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02-04-2009, 07:16 PM
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#1 | | | Genetic break through in Sabino horses APHA
i found this article in a mag that I was reading lately and found it very interesting. Thought that I would share. l
Is your horse a sabino? Geneticists are working to answer that complex question. Find out what they’ve discovered and what it
means for your breeding program.
LIKE many Paint Horse owners, I like to play “name that
coat pattern.” At the novice level of this game, you have to correctly sort the tobianos from the overos. At the intermediate level, toveros are thrown in to increase the difficulty. At the advanced level, you divide the overos
into frame overos, sabinos and splashed whites. Get bonus
points if you correctly guess the color, too. Just when I
thought I had mastered the coat-pattern game, I learned
that a genetic test recently became available to determine
whether a horse carried the Sabino 1 gene—the cause of
one of the sabino patterns.
Now, I was pretty sure that my chestnut overo mare was a
sabino, so my curiosity was piqued. Wouldn’t it be cool to
get her tested and find out for sure? What is a Sabino 1
horse? How many sabino patterns are there? What would
the test results tell me? What if my mare wasn’t a sabino
after all? And most important of all—was it worth spending
the money? Curiosity finally got the best of me, and I pulled
30 mane hairs, sent them off to a laboratory, crossed my
fingers and determined to find out as much as possible
about the Sabino 1 gene. Here’s what researchers
know about sabinos and whether the Sabino 1 test
should be a part of your breeding program. An
unexpected find That the Sabino 1 gene mutation
was discovered at all is amazing. Dr. Samantha
Brooks of the University of Kentucky was actually researching the tobiano pattern in
2005 when some unusual foals caught her attention.
“I was on a site visit with a local Tennessee Walking Horse
breeder to collect some blood from one of her tobiano horses,”
explained Brooks. “She had two all-white foals that year, both
healthy and sound, out of rather plainly marked
mares. Both mares had four socks and a blaze. Since I was
there, I collected blood from both foals and their relatives.
“Once I got back to my office and did a little research, it
became apparent that the pattern on the mares, and the white
coat of the foals, closely resembled patterns produced by
simple mutations of the KIT gene, a gene I was already
working on because it was known to be linked to tobiano.” In
the article that resulted from the research Brooks and her
colleague Dr. Ernest Bailey conducted, it is explained that the
KIT gene is responsible for causing spotting in mice, pigs and
humans, similar to the sabino phenotype found in horses. In
the mouse, the characteristics for heterozygotes are white
markings along the mid-ventral line often extending to the
extremities, white head spots and some dilution of the
remaining body color. Homozygotes are completely white with
black eyes. | |
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02-04-2009, 07:18 PM
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#2 | | | Brooks and Bailey defined sabino in the horse as a white
spotting pattern characterized by white patches with irregular
borders on the face, lower legs or belly and interspersed white
hairs on the midsection. The white areas lack pigment, both in
the hair and the skin.
They hypothesized that the KIT gene which had already been
linked to two spotting patterns in the horse, roan and tobiano
was also responsible for sabino spotting. “As it turned out, I
included the samples from the white foals with my tobiano
samples when I sequenced the KIT gene and ‘eureka’ found a
large piece of the gene missing in the white foals,” said Brooks.
“This missing piece is due to a mutation in the gene that
causes it to be abnormally processed as it is prepared to be
used to make the final protein.”
That “eureka” moment lead to a research project involving
three families of Tennessee Walking Horses and individuals
from 12 other breeds, with a total of 320 horses tested. Blood
samples, pedigree information and photographs showing coat
color patterns were analyzed. In all three Tennessee Walking
Horse families, the only spotting pattern present was sabino.
Why were Tennessee Walkers the focus of the study, when so
many sabino-type Paint Horses are available? The answer is
simple.
