Phenotypic effects of a fertility mutation in Norwgain White Sheep

Abstract

Norwegian White Sheep (NWS) is a synthetic breed that results from crosses between local Norwegian and foreign breeds. This breed is mainly kept for the meat production, and litter size is therefore a trait of large economic value. A mutation in the ovine GDF9 gene (c.1111G>A) on chromosome 5 was recently found to be associated with increased litter size in daughters of AI rams of Norwegian White Sheep (NWS). This thesis aims at estimating the phenotypic effect of the (c.1111G>A) mutation in nearly 900 NWS-ewes’. Since litter size can only be directly observed at females, genotyping of the mother is considered to give a better phenotypic estimate of the allelic effect compared to estimates based on the EBVs of the rams. The information available for these ewes was number of lambs born at age 1 year and 2 year of age. A total of 853 NWS ewes were genotyped for the (GDF9 c.111G>A) mutation by the iPLEX Gold technology (SEQUENOM). The genotyping success rate was more than 90%. The average litter size for ewes at 1 and 2 year of age was 2.061±0.73 and 2.671±0.91, respectively, showing that the average litter size was lower at 1 year compared to those at 2 years. The phenotypic effect of being homozygous for the mutant allele at age 1 and 2 year was found to be 0.54 and 0.87 additional lamb per litter, respectively. The frequency of the c.1111A allele was 0.38 in NWS ewe population. As the experimental ewes were heavily selected for large litter size and strong association between c.1111A allele and litter size is known, it can be assumed that the selection has increased the frequency among these ewes. Handling of c.1111A allele in NWS will significantly influence the future litter size in this population. The Norwegian Association of Sheep and Goat Breeders will design how to exploit the rams carrying the c.1111A allele in Norwegian White Sheep breeding program.