EVALUATING THE DORPER & WHITE DROPER PEDIGREES & GENETIC DIVERSITY

Harvey Blackburn • National Animal Germplasm Program, Agricultural Research Service, USDA • Fort Collins, CO

In looking at Figure 2, it is important to note that the rapid increase in inbreeding after generation 8 or 9 in Dorper and after generation 8 in White Dorper may be due to fewer animals being registered at the time the data was acquired, therefore more registrations may increase or decrease the averages for these generations. We can also see from these two graphs that the inbreeding trends for both breeds was similar.

As research has proceeded, it has become more apparent that the actual level of inbreeding may not be as important as the rate of inbreeding (e.g., inbreeding increase per generation). Ideally, and as a target, it is desirable to keep the rate of inbreeding to less than 1% per generation. Maintaining rate of inbreeding below 1% will also preserve genetic variability for future selection. As Figures 2c and 2d show in later generations the rate of inbreeding has increased.

Exploring the subpopulations of each breed can be accomplished using pedigree data and computing the genetic relationship among members of each breed. The genetic relationship among animals in the population can then be used to cluster each breed into subpopulations that are more related to each other than the entire population. We show four primary clusters for Dorper and three primary clusters for White Dorper (Figure 3a, b). As Figure 3 shows, each primary cluster can be subdivided into smaller groups or subclusters. As primary clusters are subdivided, the genetic relationship and inbreeding levels can increase. For example, White Dorper cluster 2 can be partitioned into four subclusters (2a,2b, 2c, and 2d). Each of those subclusters has a different genetic relationship within the subcluster and among subclusters. While the overall relationship and inbreeding among animals in cluster 2 were relatively low, 2.7% for genetic relationship and 1.35% for inbreeding, evaluating the subclusters reveals a different picture. The genetic relationship for 2a was 3% (inbreeding = 1.5%) and for 2c was 26.8% (inbreeding = 13.4%). The clusters and subclusters provide insight into the breed’s structure and can potentially be used to guide mating decisions. Continuing with the cluster 2 example, owners of animals in cluster 2c may wish to make matings outside cluster 2c to lower inbreeding levels (if so desired); while breeders with animals in cluster 2a have the flexibility of selecting mating choices within 2a, potentially without increasing inbreeding levels dramatically, or any of the other subclusters.

Figure 2. Average inbreeding by generation (2a & 2b), where generation 1 are the initial animals of the registry (e.g., parents are unknown or imported) and rate of inbreeding

As breeds, Dorper and White Dorper have low to moderate levels of inbreeding. In previous generations the rate of inbreeding has been below 1% per generation. However, in the most recent generations rate of inbreeding has increased suggesting that within both breeds genetic variation is contracting, and this can have ramifications on the rate genetic progress can be made through selection. Breeders can control these factors by using a wide variety of sires and dams in future generations. The cluster analysis demonstrates some groups within the breed have obtained high levels of inbreeding that merit management. Depending upon the breeding goals of the owners’ decisions can be made to perform future matings within clusters or to utilize different clusters as a future source of mating choices.

Breeding programs are long term endeavors. Altering the performance levels of breeds and/or individual flocks is a multigenerational effort which takes time and requires consistency in selection. The information provided in this analysis can assist in making long term breeding decisions in terms of managing inbreeding and therefore genetic diversity among the two breeds.