A Genetic Defect and its Management
By Dagny Vidinish
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
its 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
one 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 losing 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.
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
Phone: 979-845-3414
