National Scrapie Plan: Genotyping
This page aims to give some information on how genotyping
is being applied in the National Scrapie Plan.
Scrapie can be a difficult disease to control. Methods commonly employed
include the selective culling of infected flocks or family lines, prompt
removal of afterbirths from lambing pens and, in recent years, the use
of "genotyping" technology.
Scrapie develops when the normal form of the PrP protein in a sheep's brain converts to an abnormal form. The PrP gene which produces this PrP protein also determines a sheep's resistance or susceptibility to scrapie i.e. whether or not it is likely to develop scrapie in its lifetime if exposed to the infectious agent.
The sheep PrP gene has two copies (or alleles), one derived from each parent. Each position or "codon" on the gene gets translated into one of the 256 amino acids that form the PrP protein.
Scientists have identified variations in the amino acids encoded at three specific codons on the alleles. The particular codons are 136, 154 and 171.
Based on variations at these three codons five different scrapie related alleles in sheep have been identified. These are:
| 136 |
154 |
171 |
| A |
R |
R |
| A |
H |
Q |
| A |
R |
H |
| A |
R |
Q |
| V |
R |
Q |
(Key: Amino acids: A = alanine; H = histidine; Q = glutamine; R = arginine; and V = valine)
The genotype is determined by referring to the amino acids encoded at the three relevant sites on both alleles.
An animal that has inherited the same information on the alleles from both of its parents is said to be a homozygous genotype (e.g. ARR/ARR, AHQ/AHQ). If it has inherited different information it is said to be a heterozygous genotype (e.g. ARR/ARQ).
Taking the homozygous or heterozygous pairing of alleles inherited from both parents into account allows for the generation of up to 15 different genotypes. Some breeds are known to contain all 15.
Genotype testing to determine a sheep's resistance or susceptibility to scrapie uses DNA extracted from the white blood cells.
Scrapie resistant sheep can be used in a breeding programme to confer resistance to their progeny and, over time, this will impact on the incidence of disease in the national flock.
The Predicting Method Chart (201
KB) 
illustrates the principle of how genotypes can be predicted in progeny.
If you know the genotypes of your ram and ewe, you can use the Genotype
Calculator Grid (74 KB)
to work out the possible genotype outcomes of their offspring.
Page last modified:
1 August, 2007
Page last reviewed:
12 October, 2006