Equine Coat Color
Genetics
Lesson Eight
Patterns of White
Part Two -
“Overo”
In
the last lesson we discussed the tobiano coat pattern
and genetics. In this lesson we will
discuss the other coat patterns. As mentioned
last time, the Paint and Pinto registries divide patterns into tobiano and overo (or “tovero”
for horses with both). However, although
tobiano is just one simple gene, the “overo” category
includes “everything else” which is now known to be dozens of different genes.
The
registries have lists of the “differences between tobiano
and overo”, such as:
·
Tobiano
looks like a “white horse with dark spots” while overo looks like a “dark horse
with white spots”.
·
Tobiano
has white legs, overo has dark legs.
·
The white crosses the back on tobiano, and doesn’t on overo.
·
The white spots are vertically oriented
on tobiano, and horizontally on overo.
·
Tobiano
spots are smooth-edged and rounded, while overo spots are jagged or lacey.
·
Tobiano
tails are often two-toned, while overo tails are all dark.
·
Tobianos
generally have dark eyes, overos generally have blue
eyes.
And other similar generalizations. However, these are outdated, are very general
tendencies and they really only apply to one of the patterns in the “overo”
category - Frame Overo. As you will see,
the others are different, and there are also many exceptions in every
pattern. There is only one
characteristic that is uniformly true and that is face white – all of the genes
in the “overo” category tend to put white on the face, while tobiano does not put white on the face.
Many
years ago, overo was thought to be recessive.
This was mainly because of what were called “cropouts”,
where a loud body-spotted foal was born to two parents with “regular
markings”. At the time, the Quarter
Horse registry (among others) would not register horses with white body spots,
so the QH “cropouts” were registered as Paints. Other breeds were not so lucky, although most
of those denied registration in their breed registry would be accepted into the
Pinto registry. The vast majority of “cropouts” are in the “overo” category. You will recall last time we looked at the
so-called “crypto tobiano” where the horse has the
gene, but does not display any body spots, so can be
mistaken for a non-pinto. This is quite
rare with tobiano, but much more common with the
“overo category” genes. It is not known
why this is so. At any rate, as
knowledge grew, and statistics were analyzed, it was understood that it was not
recessive; it was just more prone to minimal expression.
Within
the “Overo” category, there are three main patterns. These names are based on phenotype
(appearance). Although more and more
genes are being found all the time, not all are isolated yet. The three general sub-categories of “Overo”
are: Frame, Splash, and Sabino. Within those
patterns are varying numbers of actual spotting genes.
At
this point it is worth mentioning that there are some people who believe that a
horse should only be called a specific pattern if it has tested positive for
that gene. This is wrong for many
reasons, one being that horse color and pattern names were around long before
any of the genes were discovered, and the name of a color does not necessarily
correlate with the name of the gene.
Another is that these genes are far from all known. These pattern names refer to a horse with a
specific phenotype, whether or not the causative gene(s) have been found. But if you spend time on the Internet,
especially Facebook, you will undoubtedly run across some of these
“know-it-alls” that insist on such things.
Feel free to ignore them.
Frame Overo
The
first pattern is called Frame Overo, or just Frame. This is the pattern most think of when they
hear the term “overo”. It is also called
“Lethal White Overo”, or LWO. It had
been known for many years that breeding overo x overo had the potential to
result in what is known as a “lethal-white foal”. These foals are born all-white (rarely, with
a few bits of color) and they do not survive.
This is called LWOS, for “lethal white overo syndrome” (or OLWS, overo
lethal white syndrome). It took some
time to figure out, but once it was understood that there are various other
kinds of “overos” (thus explaining the fact that
lethal-white foals comprise far less than 25% of overo x overo breedings), and that the pattern can be very minimally
expressed (thus explaining the occasional lethal-white foal coming from one
“solid” parent), then it was only a matter of time and observation before it
was well understood that only the Frame Overo pattern was the source of the
lethal gene, and that the lethal-white foals were the ones that were homozygous
for the gene. Once the gene was
isolated, testing confirmed that.
