Implications of cloning for breed improvement strategies: are traditional methods of animal improvement obsolete?
Can the optimum animal be defined? Will that definition change over time, by location, by market demand? First, assume what may be impossible, that the perfect animal can be defined or that only a limited number of definitions of "perfect" are needed. Then, can the "perfect" animal to match a definition be found? Suppose such an animal is found. Then the question to be answered before trying to clone as a method of genetic improvement becomes "Is the animal perfect because of phenotype or genotype?" In other words, the P = G + E problem exists, which requires traditional methods of genetic evaluation and testing to determine whether genotype (G) or random environmental (E) effects or a combination leads to the apparent perfection in the phenotype (P). For most traits, additive genetic variance accounts for 10 to 50% of total variance, a fraction denoted as heritability. With a simple model, the best prediction of genotypic value is to reduce the apparent phenotypic superiority by multiplying by heritability. Cloning the "perfect" animal also could capture optimum dominance and epistatic genetic effects that are otherwise difficult to select for. For some traits, maternal effects are important. In that case, clones as breeding animals must be "perfect" for both direct and maternal genotypes, or alternatively terminal and maternal clone lines would need to be developed. The use of clones to increase uniformity can be only partially successful. If heritability is 25%, then the standard deviation among clones would be 87% of that of uncloned animals. Only if heritability is 100% will clone mates have complete uniformity. Fixing the genotype could increase susceptibility to failure if environment changes or if the cloned genotype is susceptible to a new disease or if economic conditions change. Cloning, at best, is another tool for animal improvement that joins the list of previous biotechnological inventions, some of which have become cost-effective, such as artificial insemination, sexing of semen, multiple ovulation and embryo transfer, embryo sexing, and in vitro fertilization. Cloning has a place in that inventory but, in the long-term, the use of cloning will need to be managed to be cost effective for the improvement of quantitative characters.