Analysis for a major gene affecting parasite resistance in sheep

Abstract

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 apparrent 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.