The influence of the poll gene on certain production characters of Merino sheep

The results of a series of matings of Merino sheep of the form heterozygous poll rams (Pp) x knobbed ewes (pp), are presented. The phenotypic ratio poll progeny : horned progeny was 75:73 for rams and 81:94 for ewes. These were not significantly different from the expected 1:1 ratio. The 75 Pp rams had horn growth varying in length from 1/2 in. to 13 in. One known poll ram phenotype, depressions at the horn sites, was not seen. Comparisons were made within sires of Pp and pp progeny for a series of production measurements. Pp rams cut 4.09 per cent. more clean fleece weight than pp rams, while Pp ewes cut 4.47 per cent. less than pp ewes. This genotype-sex interaction was further examined by comparing sexes for the ratio of positive to negative within-sire differences. There were highly significant chi-square values for greasy fleece weight, clean fleece weight, and body weight. The differences in clean fleece weight were largely accounted for by differences in body weight and staple length. Fold score differences were negligible. The hypothesis suggested as an explanation for these results is that the Merino poll gene P is associated with a small deleterious effect when heterozygous. The effect is apparent only in ewes where the specific effect is the removal of small bony knobs. A subsidiary hypothesis is that in rams the deleterious effect of P is over-compensated by the nutritive benefits derived from the absence of the large ram horns. The practical importance of these findings is discussed.