Genetic Impact of External Sires on the USSES Flock

The ancestors explaining the genetic variability of each population were identified in order of importance to the population in terms of genetic contribution; their marginal and cumulative contributions to the population were computed. Each additional ancestor was selected based on the proportion of genes it contributes that were not yet explained by previously selected ancestors until all genetic variability in the reference population was accounted for (Boichard et al., 1997). For the 18 external sires and the 12 USSES sires, the marginal contributions for each sire were determined. For each sire, the number of breeding years, number of offspring, number of retained offspring, and GCI were obtained.

To compare the two sire groups, total lambs produced and total contribution to retained lambs for the study period were computed. Population differentiation between the two sire groups was compared using Nei’s minimum distance (Nei, 1978) and Wright’s F_ST (Wright, 1951). Nei’s minimum distance (D) was computed as

where f_ii and f_jj are the average coancestry within populations i and j and f_ij is the average coancestry between the two populations. Wright’s F_ST was computed as

where f¯ is the average coancestry for the subpopulation and f~ is the mean coancestry for the entire population.