Physiology and Molecular Genetics of Mutations that Increase Ovulation Rate in Sheep

VARIATION in litter size in sheep is controlled by both genetic and environmental factors. Most breeds of domestic sheep have one or two lambs at each lambing, although a small number of breeds, including the Booroola Merino, Cambridge, D'Man, Finnish Landrace, and Romanov, consistently have litter sizes of three or more (see Ref. 1). Attempts to increase litter size by selection within a breed result in slow progress because the heritability of litter size is low (2). Therefore, the discovery of major genes (or mutations) with large effects on ovulation rate, and thus litter size, has generated considerable interest among sheep breeders and scientists. The major gene (FecB) in the Booroola Merino breed was first postulated in 1980 (3). Since then, the search for other mutations influencing reproduction in sheep has intensified, and a number have been proposed (4–8). As an alternative to the slow progress achieved by selection for litter size, these mutations allow more rapid progress by introgression into other breeds. Back-crossing of the FecB gene into a number of breeds is now in progress (9).