The biochemical pathways contributing to growth of body tissues are regulated and coordinated by hormones. It is therefore likely that inherited variation in the endocrine system is a major factor in the physiological basis for genetic variation in growth. The endocrine system is subject to genetic variation at all levels. A complete understanding of the endocrine basis for genetic variation therefore cannot be obtained alone from correlations between growth and circulating levels of hormones. However, profound effects on productivity have been obtained by a simple change in the circulating level of a single hormone. Important relationships may therefore be found even if the mechanisms behind the observed correlations are not completely understood. Relationships between growth and circulating levels of hormones have been studied in numerous experiments, but the results have often been conflicting. The reasons for the discrepancies are likely to be confounding effects of environmental factors, physiological state of animal or sampling technique. The influence of these factors should be taken into account in the interpretation of experimental results. With the influence of these factors in mind the existing data suggest that animals with high genetic growth capacity have elevated circulating levels of growth hormone, somatomedin, thyroid hormones and possibly insulin. The influence of the confounding factors should be considered when new experiments are planned and their influence should be known and taken into account when and if the knowledge of the endocrine basis for genetic variation in growth capacity is used in breeding programs.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XI. Genetics of reproduction, lactation, growth, adaptation, disease, and parasite resistance., , 261–270, 1986
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