Genetic analysis of test-day somatic cell scores (SCS) was used as an example of a mixture model analysis with application to animal breeding. The trait SCS may be regarded as coming from two distributions depending on the udder health status (healthy, mastitic) of the cow. A hierarchical liability-normal mixture model, with random cow effects for liability to mastitis and SCS in healthy and diseased animals were developed and applied for analysis of SCS test-day data from Danish Holsteins. The estimated heritability for liability to putative mastitis (solely based on recorded SCS) was 0.14, which is substantially higher than most heritability estimates for observed clinical mastitis. The estimated genetic correlations between liability to putative mastitis and SCS were close to zero, while the estimated genetic correlation between SCS in putatively healthy and mastitic cows was moderate positive (0.55).

J. Odegard, P. Madsen, G. Klemetsdal

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume , , 26.11, 2006
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