畜牧兽医专业英语(高等职业教育畜牧兽医类专业教材)

Lesson 1.Genetics Part one Intensive Reading 1.Learning Objective After learning this lesson，you should understand the following: ⑴Define the concept of genetic manipulation of variance of phenotype. ⑵Give the reason why the meat processors often desire a uniform product. ⑶Give an overview of why we can not be achieved genetic uniformity by high levels of inbreeding. ⑷Explain why selection for uniformity is unlikely to be successful. ⑸Describe what difficulties may cause in genes of large effects on productivity. ⑹Explain why the optimum management for animals of one genotype may not be optimum for other genotypes. 2.Text A Genetic Manipulation of Variance of Phenotype Variance of phenotype in commercial production can have direct effects on profitability. Many production systems would be more profitable with increased uniformity of phenotype. Large variation in phenotype may mean that a final management system is suboptimal for all but a few animals in the system. Similarly，meat processors often desire a uniform product to optimize slaughter operations and retailing of final product. It is also possible that increased phenotypic variance would be profitable in some cases. For example，with dairy cattle，increased phenotypic variance at a fixed mean of performance of cows entering the herd would lead to a greater increase in herd mean production level (across all age classes) for a given rate of voluntary culling. The increase in herd profitability would presumably be dependent on the rate of voluntary culling and rate of change in production with age and distribution of age classes in the herd. Genetic uniformity can be achieved by high levels of inbreeding，but this inevitably leads to reduced performance (inbreeding depression) and reduced homeostatic control and consequent increased phenotypic variance. Under an infinitesimal model，selection is expected to have little effect on genetic variation (except for the temporary effect of gametic phase disequilibrium). And，selection for uniformity is unlikely to be successful since，1) homeostasis is expected to be highly related to fitness and hence show little or no genetic variation in the positive direction (e.g. increased homeostasis = reduced phenotypic variance)，and 2) homeostasis is associated with heterozygosity in naturally outbreeding species. Thus，on our present understanding，breeding programs to reduce phenotypic variance of quantitatively inherited traits are unlikely to be successful. And，programs to increase phenotypic variance，via inbreeding，would likely pay an unacceptable cost of inbreeding depression of average performance. Increased uniformity can be achieved for traits controlled by few genes，by fixation of the desirable alleles. A few examples are the coat and skin color patterns of recognized breeds，the halothane gene in swine (causing increased stress susceptibility and carcass leanness)，the Booroola gene in sheep (causing large increases in lambing rates)，the double muscling gene in beef cattle (causing substantial increases in carcass lean weight and percentage) and the dwarfing gene in poultry (used in female parents of broiler stocks to reduce body size and hence egg production costs). Segregation of genes determining coat and skin color patterns can cause the visual perception of large amounts of phenotypic variation while，in many production systems，having no effect on variation in productivity. In some situations such variation may however be associated with variation in productivity via effects on susceptibility to such productivity reducing complaints as sunburn (pale skinned pigs)，eye cancer (cattle with lightly pigmented eye surrounds) and tick burden (dark coated cattle). And，even where color pattern variation has no direct effect on productivity，it may have substantial effects on profitability of a breeding company since customers often expect or prefer a particular color pattern，even to the extent of refusing to purchase a particular stock if of an unusual or unexpected coloration. In such cases it is important not just to have the optimum color pattern on average，but to have as little variation as possible about that optimum pattern. Similarly，genes of large effects on productivity may cause management difficulties if segregating in a population since optimum management for animals of one genotype may not be optimum for other genotypes. An example might be the booroola gene. The effect of the Booroola gene varies depending on genetic background，but a typical situation might be average litter sizes of 1，2 and 3+ for the homozygous wild type，the heterozygotes and homozygous booroola. Under harsh extensive conditions，a litter size of 1 is optimum，leading to elimination of booroola allele. Under semi-intensive conditions，a litter size of 2 could be optimum for a dam line suggesting a crossing scheme between two lines fixed for opposite alleles to produce F1 dams. Under highly intensive conditions a litter size of 3 or more might be optimum leading to fixation of the Booroola allele. Perhaps more likely，a proportion of litters of 3+ might be acceptable provided there were also single litters for fostering of lambs from ewes unable to cope properly with 3 or more lambs. In such situations a segregating population might be desirable.