The direct response achieved after 20 years of divergent selection for skin fold score was examined in relation to a randomly-selected control flock. An estimate of 0.56 was obtained for the heritability of fold score in the control flock. Selection response was asymmetric, giving a realised heritability of 0.36 in the upward direction and 0.53 in the downward direction. However, the latter flock was characterised by a reduction in the realised heritability estimate over time. This was concluded to be the result of large changes in gene frequency. On the other hand, the lower estimate obtained with upward selection was attributed to a failure to account for linkage disequilibrium in the regression of response on selection differential. The potential for misinterpretation of selection response when due consideration is not given to causal factors of asymmetric response is illustrated.

B. J Crook, J. W James

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 19. Selection and quantitative genetics; growth; reproduction; lactation; fish; fiber; meat., , 53–56, 1994
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