“The problem with studying spotting patterns in the APHA
industry is that there are many, many horses with more than
one pattern,” said Brooks. “This really causes problems
because it interferes with our ability to define a phenotype. “For
a while it was more fashionable to have a solid-colored
Tennessee Walker, so now the variety of spotting genes is
limited in that population. It is easier to study one spotting
pattern at a time without worrying about other patterns
interfering. So I did not use any APHA families for the initial
inheritance studies in that paper.”
In the study, the Tennessee Walking Horses were divided into
three groups: those without the sabino spotting pattern, those
with the sabino pattern and those with mostly white body color
who were the offspring of two sabino parents. During the
course of the project, another interesting question arose. Were
those white foals possibly homozygous for this sabino gene?
The researchers hypothesized that homozygosity would result
in a phenotype with extensive white coat color, at least 90
percent depigmented from birth. If the foals were white at birth,
this would exclude the possibility that they later turned white
due to the gray gene, which eliminates a horse’s normal coat
color as it ages. (See “Gray isn’t a ‘color’” in the August 2006
issue of the Paint Horse Journal.)
All the white-colored horses that participated in the study were
tested for other genes that might imitate or conceal the effects
of a sabino gene, such as overo lethal white foal syndrome
causing frame overo spotting patterns, cream and tobiano.
DNA sequencing showed that horses expressing this particular
sabino phenotype, both spotted and the more extensive white
type, had a mutation of the KIT gene exon 17 was missing.
Brooks and Bailey were able to prove a complete link between
this mutation, which they designated as SB1, and this coat
pattern in the Tennessee Walking Horse families in the study,
which they named Sabino 1. Five horses were homozygous for
SB1, and all five were white, a phenotype they called sabinowhite.
The 68 horses with one copy of SB1 all expressed the
Sabino 1 phenotype or were multi-patterned. Some of the
multipatterned horses appeared to be all white, but they also
carried genes for frame overo and tobiano, which
demonstrates an additive effect of white spotting patterns.
But also of major importance was the fact that 13 horses
expressing sabino-type patterns did not have the mutation.
Presumably, other genetic factors can also produce what
we describe as sabino. | |
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02-04-2009, 07:20 PM
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#3 | | | • Define “sabino”
That last point brings up one of Brooks’ pet peeves. “You can’t
say ‘the sabino gene,’” Brooks explained. “It doesn’t work that
way. There are many genes that cause patterns that are
commonly described as sabino, at least five [patterns] that I
have seen. All of these have fundamentally different genetic
causes, potentially different genes, so you really have to
specify. It’s correct to say ‘the Sabino 1 gene’ (SB1) or ‘the
sabino genes.’” In the case of Brooks and Bailey’s study, they defined Sabino 1 horses as those with three of the four following characteristics: • two or more white feet or legs • a blaze or white patch extending the length of the face • jagged margins around white areas • spots or roaning in the midsection Beyond Sabino 1, it is difficult to assign definitive names and descriptions to the other patterns in this group. Common terms used in the past include “minimal sabino,” “maximum sabino,” “sabino white,” “white sabino,” “roaned sabino,” “sabino roan” and “Clyde-type sabino.” From a scientist’s point of view, these are rather imprecise. “Many people who contact me are very caught up in specific characteristics lip spots, for example, or ‘lightning strikes,’” said Brooks. “I’ve had many an owner tell me that their horse must be sabino because it has this one leg with a sock that is pointy at the top, and that this is a sabino characteristic. Well, yes and no. Many sabinos do have pointy socks, but there are many different biological explanations for pointy socks, only one of which is Sabino 1. “When I look at a pattern, I want to know what may be discernible about the genetics,” Brooks explained. “I want to know what population of horses it exists in and what mode of inheritance it is transmitted by. “The draft-type sabino, for example, is present in heavy horses, Shires, Clydesdales, etc. This population, until the recent popularity of draftcrosses and spotted drafts, was not intermingled much with the light horses. And, though it seems to be dominantly inherited, it does not produce a white phenotype.” While there is discussion that some sabino patterns may be polygenic (coming from more than one gene), this is not the case with