Non-normality in carcass quality measurements and effects on the genetic evaluation of sheep

In order to estimate genetic parameters for four traits related to carcass quality in sheep (live weight. ultrasonic fat and muscle depth and in vivo conformation, assessed at 21 weeks of age), the validity was tested of the assumptions of homogeneity of variance and normality of residuals. Animals were of the Bluefaced Leicester breed, the most prevalent crossing sire bre ed used in the UK. The fit of the model and the distribution of the residua ls were investigated when fitting a mixed (animal) model after applying Box -Cox power transformations to deal with scale-related non-normality. For ul trasonic fat depth, the log-transformation was shown to improve normality a nd decreased the correlation between hock means and standard deviations, Th e other traits did not show sufficient improvement to justify a transformat ion. For ultrasonic fat depth, the regression of both genetic and residual variance on sample means obtained by bootstrapping (sampling hocks) was pos itive whereas the regression of heritability on sample means was negative, The log-transformation made the association weaker and the regression coeff icient changed sign for genetic variance only. The four traits all had mode rately high heritabilities. Log-transforming ultrasonic fat depth caused th e heritability to decrease from 0.43 to 0.30. Conformation had the lowest h eritability (0.20) but had high positive correlations with the other traits , suggesting that improving conformation could lead to more muscling but ma y also increase the amount of fat in the carcass. The log-transformation ha d a major effect on ranking of individuals based on estimated breeding valu es, particularly fur high selection intensities. (C) 2001 Elsevier Science B.V. All rights reserved.