A series of crosses were made using a Merino ram whose progeny displayed an exceptionally high level of resistance to infection with Haemonchus contortus. The ram's estimated breeding value for faecal egg count (in eggs per gram on a square root scale) following artificial infection was -55.4, compared with means of -29.2 in his progeny and 0.0 in unrelated animals. Although this phenomenon was strongly suggestive of a major gene for resistance, no segregation was apparent in the various progeny crosses. Heritability of faecal egg count was estimated at 0.24±0.04, or 0.21^0.05 when relatives of the exceptional ram were excluded. A major gene index was tested and did not provide clear evidence of a single gene with large effect.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XV. Beef cattle, sheep and pig genetics and breeding, fibre, fur and meat quality., , 131–134, 1990
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