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Variance components were estimated for maturing age, first litter size, and animal size in Finnish minks. The fitted animal models had direct genetic and maternal genetic effects, litter effects, and maternal environmental effects. Multivariate analysis was performed to determine covariances between the traits. Maternal effects represented a significant source of phenotypic variance in the maturation age and animal size. For litter size, maternal effects were not as clear. Moreover, in maturation age and animal size, the covariance between the direct additive effect and the maternal additive effect was negative. In addition, litter effect variances were larger than maternal variances for all traits. Therefore, it is crucial to also estimate environmental effects common to littermates for these traits. Direct heritability and the response to selection are overestimated, especially for maturation age and also for animal size, when maternal and common litter effects are not considered. Key words: fertility, genetic parameter, maternal effect, mink

In a five-generation selection experiment, separate lines of standard mink (Mustela vison) were subjected to selection for litter size at 3 wk (F line), body weight in September (BS line), underfur density (P line), or combined selection for litter size and body weight (I line). One unselected line served as a control (C line). The present paper focuses on correlated responses to selection regarding fertility and fitness traits, fur quality, and body size traits. Genetic and environmental parameters were estimated with REML (Restricted Maximum Likelihood) techniques using a multi-trait, reduced animal model in a derivative-free way. Genetic and phenotypic correlations were estimated from four subsets of data consisting of 1) female September weight, litter size, and kit mortality; 2) body size traits; 3) September weight and fur traits graded on live mink; and 4) fur traits graded on live mink and skins. September weight was found to be negatively correlated with fertility and fitness traits as well as with fur traits. Selection for underfur density resulted in an improvement in guard hair quality and in general impression of the fur and almost eliminated the fur defect metallic sheen.

In a five-generation selection experiment, separate lines of mink (Mustela vison) were subjected to selection for improved litter size at 3 wk (F line), BW in September (BS line), and underfur density (P line), and combined selection for litter size and BW (I line). Underfur density was subjectively judged on live animals. One unselected line served as a control (C line). Significant changes were achieved in each trait: litter size in the last generation was 5.3 in the F line vs 3.7 in the C line; September weight in males was 2,254 g in the BS line vs 1,979 g in the C line, and the underfur density score, graded using a 5-point scale, was 4.1 in the P line vs 2.9 in the C line. In the combined line (I line) litter size was only slightly improved, whereas BW was substantially increased (male mean = 2,194 g). A univariate animal model was used to predict genetic values and to estimate variance components with a REML procedure. Heritability estimates were .14 |+ or -~ .09 for litter size, .39 |+ or -~ .06 for September weight, and .21 |+ or -~ .06 for underfur density. It was confirmed that the reproductive performance of heavy of fat animals was poor. Responses were higher than predicted when selecting for September weight and underfur density. In the last generation the average breeding values, relative to the base generation, were + .8 kits for litter size (F line), +365 g for male September weight (BS line), and +1 point for underfur density (P line). The study suggests that negative maternal effects on litter size may exist in mink.