Carrie Wilson USDA

The genetic diversity available within each sheep breed provides producers with tremendous opportunity to make genetic improvement. The snapshot of the genetic diversity currently available is extremely valuable because it provides a benchmark for comparing the consequences of selection over time in individual breeds. To establish that starting point, researchers used both pedigree- and molecular-based information to assess genetic diversity in the four breeds involved in Sheep GEMS -- Genetics, Environment, Management and Society.

For each of the breeds that were evaluated - Katahdin, Polypay, Rambouillet and Suffolk - researchers found substantial genetic diversity, which coincides with low inbreeding levels. Inbreeding arises from the mating of relatives, which leads to an increased chance that a lamb inherits identical copies of an allele from both its sire and dam. Such an increase in homozygosity is not necessarily bad; it is almost inevitable in a selection program where producers retain rams and ewes with more favorable genotypes - or packages of alleles - for breeding.

However, inbreeding does come with risks. Often, deleterious alleles are recessive. When that is the case, with inbreeding they will appear together in a homozygous state more often. That results in an expression of their deleterious effects. Such negative consequences of inbreeding are most seen in fitness traits such as health and reproductive success.

Increased inbreeding also coincides with reduced genetic diversity in a breed in general. The ability to achieve good genetic progress in the long term depends on having genetic variation to work with. So, in a well-designed breeding program, producers need to balance an increase in inbreeding with strategies to track and maintain genetic diversity.

Another important concept related to genetic diversity is the effective population size - Ne - which is the number of individuals that effectively participate in producing the next generation. In other words, it is an estimate of the number of active breeding animals. The Ne is usually much less than the actual size of the population. Producers expect a Ne of 50 to lead to a rate of inbreeding of only one percent per generation. Consequently, it is recommended to have a Ne of at least 50, preferably at least 100.

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For Katahdins, inbreeding in the most recent generation averaged 1.7 percent with a rate of change of inbreeding of 0.025 percent per year. The Ne for the breed's pedigree estimate ranged from 42 to 451 while the molecular-based estimate was 150 animals. Those results are well within the recommended values and suggest the Katahdin breed has sufficient genetic diversity moving forward.

For Polypays, current pedigree-based inbreeding was 3.5 percent with an annual rate of inbreeding of 0.069 percent per year. The Ne ranged from 41 to 249 for pedigree-based methods and 118 for the molecular-based method. Furthermore, from the analyses of Polypay, the breed has become differentiated from the foundation flock at the U.S. Sheep Experiment Station, likely due to different selection objectives among National Sheep Improvement Program flocks.

For Rambouillets, the current pedigree-based inbreeding was two percent and the rate of inbreeding per year was 0.079 percent. It was estimated the average pedigree-based Ne to be 165 animals while the estimated molecular-based Ne was 392 animals. Therefore - like in Katahdins and Polypays - Rambouillets are genetically diverse.

For Suffolks, the pedigree-based inbreeding was 5.5 percent for the most recent year while the annual rate of inbreeding was 0.015 percent. Researchers computed the Ne using multiple pedigree-based methods, resulting in a range of 28 to 244. With the molecular-based method, the Ne was 79.5. The Suffolk analysis showed some disconnectedness within the breed, which is great for genetic diversity but less appealing for genetic evaluation, which relies on pedigree ties across flocks.

The take-home message from the studies is the genetic diversity of those four breeds is substantial, and producers can feel confident moving forward with genomic selection. However those analyses should be repeated every 10 to 15 years to ensure producers continue to maintain that genetic diversity.

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