Already in 1995 the Swifter Herdbook started blood tapping for examining of the DNA for scrapie sensitiviness.

 The PrP-gene of sheep plays an important role in the development of scrapie. Like in other genes, mutations exist in the PrP-gene. It has been found that in Switers, as well as in Texels, but also in other breeds, the sensitiveness to scrapie has a genetic cause. In Swifters and in Texels, the VRQ-allel is coupled to a high sensitiveness to scrapie. Over 95% of all examined scrapie sheep (Swifters/Texels) have at least one VRQ-allel.

The ARR-allel is coupled to a lower sensitiveness (resistance?) to scrapie. Less than 5% of the scrapie sheep (Swifters/Texels) have a ARR-allel. The ARQ, AHQ and ARH-allele seem to be neutral, because they exist in both healthy and scrapie sheep. These allele are not coupled to sensitiveness or resistance.

Sheep that are homozygote for the VRQ-allel (genotype VRQ/VRQ) are most sensitive. On a scrapie farm, these animals develop scrapie when they are about 25 months old. The heterozygote VRQ-sheep (genotype VRQ/ARQ) are less sensitive, but can develop scrapie, 35-60 months after birth. In VRQ/ARR-sheep the "resistant" ARR-allel is dominant. Almost 100% of the VRQ/ARR sheep is healthy after 70 months, despite the presence of the "sensitive" VRQ-allel.

Already from 1997 the Swifter Herdbook has decided only to breed with rams that do not have the VRQ-allel.

From 2001 onward, sheep with genotype ARQ/ARQ will not be allowed as a new breeding ram. The goal of the herdbook is to breed a population with ARR/ARR as the only genotype. Breeders will normally only use rams with genotype ARR/ARR and from 2008 it's obligatory.

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