Like
all pinto patterns, Frame can range from just a little white to mostly
white. This gene puts white on the face
first, often with one or both eyes blue, whether the skin surrounding the eye
is white or dark. It’s not uncommon for
the face white to be more extensive on one side than the other. Body spots tend to start on the side of the
neck and barrel, and spread out in a roughly horizontal direction. The white typically does not cross the back between
withers and tail, and the tail is almost always dark. The chest and back side of hindquarters
usually remain dark, hence the name Frame – it’s as if the white parts are
“framed” around the edges by dark. The
edges of the white markings are typically not smooth and rounded like some other
patterns, nor roany or lacey like some others. They tend to be rather jagged with a “torn
paper” type of look. Frame does not put
any white on the legs (in the absence of another different white-spotting or
white-markings gene, of course). Just as
we saw last time that lots of white on the legs combined with a dark face is
highly suspect for Tobiano, the reverse is true here
– a large amount of face white combined with none on the legs is highly suspect
for Frame, especially if the eyes are blue.
Keep
in mind, all these characteristics are for a horse that is Frame only. This is not all that common in the Paint and
Pinto breeds since there are various other patterns and horses have often been
bred for as much “color” (actually, white!) as possible, so many of them have more
than one kind of pinto gene.
Frame Overo examples:
How it works
Frame
Overo is, like Tobiano, caused by just one gene. It is an incomplete dominant (you will
remember from the discussion about Cream, that this means a gene that has one
effect when heterozygous, and another when homozygous). The gene is called “O”. So “o/o” would mean a horse with the
recessive, normal allele, and “O/o” would be a horse with one copy of Frame
Overo, which will result in white spotting (to greater or lesser degree), and
“O/O” would be a horse that is homozygous for Frame, and would be a
lethal-white foal which is always fatal.
The gene has no harmful effect when heterozygous – only when homozygous. It is highly recommended that every breeder
of Paints and Pintos, as well as other breeds that have Frame in their gene
pool, should test every horse before using it for breeding. This is because the Frame pattern is well
known to “hide” on minimally-expressed individuals, or horses that have one or
more other spotting patterns as well.
The gene can be expressed so minimally that there is no outward sign of
it at all (this is admittedly rare, but losing a foal in this way is tragic for
all concerned).
The
Frame Overo gene was one of the first color genes to be isolated, in 1998. Unlike other colors where the primary motive
was curiosity and preference for one color over another, this one had the
urgency of saving lives because of the LWOS issue. Now that there is a test, it is a simple
matter to make sure to never breed two “O/o” horses together. The gene is on Chromosome 17 at the locus
which controls the “Endothelin Receptor b” (EDNRB). This gene is believed to be a relatively
recent mutation in North America. It
does not occur in breeds originating elsewhere. It is fairly widespread across
many breeds, though, even some that dislike spots. This is attributed to its ability to “hide”
(be minimally expressed) for even many generations.
Most
testing labs report the results as “O” for the dominant Frame allele, and “N”
for the recessive non-Frame allele, rather than using “O” and “o” as genetics
terminology would. They feel that it is
easier to understand that way.
Because
the homozygous Frame Overos all die, you can see from
the Punnett Square that breeding two Frame Overos
together does not increase your odds of getting a colorful foal any more than
breeding Frame to solid. It is 50%
either way.
|
|
O |
N |
|
O |
O/O |
O/N |
|
N |
O/N |
N/N |
O/O = lethal-white (25%); O/N = Frame Overo (50%); N/N = solid (25%)
|
|
O |
N |
|
N |
O/N |
N/N |
|
N |
O/N |
N/N |
O/N = Frame Overo (50%); N/N = solid (50%)
Splash, AKA Splash-White, AKA Splashed White
Splash
was once thought to be rarest of the “overo” patterns. However, as more was learned about it, it was
found to be more widespread than previously thought.
The
name is thought to have arisen because the general appearance is one of a horse
that walked through white paint and it “splashed” up onto him from the bottom
up. Additionally, it tends to look as
though the horse dunked its head right into a bucket of white paint. The edges of the white spots tend to be
fairly smooth, like the Tobiano, when no other
patterns are present.
The
“classic” or “typical” Splash pattern is actually caused when the horse is
homozygous for the Splash gene. This
description refers to that phenotype.
The
white starts on the legs and head, and spreads up onto the belly and
sides. White forelegs
up over the knee level, with a horizontal edge, is common for this
pattern and not others. The hind leg
white will generally run up the front of the leg. The face is generally all or mostly white,
with the white commonly ending in a smooth horizontal line about where a browband would rest.
