Genetics of increased importance of health traits

Abstract

The purpose of this study is to present mechanisms of possible negative effects of genomic selection in secondary traits, especially in health traits, and present steps to mitigate those effects. Selection on production traits causes corresponding declines in antagonistic fitness traits. Under traditional selection, declines were initially compensated or slowed down by continuously improving management. When the deterioration was too strong to ignore, the fitness traits were added to a selection index. Under genomic selection the genetic progress for production traits accelerates causing accelerated declines in fitness traits, and management modifications are not fast enough to compensate. Including the fitness traits in the selection index is only partially effective because of lower accuracy of evaluation due to lower heritability and fewer phenotypes. In particular, genomic selection can give rise to a sudden onset of categorical traits such as diseases. If deteriorating traits are underlying values for categorical traits such as a disease status, such traits are not expressed until a threshold is crossed. Then the incidence can increase rapidly. Mitigating the negative effects of genomic selection for health traits will require several steps. First, the selection pressure on major traits that are antagonistic to the deteriorating health traits needs to be reduced, allowing for management modifications to catch up. Second, selection for health traits needs to intensify, with weight on each trait calculated from recent genetic parameters. As genetic parameters can change rapidly under genomic selection, a challenge is to estimate these parameters for the last generation using complete data, as required for unbiased estimates. Third, methods for evaluating categorical traits can be improved. If the evaluation for health traits is not true multitrait with the production traits, the index weights need to be modified to account for ignored genetic correlations.