Equine Coat Color Genetics
Lesson Seven
Patterns of White
Part One – General Overview and Tobiano
Last
lesson we talked about genes which add white hairs to the horse’s coat, usually
mixed in rather uniformly, without affecting the pigment in the skin
underneath. The other category
of “white adding” genes are the kind that cause white areas of any size
with depigmented skin underneath. There
are many different genes that do this.
One important thing to keep in mind is that, like grey and roan, these
patterns are superimposed on top of whatever color the horse would have been
without them. On horses with large
amounts of white spotting, it can be hard to tell what the base color is, but
every horse has one.
If a grey horse
has white pattern markings, the colored areas will gradually fade as the horse
goes grey, until the whole horse is white, making it hard to tell where the
spots were. But the skin will remain
pigmented under the grey parts, so the spots can be seen when the horse is wet
or shaved.
Broadly
speaking, white patterns are broken down into three general types: White markings, pinto type spotting, and
appaloosa type spotting.
“Regular” white
markings such as stars, snips, and socks occur in almost every breed and the
genetic mechanism is unknown. Some of
the known spotting genes do cause markings such as these when minimally
expressed, but the vast majority of horses with ordinary markings do not carry
a known spotting gene. There are a wide
variety of names for the various types of markings. Most registries have their own
terminology. The Thoroughbred registry
has one of the largest vocabularies for describing white markings. You can see them here:
and
here:
(If those links
no longer work, you can search the jockeyclub.com site for “registration” and
“how to identify”.) Most every other
major breed registry has a similar guide on their website. We won’t go through all the names for all the
various types of markings, since they vary somewhat from one breed to another
and from one place to another, and since the genetics of them are unknown at
this time.
Appaloosa type
spotting is unique and will have its own section.
The third broad
category is generally called “pinto” spots but has many other names as
well. In Europe, the appaloosa type are called “spotted” and the pinto type are called “coloured”. It has
been called “broken” coated (as in, the dark areas are “broken up” by white
areas) in some places. In England in the
past, black and white pintos were called “piebald” and any other color and
white was called “skewbald”. (This
terminology persists to some degree in some places, but is discouraged since it
tells you nothing about the actual pattern involved.) Another term you may come across is
“particolored”. In America they are
commonly called paint or pinto.
All of the color
patterning genes can occur singularly or in any combination. A horse
carrying both appaloosa and pinto genes is referred to as a “pintaloosa” and is unregisterable by
the American Paint Horse Association, Pinto Horse Association or Appaloosa
associations. These breeds are rarely
crossed. However, Miniature horses do
carry all these genes, and pintaloosas are welcome in
their registry, so there are some out there.
One
important note about white spotting genes. They do not control “how much” white the
horse has, only the “location and character” -- where on the body it will be,
and what it will look like. Horses with
more than one kind of pattern gene tend to have more white, but not
always. It is poorly understood what
makes one horse have much more white than another, and it may be to some extent
environmental or pure chance, since clones do not have exactly identical
markings. However a general tendency
toward lots of white or very little does seem to run in families to some
extent, so probably has a genetic basis.
This explains how some of the pinto patterns were thought to be
recessive for many years. It could
“hide” on a minimally-marked horse (one that does in fact have a pinto gene,
but it looks like ordinary markings) and then when crossed with a horse that
happens to have genes for “lots of white” – voila! A loud, body-spotted pinto
results.
The Paint/Pinto Coat Patterns
First let’s
discuss the terms “Pinto” and “Paint” and mark the difference in the use of
these terms. When not capitalized, they
refer to a horse of any breed with white body spots of any pattern. Paint is more common in the West, and pinto
in the east, but “pinto” is a general term for any white spotted horse. When capitalized, it is generally understood
to mean a horse that is registered in that registry. The Pinto Horse Association (PtHA) is a registry for patterned horses of many breeds,
with the exception of any Appaloosa, draft or mule breeding and/or
characteristics. More info: http://www.pinto.org/index.php/about-us/the-breed The American Paint Horse Association (APHA)
is a “stock horse” breed which only includes horses of APHA, Quarter Horse, or Thoroughbred blood.
More info: http://apha.com/association/history/
These registries
are widespread and popular, and have worked to increase the horse-owning
public’s understanding of pinto patterns.
Unfortunately, when it comes to genetics, they are both somewhat “behind
the times”. The Paint and Pinto
registries both break down patterns into two basic types: Tobiano and
Overo. (A horse with both patterns is
called “Tovero”.)
