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 normal.
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
Many in the Philippines have little background knowledge on this topic.Known from the American side that some carriers of G6S came into country under USPL 480 goat export/im port program and some of those who have imported as private.Should become interesting in a few years time if anything starts to show up or covered up and which Nubian bloodlines are more problematic and avoid from those breeding known bloodlines.The same can be said with the Australian Anglo Nubians,some probably came in as carriers and when 2 carriers are bred,problems will show up and questions will be asked.The Philippines might have to set up DNA testing to prove the Normals from the Carriers and if any are Affected.
