Sheep & Goats

Selection for scrapie resistance in Icelandic sheep: very intensive selection for a very rare allele

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Abstract

The Icelandic sheep population is a closed population with around 300,000 ewes. The R171 allele in the PRNP gene, connected with resistance to scrapie, was first discovered in Iceland in 2022 at a very low frequency. Since then, selection for resistant genotypes has been intensive, causing concerns about future inbreeding and loss of genetic diversity within the population. To counteract this, it was recommended to breeders, based on results from simulations, to use young breeding rams and each ram only for a single year. Furthermore, some breeders also selected PRNP alleles, other than R171, that are connected to partial resistance to scrapie. The aim of this study was to investigate the effects of intensive selection for scrapie resistance on allele frequencies, inbreeding, and mean relationship within the Icelandic sheep population. The number of alleles that each sheep carries (i.e. gene content), for each of the 7 different PRNP alleles, was predicted for the whole population based on registered pedigree and genotypes of 219,396 genotyped sheep using a linear mixed model. Relationship coefficients, inbreeding coefficients and age of parents were calculated based on registered pedigree. Allele frequencies, average relationship and inbreeding coefficients were calculated for breeding rams and breeding ewes for each year of birth from 2021 to 2025. Each cohort included 5,500 to 7,050 rams and between 55,000 and 70,000 ewes. For calculations of relationships and inbreeding coefficients, only sheep with pedigree completeness index for five generations above 0.95 were included. The allele frequency of R171 rose from below 0.005 in the 2021 and 2022 cohorts to 0.20 and 0.11 for rams and ewes, respectively, in the 2025 cohort. The frequencies of other alleles connected with lower risk of scrapie also increased. The generation interval decreased, especially for sires of rams for which the average age decreased from 2.83 years in 2021 to 1.72 years in 2025. The mean relationship among rams increased from 2022 to 2024. This was most likely caused by the extensive use, enabled by artificial insemination, of the few rams carrying the R171 allele that were known in 2022, and large numbers of their sons and grandsons being used in the following years. Still, the average inbreeding coefficients were lower for 2024 and 2025 than in 2021 and 2022. The selection for scrapie resistance in Icelandic sheep took a fast start despite a very low allele frequency of R171 in the beginning. Mean inbreeding has decreased but increased mean relationships within the youngest cohorts can cause inbreeding in the future. Intensive selection for scrapie resistance will continue and meanwhile monitoring of inbreeding and genetic diversity in the population is essential.

Keywords: 2026

How to Cite:

Eiríksson, J., (2026) “Selection for scrapie resistance in Icelandic sheep: very intensive selection for a very rare allele”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2286662. doi: https://doi.org/10.31274/wcgalp.24085

Rights: 1