Sabino 1, which is caused by a single gene SB1. The Sabino 1 has a semi-dominant mode of inheritance. This means that heterozygotes do not look identical to homozygotes. Although the presence of the single, dominant allele, in this case SB1, is clearly visible, it’s effect is not as strong as when two dominant alleles are present. Compare this to the tobiano pattern, which is caused by the action of a single, dominant gene. It is impossible to tell with absolute certainty if a tobiano is heterozygous or homozygous just by looking. In the case of a Sabino 1 horse, the homozygous horse is clearly different from the heterozygous horse. The first is completely white or nearly all white. The second always expresses the Sabino 1, but is easily distinguishable from the first. Even though we classify the Sabino 1 horse as an “overo” pattern, the SB1 gene is not associated with overo lethal white syndrome. In homozygotes, Sabino 1 produces a perfectly viable white, or nearly all-white, horse. Of course, if a horse carried genes for SB1 and OLWS, it could potentially produce a lethal white foal. • A widespread phenomenon? At this point, no one knows how prevalent the Sabino 1 gene mutation is in a specific breed or in the general horse population. Brooks and Bailey’s study included 320 individuals from 13 different breeds. Tennessee Walking Horses made up the majority of subjects, with 110 tested. Among those, 67 were not carriers, 39 were heterozygous and had one copy of the SB1 gene, and four were homozygous with two copies of the gene. | |
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02-04-2009, 07:21 PM
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#4 | | | Of the 27 Paint Horses included in the study, 23 were not
carriers, four were heterozygous and none were homozygous
for the SB1 gene. Brooks states that this doesn’t necessarily
mean that there are no homozygous Sabino 1 Paints. The
researchers simply were not looking for them. Interestingly
enough, the SB1 gene mutation turned up in quite a few of the
American Miniature Horses in the sample. Of the 29
miniatures tested, 18 were not carriers and 11 were
heterozygous. Animal Genetics, Inc., located in Tallahassee,
Florida, has been offering the Sabino 1 test for a little over a
year, so the number of horses tested thus far 276 is still
somewhat small. While this sample is not broad or random
enough to extrapolate the results to the general horse
population, the results are nonetheless interesting.
The breed with the highest number of horses tested, the
American Miniature Horse, had a significant number of
individuals carrying the gene mutation. More than one-third of
the 88 tested 33 were heterozygous for SB1, and three more
were homozygous. Five out of the 51 Gypsy Vanner Horses,
the second most prevalent breed tested, were heterozygous.
Results were similar for Paint Horses, with five out of 42
individuals carrying one copy of the SB1 gene. While none of
the Thoroughbreds were carriers, one of the 10 Quarter
Horses tested turned up homozygous for the SB1 mutation,
meaning that this individual could pass the gene on to all of its
offspring. Although Tennessee Walking Horses were the focus
of the Brooks and Bailey study, thus far Animal Genetics, Inc.
has only tested four. All were found to be heterozygous for
SB1. From a statistical standpoint, we cannot make any
generalizations from the information provided by Animal
Genetics, Inc. However, based on these findings, it is safe to
say that some Paints and Quarters carry the Sabino 1
mutation. But again, this particular mutation does not explain
all sabino phenotypes
• Implications for breeders
Do you need to test your horse for Sabino 1? Ultimately, only
you can answer that question. While the test identifies the gene
that only produces one of the sabino phenotypes, there are still
plenty of valid reasons to have it done. Once you get beyond
the general categories of tobianos and overos, determining a
pattern based strictly on photographs has its limitations,
especially in horses expressing multiple patterns. The Sabino
1 pattern can mimic others, and it is sometimes difficult to
differentiate between a sabino and a frame overo. Brooks also
found that in Tennessee Walking Horses, many carriers for
SB1 were misclassified as roans, which created some
confusion when they later produced white foals.
“The test can certainly be used to identify SB1 carriers in multipatterned
individuals where there’s not enough color left to
identify the pattern,” said Brooks. “For example, say a breeder
went out and bought this great white stallion thinking he was a
medicine hat and expecting to cross him with solid mares and
get tobianos, toveros and frame overos. But lo and behold, he
gets a bunch of foals with four socks and a blaze. The test
could have identified the stallion as a Sabino 1 homozygote
and not a medicine hat.”
