EVALUATING THE DORPER & WHITE DOrPER PEDIGREES & GENETIC DIVERSITY

Harvey Blackburn

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

All closed populations of livestock are confronted with the issue of increased levels of inbreeding over time since their populations are closed to the introduction of new genetics from other breeds. This is particularly true of breeds that were imported with few founder animals and therefore a limited breadth of genetic diversity among the imported animals.

Breeders need to be aware of their animals’ inbreeding levels for several reasons:

1. Inbreeding may be used to increase similarities among animals and therefore be used in marketing.

2. Inbreeding can help to identify animals that are carriers of undesirable mutations.

3. As inbreeding increases, genetic variability can be reduced and therefore slow genetic progress.

4. Increased inbreeding can lead to depressed animal performance and/or fertility.

Part of our work at the National Animal Germplasm Program is to quantify genetic variability of U. S. livestock breeds and to conserve genetic variability through collecting genetically diverse germplasm. Therefore, we gained access to the pedigree files from your Association. We used pedigree information to determine the genetic relationships and inbreeding levels for all animals contained in the Association’s pedigree files. Furthermore, this data was extended to group individual animals sharing similar genetics into groups or clusters. With information computed from pedigree files, Dorper and White Dorper breeders have a baseline of inbreeding information concerning their breeds and potentially new ways of designing mating programs that may maintain genetic diversity among these populations.

The pedigree files supplied to us contained animals born from 1965 to 2020. The pedigrees for Dorper and White Dorper were treated as separate breeds. ForDorper, there were 4,373 unique sires and 27,004 unique dams; and White Dorper had 2,288 unique sires and 16,900 unique dams used in the analysis.

Over time, more animals were inbred at some level (Figure 1a, b) for both Dorper and White Dorper. This type of result is typical for all closed populations, and the vast major of animals in both breeds have an inbreeding coefficient that was equal to or less than 5% (Figure 1c, d). Usually, inbreeding depression does not impact performance for most traits until it reaches 12 to 15%.

That said, with advancing generations the peak seen on both graphs will shift to the right indicating higher levels of inbreeding among the entire population.

To understand inbreeding trends over time, animals were assigned a generation number (e.g., animals in the pedigree with no known parents were considered generation 0 and generation 1 were the progeny of generation 0). Figure 2a,b show how inbreeding changed as generation number progressed.