The eyes are usually blue, and there is very often a ring of dark
“eyeliner” around them. This is unique
to this pattern. The tip of the tail is
often white. This is also very unusual
on any other pattern (tobianos with two-toned tails
typically have white at the top and color at the bottom).
One
interesting thing about Splash is that they are often deaf. Deafness has not been associated with any
other horse color. In other animals,
white coats and blue eyes are commonly associated with deafness. It may be that Splash works in a similar
manner, but not all of them are deaf. It
doesn’t seem to matter whether the ears are white or not,
or whether the horse is heterozygous or homozygous for Splash, but it does tend
to run in families. If neither parent is
deaf, it appears the foal is very unlikely to be deaf, no matter how much white
it gets.
How it works
Splash
is another incomplete dominant, like Frame and Cream. When homozygous, it produces the “classic”
example of the Splash pattern, as described above. But when heterozygous, it is far less
expressed. The appearance of a
heterozygous Splash with lots of white, and a homozygous Splash with little
white, overlaps to some degree. But a
horse with large body spots is surely homozygous, and heterozygous ones can
appear to be a solid horse. This is
undoubtedly one of the reasons that “overo” was thought to be recessive at one
time, and why Splash was thought to be relatively rare. In breeds which do have Splash in their gene
pool, but do not accept “pintos”, meaning white spots on the body, these
“pintos” would occasionally appear from two “solid” parents. Once the nature of Splash was understood
(that it was an incomplete dominant), this made perfect sense. The heterozygous ones just didn’t look like a
Splash (or a pinto, for that matter).
When
heterozygous, Splash can have quite a bit of face and leg white, but it can
also have very little. They may or may
not have blue eyes. If the white covers
the eyes and the eyes are blue, they often have the “eyeliner”, but not
always. They may or may not have the
white tail tip. There is usually no body
white, aside from perhaps a little on the underside of the belly. Horses with one Splash gene can be expressed
very minimally, just like Frame, thus allowing it to “hide”.
In
2012 researchers set out to find the Splash gene, and came up with a
surprise. They actually found more than
one gene that causes the Splash phenotype.
(There are also horses out there with the classic Splash pattern, but
testing negative for the known genes, so it is expected they will find
more.) Horses with the pattern are
still all called “Splash”, but if it is known through testing which gene they
have, it can be specified.
Even
more surprisingly, the mutations were found at two different gene locuses. The first
one, named SW1, is located at the MITF gene on chromosome 16. It is widespread in many breeds around the
world, so has been around for quite some time.
It is by far the most common one, and it behaves as described above.
The
second gene, called SW2, is located at the PAX3 gene on chromosome 6. The researchers believe that this mutation
occurred in a Quarter Horse mare foaled in 1987 named Katie Gun. All of the horses testing positive for this
mutation are descended from her. They
did not find any horses homozygous for SW2, and speculate, based on the way a
similar gene works in mice, that it may be an embryonic lethal. (Time will tell. A mare that is only 30 years old is unlikely
to be doubled up in pedigrees yet.)
Horses with this gene appear very similar to those with one copy of SW1,
perhaps somewhat more likely to be fairly minimally expressed, and are often
deaf.
Horses
which have both SW1 and SW2 do not appear to have large amounts of body
spotting like the SW1/SW1 do. The ones
observed so far look not much different from a heterozygous SW1 with a good
deal of white.
A
different mutation was identified at the MITF locus and named SW3. Not much information was given about this
gene, but they called it “extremely rare” and suggest that it too may be
embryonic lethal when homozygous. One
photo was shown of a horse with this gene, which appears to have a fairly
normal Splash-type phenotype, and a foal of that horse which has both SW1 and
SW3, and is all white.
Macchiato
An
oddly marked Franches-Montagnes horse was born in 2008 to two
solid parents. His pattern looks more
like what would fall under the “sabino” category, but
he also has an odd pigment dilution, and is deaf. The mutation which occurred in this horse to
cause his pattern was unknown at the time, and was named “Macchiato”. The researchers discovered he had a unique
mutation at the MITF locus. Technically that would make him a type of Splash,
although researchers declined to name this SW4, perhaps in deference to what
his breeders were already calling the color.