Over the years it has been found that there are many more white-spotting
genes than just two. Tobiano
remains separate and is caused by only one gene, but the “Overo” category
really just means “anything other than tobiano” now,
and there are many genes in that category.
We will leave them for next time, and focus on just Tobiano
this time.
What a Tobiano
Looks Like
The pattern of a
tobiano (pronounced tow-bee-yah-no) is evident
from the moment of birth. The white areas characteristically cross over the
back and produce four white legs at least below the knees and the hocks. Tobiano has often
been said to look like “a white horse with dark spots” (as opposed to overo,
which is opposite), however, this is rather a matter of opinion.
Typical
tobiano features - white legs, white crossing over
the back, a neck shield and a dark head.
The dark (base
color) body spots have smooth and distinct edges which are usually rounded
rather than jagged. The chest may be
dark creating a "shield" and at least one flank is usually colored.
The head of a tobiano is usually dark, and Tobiano does not put white on the face. (They may have regular white markings like
any non-pinto horse, of course.) This is
true even in mostly white tobianos, as the dark color
of the head is the last place to be covered up by white. Tobiano does not
cause blue eyes, so the eyes are usually dark, although blue eyes can pop up in
rare cases in any horse. The tail may be
two-toned, which is rarely seen with other patterns.
Tobiano
markings may range from an almost solid white horse with color only on the ears
to a almost solid animal
with white only on the legs. The APHA
shows typical Tobiano patterns as:
Images
of Tobiano coat patterns. The consistency are
the white legs and a dark head.
How the Tobiano
Gene Works
The tobiano spotting pattern is caused by a large chromosomal
inversion (you could think of part of the chromosome being upside down) in the
“KIT” complex on chromosome 3. The tobiano gene is dominant so is called “T”.
(Sometimes you will see it written as “To”).
A heterozygous tobiano would be Tt. Fifty
percent of the foals produced from a heterozygous tobiano
should have the tobiano pattern.
To the delight of Paint Horse
breeders, the homozygous tobiano exists.
Homozygous tobianos result from the mating of two tobiano parents and, per a Punnet
Square, one out of every four foals produced by two heterozygous tobiano parents should be a homozygous tobiano.
Having two
copies of the tobiano gene does not increase the
amount of white on the horse, but, the individuals carrying this genotype can
often be identified by the small dark hairs scattered in clusters in the white areas
of its coat. Most, but not all homozygous tobianos
have these "ink spots" or "paw prints", and most, but not all, heterozygous tobianos do not
have them. There are documented
exceptions, though, so it’s not a “for sure” thing.
CAN
DO FULL COLOR (APHA)
A homozygous tobiano
stallion with characteristic ink spots.
Homozygous tobiano
are prized for their ability to produce tobiano offsprings. Statistically, every foal produced from the
mating should receive one copy of the dominant tobiano
gene "T", thus creating a tobiano.
Sonny
Double Dee (APHA)
Homozygous
tobiano mare with paw prints on hip.
Minimal White Tobianos
Since white pattern genes such as Tobiano do not determine “how much” white the horse will
have, occasionally there will be a tobiano that does
not have any white spots on its body.
These horses are often referred to as “crypto
tobianos” or “slipped tobianos”. (Think of it as the
“white” has slipped down the body to the lower legs.) For these horses,
the tendency of the tobiano to have a dark
head and white legs holds true even if the white legs are no more than white
pasterns or fetlocks. Horses with ordinary markings tend to have similar
amounts of white on both legs and head. A small star and a small sock or two, or a large blaze with four
stockings, for example. This
combination of white legs and a dark face is highly likely to be a tobiano.
An example of a “Slipped Tobiano”.
Notice
the lack of Pinto markings on the body and white limited to the legs only
The Paint and Pinto
registries have strict rules about minimum sized body spots that make a horse
eligible for registration. A horse
without body spots is called “Breeding Stock” or “Solid” even if they actually
have a pinto gene. Even though horses
such as these are registered as “Solid”, they are, in fact, tobianos
and do pass on the dominant Tobiano gene 50% of the
time (or 100% of the time if they are homozygous, TT).
Testing for the Tobiano
Gene
The inversion
associated with the Tobiano pattern was identified by
researchers at the University of Kentucky.
Since the gene has now been isolated, there is a DNA test, available at
most labs. One example: http://animalgenetics.us/Equine/Coat_Color/Tobiano.asp
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