Pedigree Dorset Horn sheep in Australia. 2. Breed structure and genetic analysis

Abstract

Flock Books were used to determine the breed structure of pedigree Dorset Horn sheep in Australia. Four-generation ram sample pedigrees were used to determine important flocks in 1954, 1969 and 1974. Two-line ram sample pedigrees traced to foundation or imported animals were used to determine important animals and the level of inbreeding and its components in the same years. A hierarchical breed structure was found to exist. Only 15 to 18 per cent of flocks were Breeders' Flocks, i.e. supplied rams to other pedigree flocks, and only one-third of these supplied rams to other Breeders' Flocks. The effective number of flocks supplying sires in the third and fourth generations was two to three in 1954 and four to five in 1974 and 1969. One flock, Newbold, had a genetic contribution of over 40 per cent in 1954. Two flocks had a genetic contribution of over 20 per cent each, with five flocks having a combined genetic contribution of over 62 per cent in 1974. Total inbreeding was 12.5 per cent and increased by 1.5 per cent per generation in the five years to 1974. Current inbreeding was 1.85 per cent in 1974 with 5.0 per cent of matings being parent-offspring and 3.7 per cent paternal half-sib. Expected long-term inbreeding (7.9 per cent) was the major component, which was caused by few animals being used extensively in the development of the breed. One sire had a direct relationship of 30 per cent to rams registered in 1974. Implications of the high level of inbreeding are discussed as well as development of breed improvement schom3s within the existing breed structure.