“Macchiato”
Additional
research later found one more Splash mutation at the PAX3 locus, which is
described thus far only as “a very rare form of
splash white found in Appaloosas”. More
info will be added when it is published.
Sabino
“Sabino” is now a phenotype term, not a
genetic term, just like “overo”.
At one time it was thought to be one gene, but as the widely varying
types of expression mounted up, and genetic testing in mice discovered dozens
of different genes causing similar-looking white spotting, it became obvious
that this category would be a large one indeed, and so it has turned out. We now know there are many different
mutations causing the sabino patterns, all (so far)
at the same locus.
The
basic sabino appearance is a horse with white
starting on the legs and face, and spreading to the belly, flanks, and further
in some cases, up to and including all white.
The edges of the white spots are usually lacy or roany,
sometimes with extensive roaning. The eyes are usually dark, no matter how much
white the horse has. (Blue eyes have
been known to pop up randomly in some lines.)
The more minimal expression would be a horse with “lots of chrome”,
blaze and stockings, with jagged or lacey or roany
edges, and perhaps a little belly spot.
A louder one would have more face white, stockings extending above the
knees/hocks (typically running up the back of front legs and the front of back
legs), more white on the belly, and perhaps more roaning
around the flanks, chest, and belly.
Really loud ones will have wild spatters and splotches of white on the
body, and some may have roaning all over. And maximally expressed ones will be nearly
all white, with varying amounts of small dark speckles, perhaps a “medicine
hat” marking (base color only covering the ears and surrounding skin, like a
hat). Some are heavily roaned all over, and these have been called “sabino-roans”.
When
you think of sabino, think of Clydesdales. They are almost all sabino. Most have big blazes and stockings, many have
belly spots, some have extensive roaning (in fact
many are registered as “roan” although true Roan does not exist in the breed),
and a few are all or nearly all white.
The gene causing the sabino pattern in this
breed has not been isolated yet.
These
patterns are present in nearly every breed of horse except for a few that have
strongly selected for minimal or no white markings for many years. Like the other “overo” category patterns,
they can be expressed so minimally as to appear solid, or “just ordinary
markings”, and then be expressed much more in the offspring, leading to
“surprise” pinto foals. At this time
there is no effective way to draw a line between “regular markings” and
“minimal sabino type markings”. As more and more gene variations are found,
we may someday know for sure which horses with “lots of chrome” have the
genetic potential to produce loudly-spotted or even all-white foals, and which
will never go beyond a certain point.
But that does not look likely any time soon.
How it works
In
2005, a gene was found in the “KIT” complex on chromosome 3, which causes a sabino phenotype, and it was named Sb1. Not all sabino-patterned
horses had this gene, and the researchers fully expected there would be more
genes found. But there’s a twist in the
tale, which we will get to in a moment.
Sb1
is another incomplete dominant, but with quite a lot of variability. The heterozygous horses typically have lots
of leg and face white, lots of roaning, maybe a belly
spot. The homozygous ones are typically
all or mostly white. The gene is
widespread but only in a limited number of breeds. Note that although the homozygous horses are
(more or less) all white, there are no health issues
associated with the color in this case – unlike the LWO homozygous Frame.
Meanwhile,
a couple years after Sb1 was found, a different lab was searching for mutations
in the same exact gene – the “KIT” complex – in horses that were all white or
nearly so. They found four different
mutations that consistently caused a white phenotype when only one copy of the
gene was present (unlike two copies with Sb1) and they chose to name these “W”
for “White”, 1 through 4. At the time
they used the name “Dominant White”.
Dominant, because it was a simple dominant, only needing one copy of the
gene to have the effect, and White, because the horse was usually all or nearly
all white when they had it.
