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DNA
Testing as a Management Tool By
Tom Drudik
(originally written in February 2000, it's even more relevant today.)
Two conditions that can cause large economic losses to the sheep industry are Scrapie and Spider Lamb Syndrome.
Scrapie
is an infectious disease of sheep and goats that causes a degeneration of the
central nervous system. Research
suggests that it is not a genetically transmitted disease, but rather, a disease
with genetically influenced susceptibility.
In this case the infectious particle is a prion protein.
All proteins are coded for by genes that are stretches of DNA.
In the case of scrapie susceptibility the Codon 171 test is taken.
A codon is a stretch of DNA that codes for a single amino acid. Argine containing variant at codon 171 result form an R
allele of the prion gene. Other
alleles know at that site is Q and H. Both
Q and H are known to be susceptible to conversion to scrapie prions.
The three genotypes produced at this location is RR, QR, and QQ.
A considerable body of evidence exists indicating that both QR and RR
sheep do not develop scrapie where as the QQ genotype is associated with scrapie
susceptibility. This does not
necessarily mean they have scrapie, it means they may come down with it if
exposed.
Popular media has focused more and more attention on Scrapie and its possible relationship with BSE (Mad Cow Disease), which will further intensify the need for producers to eliminate stock with QQ genotype from their flocks.
Here again, it is in the producer’s best interest to know the genotypes of the breeding flock. This is especially important if one’s business is selling breeding stock. It is important for all sheep flocks to strive for a flock with as many animals as possible possessing at least one R in their genotypes. The easiest way to achieve this is to always use/purchase RR rams for breeding, which is why each of our current and future stud rams will have the RRNN genotype.
The
Spider Lamb Syndrome disorder (ovine hereditary chondrodysplasia) is a result of
abnormal transformation of cartilage to bone, resulting in severe skeletal
abnormalities. It is thought to
have occurred as a genetic mutation in the late 1960s. However, it was not until
the mid 1980s that wide spread reports of the problem began to surface in
certain breeds.
A
correct diagnosis of Spider Lamb Syndrome is absolutely essential so that the exact problem is known and
proper action taken. In the past,
one had to rely on x-rays to help diagnosis the condition in a young lamb and
most often with age, the lamb would exhibit many other distinguishable signs.
Historically, several factors have led researchers to decide that Spider
Lamb Syndrome is generally transmitted as an autosomal recessive gene.
A
brief background in how recessive genes interact with other genes may help
explain how Spider Syndrome lambs occur. First
of all, genes occur in pairs. Thus
the recessive gene that causes Spider Syndrome, denoted a (S), has a counterpart
normal gene (N), which is dominant over it.
The term recessive indicates that the effects of the S gene will not be
seen when an animal also possesses the N gene.
Since N is dominant, it dominates what is seen in the animal.
As genes occur in pairs the three genotype combinations which are
possible are NN, NS, SS. Animals
with the NN and NS genotypes appear normal.
From this, one can see the problem in dealing with a recessive gene is
that the NN and NS genotypes cannot be distinguished by merely observing the
animal. If normal appearing parents
produced a Spider lamb, each one had to be a carrier of the condition because
each parent contributes one gene. In
this case they each contributed an S. In
this example, one would know if the parents are carries of the spider condition;
but yet, one adds in the probability of receiving the N genes for a few lambing
seasons and then show up as a spider carrier.
The result of discovering this may mean a big economic loss.
Recent
research has resulted in a quicker way to determine an animal’s genotype
concerning the spider condition, DNA Testing.
DNA testing can play a big role in making management decisions on how to
eliminate S genes from the flock or negate the effects the S gene may have in
the flock. The simple blood test
can determine the status of an individual early in life and can be managed
accordingly. If an animal is a
carrier it can be sold for market or it may be mated to only NN animals that
would result in it never producing a spider lamb.
One must keep in mind that by doing the later 50% of the offspring may be
carriers.
DNA Testing is simple, and although it is not cheap, DNA testing can and should be used as a management tool for improvement of the sheep industry.
(for more information on how traits are inherited see our May 2001 Article.)
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