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A Genetic Defect and its Management
by Dagny Vidinsh
All animals, including dairy goats, have numerous genetic defects of varying
severity. We are all familiar with the occasional multiple teats, for
instance, and with such defects as undershot and overshot jaws. Other
defects are rapidly fatal, and it often is unclear whether the death of a
kid should be attributed to genetics or to misfortune. The exact inheritance
of many of these defects is often obscure; for instance, although most
people believe that multiple teats show up when both parents carry a gene
for this trait there is evidence that in some cases they are actually caused
by environmental factors. In order to manage these undesirable genes
breeders usually have to fall back on the "don't repeat that breeding"
strategy, which is very crude and unsatisfactory.
This article will describe a recently discovered genetic defect which is
easily managed and eliminated because it's mode of transmission is
straightforward and, more important, because a foolproof DNA test is
available to identify carriers of the gene.
This defect's full names are mucopolysaccharidosis IIID, or G-6-Sulfase
deficiency, and it is usually referred to as G-6-S. It was first identified
in 1987 at Michigan State University, and subsequently the researchers
tested nearly a thousand goats in Michigan and concluded that about 25% of
Nubians carry this gene. All cases are the result of a single mutation, and
appear to be confined to Nubians and their crosses; other breeds were tested
initially and they do not have this particular defect.
The affected goats lack an enzyme (G-6-S) and this results in a variety of
symptoms of varying severity. The main symptom exhibited by affected goats
is failure to grow. Sometimes the kid is smaller than normal at birth, and
grows slowly. Some breeders
have reported kids which grew normally for the first three months and then
stopped growing. Other affected goats grow to what appears to be normal size
but is in fact small for the particular bloodlines. They lack muscle mass,
appear "slab-sided", sometimes with blocky heads. Immune function appears
to
be compromised, and sometimes they become deaf or blind. The longest-lived
goat known to be G-6-S affected died at just under four years of age, and
death is usually due to heart failure. Unfortunately affected animals can
and do grow up to breed, although they often experience
reproductive problems.
The same symptoms can have many other causes, so that
affected animals are seldom recognized as having a genetic defect.
Often they grow normally for the first few months and may be sold before any
problems become apparent. In that case the breeder may blame the new owner
for the goat's failure to thrive and early demise.
Every animal has two genes for every trait, one inherited from the dam and
one from the sire. In turn, that animal will pass only one of those genes to
each offspring, and which one it will be is a matter of chance, like the
flip of a coin. On the average, half the offspring will inherit one gene and
half the other. If the two genes are different, then there is a question as
to which of them will determine how the animal actually looks or functions.
The defective G-6-S mutation is a simple recessive gene, which means that a
goat which has only one copy of it will appear perfectly normal and will not
show any of the symptoms described above. Such a goat is referred to as a
"carrier". A goat which inherits the defective gene from both parents shows
symptoms and is referred to as "affected". A "normal" goat, in this context,
is one who has two copies of the normal gene.
If a normal goat is bred to a carrier, then all offspring will inherit a
normal gene from the normal parent. The carrier parent will pass a normal
gene to half the offspring, and a defective gene to the other half. Thus
such a mating will, on the average, produce half normal kids and half
carriers, and no affected ones. If two carriers are bred to each other, then
one quarter of the kids will be normal, one half will be carriers, and one
quarter will be affected. If an affected goat is bred to a normal goat, all
offspring will be carriers. An affected goat bred to a carrier will produce
half carriers and half affected.
As stated above, research shows that 25% of Nubians carry the defective
G-6-S gene. Almost all of these are carriers, since most of the affected
animals which are born would be culled, and the rest die early. Most people
find it surprising that something which is in one quarter of the population
can have escaped notice for so long. However, random matings in such a
population would result in only one out of sixteen being carrier to carrier,
and only one quarter of the kids from these breedings would
be affected. Thus only one kid in sixty-four (1.6%) would be affected.
Given the variable and obscure symptoms of G-6-S affected kids, it really is
understandable that most Nubian breeders believe that they have never
encountered affected kids.
However, many Nubians are line-bred, and this practice will concentrate
certain genes in some lines while eliminating them from others. It has been
observed that the G-6-S mutation is very prevalent in the same lines which
are known for high milk production. Thus breeders who have been selecting
for milk may have inadvertently also been selecting for the G-6-S defect.