However,
this created a headache for horse color genetics researchers. To understand why, it’s necessary to look
into the past for a moment. We have
touched on the fact that before any genes were actually isolated, horse color
gene theories came and went as observational and breeding studies proved or
disproved them, and a starting point was often known color genes in other
species, especially mice, which have been more thoroughly studied. There is a gene in some other species called
Dominant White, “W”, which is entirely epistatic to all other colors, and
creates a pure white individual when one copy is present, and is a homozygous
lethal. This was thought to exist in
horses, and books through the 70s and 80s state it as fact. A study was done in the 60s that was claimed
to prove its existence. (However, with
hindsight, we now know that the study had some serious issues and really didn’t
prove anything.) When a pure white horse
would pop up in various breeds, it would be labeled as a “Dominant White”
without regard for the most obvious fact of all about a dominant gene – that
one parent must have it in order for the foal to have it. And these white horses came from two
non-white parents…. but often from at least one parent with “lots of chrome” or
a loud sabino type pattern. Through the 90s horse color researchers came
to the conclusion that there was no true “White” gene in horses – just as there
is no true albino in horses – because they do not produce as such a gene would,
i.e. 50% pure white foals and 50% solid dark foals. Instead they produce varying amounts of
splashy white pintos, solids, and occasionally whites,
and the all-white ones usually have at least some, and often quite a lot, of
residual color, as patches of color or speckles of dark skin. The old “Dominant White” label was dropped,
since the gene of that name in other species is not the same as what we find in
horses. The term “maximum sabino” became preferred, even after Sb1 was found, and
with it the knowledge that there was more than one gene out there causing the sabino patterns. It
was important to use a different term so that people would understand that this
is not “a gene that makes the animal pure white” but rather a “white spotting
gene that can sometimes cover the whole horse with one big white spot”. A subtle difference
perhaps, but important.
So
the horse color genetics community was widely disappointed that the researchers
chose to resurrect that old term and call these new mutations “Dominant
White”. But they were apparently unaware
of the controversy, and the first few mutations they found did result in a
mostly-white horse most of the time, when they had one copy of the gene.
A
couple years later, several more unique mutations had been found in the same
“KIT” complex and had been labeled W5 through W11, but now the name was
beginning to be a problem, because some of these mutations caused all-white
phenotypes and some didn’t. Most were
varying levels of sabino-roan or sabino-spotted
phenotypes. Some produced white coats
frequently, some occasionally, some never. The old “Dominant White” label was not
working, so it was discarded without fanfare and replaced with “White Spotting”
as the name for the W series genes. Most
horse color experts believe they should have gone back to the “Sb” label for
the clearly sabino type markings, but for whatever
reason that did not happen and they just kept adding numbers and calling them
W.
The
20th such gene to be isolated, named W20, was perhaps the catalyst
for the name change. This gene does not
fit into the “Dominant White” category at all, for it does not cause all-white
on its own, nor even body spots or roaning, but
rather more like “ordinary white markings” on face and legs. It also can be homozygous with no problem
(some of the other W mutations are theorized to be homozygous lethal). By itself, it does not seem to do much, but
when combined with another of the W mutations, it seems to boost the pattern. There are all-white horses which are both W5
and W20, or W20 and W22.
All
this genetic discovery has shown us that the “KIT” locus is a very large, very
unstable one, which apparently is quite prone to mutations, and mutations here
appear to (so far anyway) only affect the horse’s pigmentation, with no ill
effects elsewhere. Most of those found
so far are fairly recent, with the “founder” (horse it first occurred in)
known. W20 is the only one so far that
is very widespread and probably quite old (another reason most experts feel it
should have been named Sb2 and not W, but it’s the discoverer’s choice what to
name a gene). Many of the others can be
traced back to a specific horse, which was born white or near-white or loudly
spotted, from solid parents. It’s also a
very large locus, so a horse can actually carry more than just two
mutations. At least one has already been
tested that is homozygous for W20 and heterozygous for W22. This means that the two mutations are located
far enough apart that they don’t “occupy the same address”. This is unusual. There are still plenty of sabino-type
patterned horses out there that test negative for all the currently known
spotting genes, so we fully expect that more will be found.
Face White
As
mentioned, the one characteristic that all of the “overo” genes have in common
that Tobiano does not, is face white. The three general sub-categories of overo
patterns all put white on the face to some degree, but they vary in the
appearance of that white. It’s interesting
to compare and contrast them. Of course,
they vary a lot, and can be very minimal, but when considering each pattern
separately at a moderate degree of expression, they do show different
tendencies. Splash tends to have a
smooth line across the forehead, tends to go right over the eyes, and blue eyes
with “eyeliner” are common. Frame tends
to go down the side of the face between eyes and nose, and blue eyes are common
even if surrounded by dark skin. Sabino tends to wrap around under the jaw, seems to avoid
the eyes, which are almost always dark, and is usually jagged or lacy.
