by completely eliminating affected animals and carriers may seem reasonable, it is neither practical nor desirable. Because each animal carries nearly 20 000 different genes, ridding lines of all deleterious alleles is not possible. If carriers can be determined, however, breeding phenotypically normal dogs is a real possibility by never breeding two carriers together. If we are aware of heterozygotes, we can safely breed carriers with known normal individuals, and we will never see cases of the disorders we are trying to avoid. Is that not what genetic counseling is all about?

Trying to overcome polygenic traits takes longer and is more troublesome. Obviously, the higher the heritability of a trait and the more ruthless we are in selecting superior individuals for breeding, the more successful our selection process will be. This response to selection (R) can also be described as an algebraic function, R = h ² S, where R is the response to our breeding strategy, h ² is the heritability of the condition, and S is the selection differential, the phenotypic superiority of the parents we selected versus that of the general population from which they came. The result is that the mean improves; it is not that the pups will be superior to their parents.

Hip dysplasia provides a good example of how selection pressure improves the standard, albeit slowly. Let's say we create a scoring system for hips in which 0 represents severe dysplasia and 100 represents perfect hips. In our ACCT, the mean hip score for the population is 55 and the heritability of the trait is believed to be 0.25. To try to improve hip joint morphology in the breed, we mate a stud with a hip score of 88 with a bitch with a score of 92. The average score of the parents is 90, which is 35 points higher than the mean for the breed. Should that produce terriers with great hips? The response to selection (R) is 0.25 × 35, or 8.75. Thus, in the litter produced, the mean hip score for the pups is predicted to be 63.75 (55 + 8.75). We have managed to shift the whole bell‐shaped curve of hip scores to the right, but not by the magnitude you may have expected. Now you see why creating real improvement with polygenic traits takes so long, even when using superior breeding animals.