Fortunately it appears that the two traits are actually independent, that
you can cull the G-6-S carriers without at the same time culling the high
producers.
Usually it is difficult to eliminate a genetic defect without loosing all
the good genetics for which a line is known. For instance, if a buck throws
double teats, then there is no way of knowing which of his offspring will do
the same and which will not. You can cull him, but that seems rather
heavy-handed since the bad gene will undoubtedly live on in some of his
relatives. With G-6-S we are very fortunate to have a foolproof DNA test
available which will tell us whether a goat is normal, or a carrier, or
affected. This test makes it possible to save the good genetics and
eliminate the defective gene if that is our wish. If a superior animal is a
carrier, then we can test the kids and manage them in such a way as to avoid
the birth of any affected individuals.
What is a good management strategy? What is the most efficient way to save
the good and get rid of the bad? The usual recommendation for such testable
defects is to cull carrier males, but not the females. Remember that if
a normal buck breeds a carrier doe, then only half the kids will be
carriers, and none will be affected. Thus if there are some carrier females
in the herd, then using only normal bucks will reduce the incidence of
carriers
in
the next generation by one half. The average herd would start with 25%
carrier females, and if only normal bucks were used the next generation of
females would be down to 12.5% carriers, and the next generation to 6.25%,
etc. This is in sharp contrast to what a carrier buck would do in the same
herd: if used to breed all the does, his daughters would be 50% carriers and
6.25% affected. Clearly there is much to be gained by testing buck kids and
retaining only normal ones for breeding.
While it is relatively easy to cull a buck kid, one might hesitate to do the
same with a proven sire. In particular, there are some very popular bucks
whose semen commands a high price and who are carriers for the defective
G-6-S gene. A reasonable strategy here would be to use these bucks only on
normal does, thus avoiding affected kids. Then one would test the kids and
cull carrier bucks.
Although the DNA tests are expensive, if testing one's bucks prevents the
birth of even one affected kid then it is cost effective. Unlike tests for
diseases, a genetic test does not need to ever be repeated. Also, the DNA
tests are completely accurate, there are none of the gray areas which can be
so frustrating. There is no need to test the kids if both parents are known
to be normal. One can work back from one's foundation animals and if there
really is no problem in the herd then it may be possible to establish that
at reasonable cost. Normally whole blood is used for the test, but semen can
also be used. If an AI buck is a carrier, that can be established by finding
a carrier offspring out of a normal doe, but no number of normal offspring
will prove that a buck is nomal.
A number of breeders have expressed the opinion that the G-6-S defect is no
more of a problem than many other genetic defects, and therefore does not
merit any particular attention. They evidently miss the point that it is the
availability of a DNA test which makes this defect special. One can use
goats from
bloodlines which are known to have a high concentration of the G-6-S defect
completely safely by just testing the particular individuals and either
rejecting carriers or using them with proper precautions. There is nothing
to be gained by trying to sweep G-6-S under a rug, and much to be gained by
sharing information about it.
One may wonder why a DNA test has been developed for such an obscure defect,
and no help is available for, say, multiple teats. The answer is simple-
humans don't have a problem with multiple teats, they do with G-6-S. The
same genetic defect, when found in humans, is called Sanfilippo IIID; the
affected child appears normal at birth but soon stops growing, looses muscle
mass, has neurological deterioration and dies. When the same genetic defect
was discovered in goats researchers used them as models for treatment, and
goat breeders in turn benefited from their discoveries.
Copyright Dagny Vidinsh, 2001
____________________________________________________________________________
Testing for G-6-S is done at the Texas Veterinary Medicine Diagnostic Lab
(TVMDL) at a cost of $30 US. The test requires whole blood or semen. The
address is:
TVMDL
1 Sipple Rd.
College Station, Texas, 77843
ph# 979-845-3414
The samples should be submitted to the attention of Dr. Loyd Sneed, PhD,
with
instructions as to what tests are required and how the results are to be
reported. Dr. Sneed can also be reached at L-SNEED@TVMDL.TAMU.EDU
When shipping from Canada, it is advisable to first obtain a copy of TVMDL's
import permit for blood samples.
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