Analysis of the influence of the PrP genotype on the litter size in Polish sheep using classification trees and logistic regression

The relation between prion protein (PrP) genotype class and litter size was analysed in Polish Merino, Black-headed, Ile de France and Berrichon du Cher sheep in Poland. Litter size was analysed with the classification tree technique and multiple logistic regression for effects of breed, age of ewe, PrP genotype, litter size of ewe at her birth, and at her rearing and year of ewe's birth. The resulting classification tree was 6-level deep and contained 21 leaves. Age, breed and year of birth were more important determinants of litter size than PrP genotype. Importance of PrP genotype in the tree development was 0.560 relative to age of ewe. While the logistic regression analysis showed that…