Author
 |
Van Vleck, Lloyd |
|
Submitted to: Journal of Animal Science Supplement
Publication Type: Proceedings Publication Acceptance Date: 3/24/1999 Publication Date: N/A Citation: N/A Interpretive Summary: Cloning, on first impression, suggests a perfect way to improve performance of farm animals. As with many first impressions, the wrong conclusions are often drawn. These first impressions might include the following. 1) Clone mates (members of the clone) are identical in all respects, i.e., are uniformly the same. 2) Clones are superior, i.e., how they will perform is known exactly. 3) A less extreme view is that superior records of the clone originator (clone origin) would guarantee that all members of the clone are genetically superior to other animals. Thus, these first impressions seem to imply that what breeders need to do is to find the perfect animal so that no further effort to breed better animals will be needed, i.e., when the perfect animal is found, and can be cloned, then traditional methods of breed improvement will be obsolete. The purpose of this article was to show that these perceptions are generally incorrect and that traditional methods of animal improvement and genetic evaluation will be needed for effective implementation, and for economic evaluation of costs and expected returns, of cloning technology. Technical Abstract: Can the optimum animal be defined? First, assume what may be impossible, that only a limited number of definitions of "perfect" are needed. Then, can the "perfect" animal to match the definition be found? Suppose such an animal is found. 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. Thus, the best prediction of genotypic value is to multiply apparent phenotypic superiority by heritability. When maternal effects are important, 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. 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 has a place in the inventory of previous biotechnic inventions such as artificial insemination but, in the long-term, use of cloning will need to be managed to be cost effective for improvement of quantitative characters. |
