ABSTRACT
Improvements in growth rate and associated characteristics using modified selection programs or by a transgenic approach will depend on having a detailed knowledge of the genetic control of growth, an understanding of the biology of the regulatory factors, clones of the genes encoding these factors, and detailed knowledge of the regulation of their expression. A number of different approaches to identify and understand “trait genes” associated with growth have been attempted. One way is to analyze genes and gene products known to be associated with growth, such as growth hormone, the growth hormone receptor, and the insulin-like growth factors. This strategy is illustrated in this chapter by discussion of the somatotrophic axis in poultry. Other alternatives include the identification of the genes through their actions, for example, by dissection of growth into physiological components, or the assessment of the expression of a large number of genes and gene products at random and looking for those that are associated with growth. In order to use these other approaches, we have to modulate growth genetically and look for the response in expression of trait genes. One way of doing this is by using a transgenic biology, but since this is not well established in poultry, another approach is to use animals divergently selected for growth. We have used this strategy for the last few years and have interesting results at both the cellular and molecular levels. A number of examples, such as genetic differences in growth hormone, ornithine decarboxylase, and rates of skeletal muscle cell division have been demonstrated. The results of these studies suggest that a complete understanding of genotype and phenotype allowing identification of genes and gene products that regulate growth will enable the full potential of transgenic biology to be fulfilled in avian species as the methodology becomes fully developed.