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02-04-2009, 07:22 PM
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#5 | | |
Brooks offers another example. “You could also use it to
identify minimal white SB1 carriers that would produce more
color when bred to spotted stallions than your average solid
horse. Say you have an SB1 heterozygous mare who has four
socks and a blaze, but so little body white that she’s
considered solid. Half of her foals, when bred to a tobiano
homozygote, will have a more lively tovero (tobiano/sabino)
pattern, while half will have only a tobiano pattern. A true solid
individual with no spotting genes could not contribute to the
spotting and would produce only tobianos when bred to the
same stallion.” The test also allows breeders to differentiate
between a horse that is homozygous for the Sabino 1 gene
and one that is heterozygous for the dominant white gene.
While the first, when crossed with a solid horse, would produce
a Sabino 1 phenotype foal 50 percent of the time, the second
would produce an all-white foal 50 percent of the time.
For the Paint breeder, there’s a world of
difference. Knowing that your horse is an SB1
carrier can also reduce some of the element of
surprise come foaling time. By crossing two
SB1 carriers, you have the possibility of
producing a completely white foal that could
easily be mistaken for one afflicted with lethal
white syndrome. The homozygous Sabino 1
horse, however, is viable and a valuable
addition to a breeding program aimed at
producing spotted patterns. By running tests
for both SB1 and OLWS, a breeder can avoid producing a foal
with lethal white syndrome and explain the genetic origin
of white offspring. However, it’s important to keep in mind that
the Sabino 1 test has its limitations. “Many people assume that
because their horse has four socks and a blaze it must be
sabino and get upset when their horse tests negative,” said
Brooks. “Testing negative for SB1 doesn’t necessarily mean
that your horse isn’t ‘sabino,’ but some people have a hard time
with that.”
• Ongoing research
According to Brooks, some additional work is planned to
further study what if any health effects may be associated with
SB1. In mice, some mutations in the KIT gene that causes
sabino-like phenotypes result in anemia, mast cell deficiency,
sterility and deafness. However, it is unknown whether sabino
and sabino-white horses suffer from any kind of health defect.
The owners of the horses sampled in Brooks and Bailey’s
research projectdid not report any symptoms of anemia,
deafness or sterility.
While she can’t release any specific details, Brooks is currently
investigating what effects SB1 may have on the immune
system and the inflammatory process. This is where Paint
Horse owners can play a role.
“I am in need of horses, and their relatives, who have tested
homozygous for SB1,” Brooks said. “Participation would only
require a blood sample and a short questionnaire. It does not
appear that there are any severe health deficits in SB1 horses.
In fact, if some of my hypotheses prove true, they may be
protected from certain inflammatory conditions.” If you own a
horse that has tested homozygous for SB1 and are interested in
participating in the study, contact Brooks via e-mail at
samantha.brooks@ uky.edu or call (859)2574757, extens 81174.
• Getting results
As for my chestnut overo mare, test results showed she does
not have the Sabino 1 gene. Once I overcame my initial
disappointment, I reminded myself that it doesn’t necessarily
mean she isn’t a sabino or that I just lost a round of “name that
coat pattern.” What I learned during this experience more than
justifies the cost of the test. My curiosity is satisfied.
Clearly, a great deal remains unknown about the group of
patterns we commonly place under the umbrella term of
“sabino.” “I hope it will be possible one day to differentiate
between all the sabino patterns by genetic testing and then
name them accordingly,” said Brooks. Until then, the discovery
of the Sabino 1 gene and the resulting test is one more tool
that you can use to understand and more accurately predict
your percentage of spotted foals.
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02-25-2009, 12:12 PM
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#6 | | | Interesting... and long, haha. | |
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03-01-2009, 09:45 PM
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#7 | | | Very long but well worth the read  | |
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