Sabino face Frame face Splash face
Manchado
In
addition to the general categories of Frame, Splash, and Sabino,
there is one more unique and rare pattern.
It does not fit neatly into any of these categories and the genetic
mechanism is unknown, but it is generally considered to fall under the “overo”
umbrella since that just means “anything not tobiano”. This pattern is called Manchado. It is unlike any other pattern. The white tends to start along the topline
and spread down, more like tobiano. They typically have round or oval spots of
color within the white areas, which can resemble appaloosa spotting more so
than pinto spotting at times. The
pattern itself does not appear to put white on either the face or legs (they
seem to have only “ordinary markings” such as star, blaze, socks, etc.) but it
very often does put white on the tailhead (the whole
tail may be white) and most oddly, on the ears and forelock area, which is
generally the last place all the other pinto spotting genes spread to. This pattern has only been found in
Argentina, although in various breeds, and it is so rare and sporadic that it
is thought to be recessive, or possibly even have an environmental factor.
There
is an interesting article about the subject here: http://equinetapestry.com/2011/06/speculation-about-the-environmental-nature-of-manchado/
Combining Pinto Genes
One
of the most interesting things about the pinto pattern genes is that they are
located in different places, so they can occur in combination. A horse could carry as many as half a dozen
different pinto genes. Since the
patterns tend to put white on somewhat different parts of the body in many
cases, a horse with multiple different spotting genes will tend to have more
white on its body. The name given to a
horse with at least one Tobiano gene and any one or
more of the overo category genes is “Tovero” (or some
call it “Tob-Overo”).
This name was invented by the registries – both Paint and Pinto began
using it about the same time – and since the genes were unknown at the time,
these registries really only use the term for a really obvious example. A tobiano-patterned
horse with a white face, for example.
If you had a tobiano looking horse who was known by testing to also carry an “O” gene, but did
not outwardly show it (just a small blaze, say) the registry would not call it Tovero. In other
words, it’s a phenotype term, not a genetic term.
SACRED
INDIAN (APHA)
The
only colored hairs on this stallion are the dark hairs around his ears.
He is homozygous for Tobiano and also has one copy of
the Frame Overo gene. He likely carries
other spotting genes as well.
It
is not just possible, but likely, that many “overo” horses have more than one
gene creating their patterns. Here is an
example of a Paint stallion that has one copy each of Frame and Splash (SW1),
as well as at least one sabino type pattern
gene. You can see the different
characteristics of each on his body, and he has sired foals with only one, or
two (in various combinations), or all three patterns. It’s not always possible to tell by looking,
but if a horse has enough offspring, it can be sorted out which patterns he
has. This stallion sired over 100 foals,
many from solid mares, so it became obvious over time that he had all three
patterns.
From
a color breeder’s point of view, intriguing statistics are derived from horses
carrying more than one different pinto gene. A horse carrying two separate
pinto genes should produce a pinto offspring 75% of the time from solid mates.
A horse having three independent pinto genes should produce a pinto foal 87.5 %
of the time from solid mates. For the
horse carrying four different genes, the percentage of pinto foals from
solid partners hits an amazing 94%.
Adding one or more pinto genes in the other parent, and you could be
nearly guaranteed of getting at least one pinto gene in your foal, even if
neither parent is homozygous for anything. (Of course, when it comes to the
“overo” category genes, just having one copy of one gene is no guarantee of
getting a “pinto”, meaning a horse with white spots on the body. As we have seen, these can be surprisingly
minimal in expression.)
The
problem is that multiple copies of pinto genes often produces
more white on horses. This can include mostly to all white. And a
horse that is all white, even though this is caused by pinto genes, is
considered “solid” by the Paint registry.
This is something to be avoided in most breeders’ opinion (horses
registered as “solid” have fewer opportunities in the show ring). Additionally, many people do not like white
horses. They are hard to keep clean, and
may have issues with sunburn. Pink skin
around the eyes is more prone to developing skin cancer. When breeding pintos, though, it’s pretty
much impossible to predict the outcome.
So, if a person really wanted an all-white horse, or really wanted one
that’s mostly dark but with a few colorful spots, they would be well advised to
go out and buy one that they like, rather than trying to breed for it.