Identification of Merino rams homozygous for the poll gene

Abstract

A total of 23 ram lambs were produced from matings between homozygous polled Merino rams (PP) and heterozygous polled Merino ewes (Pp). The genotype of the ram lambs was established by progeny testing, and a comparison between genotype and phenotype was made. At birth, seven out of nine PP ram lambs showed clean depressions at the horn sites, whereas all of 14 Pp rams showed knobs set in depressions. At 15 months of age, the range of phenotypes with regard to horn growth was: PP rams: Least expression – a smaIl knob set in a depression; no horn growth. Greatest expression – a scur ½ in. long (horn volume. 0.012 cu. in.) attached to a large knob set in a depression. Pp rams: Least expression – a scur ¾ in. long (horn volume, 0.180 cu. in.) set in a depression. Greatest expression – a scur 8 in. long (horn volume, 12.400 cu. in.) with absence of depressions. From examination of horn growth in a group of 36 Pp rams, it was concluded that in diagnosing the genotype of 15-month-old poll Merino rams, approximately 3% (0-15%; P < 0.05) of hetorozygotes would be misclassified as homozygotes if the dividing line between genotypes was taken at 0.05 cu. in. of horn growth. In a second group of 196 Pp rams, observations on scur length indicated that a little over 1% of overlap types were present. On discussing this information relative to the establishment of homozygous polled Merino flocks, the following conclusions were drawn:(1) A large percentage of Pp rams could be diagnosed at lamb marking by the presence of horn growth on large knobs set in depressions (scores DK + or DS). (2) The measurement of horn volume at 15 months of age will differentiate Pp and PP types with accuracy equivalent to a progeny-testing scheme involving five lambs per sire.