Patterns of White
Part
IV of a series on equine color genetics.
By Tracy
Williams
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| In
our final color genetics discussion, we will be exploring many
of the flashier shades in the equine color scheme: gray, roan,
paint/pinto patterns and leopard spotting. Genetic control of
these shades is different than that of the base colors we have
previously discussed in past articles. Base color is the result
of gene combinations, separate genes that interact to create
the color you see. For example, a dun horse is the result of
genes at the Agouti locus interacting with the genes at the
Dun locus (see Part II). In contrast, each white pattern results
from separate factors that act independently from each other.
In other words, while it is only possible for a horse to have
one base color name (bay, chestnut, dun, etc), it is possible
for a horse to display more than one white pattern.
White
patterns are generally broken down into two major categories:
individual white hairs mixed in with colored hairs and discrete
patches of white hairs. The latter group contains many subcategories
including both asymmetrical white spotting (paints and pintos,
for example) and symmetrical spotting (leopard complex, for
example).
Intermixing
of White/Colored Hairs
Gray
Gray is the most common white pattern. Most gray horses are
born an initial base color but progressively gray then whiten
as they grow older, usually acquiring dapples in the process.
The speed of this development depends on the breed and the
individual, but no matter how white the coat becomes, the
skin usually stays pigmented. The graying process is also
somewhat affected by the base color of the foal, indicating
there might be some genetic interaction between base color
genes and the gray gene. Chestnut, bay, brown and black foals
often will darken first before turning gray, which makes it
difficult to tell what the base color would have been. In
contrast, light colors like dun, cream, champagne and silver
dapple usually won’t darken, which creates interesting
intermediate shades as the gray slowly mixes in.
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Shades
of Gray |
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| Gray
horses are typically darker in their younger years and
progressively whiten with age, usually acquiring dapples
in the process. 1997 Kentucky Derby winner Silver Charm
was born a jet-black foal. By the age of 3 he sported
a dark dappled coat. Today, at age 14 the famous stallion
is nearly white. |
|
There are two main types of gray horses. The first type loses
color in the mane and tail and can become completely white with
age, although still retaining pigmented skin. The other type
maintains black pigment in the mane and tail, and the graying
process tends to stabilize before the horse turns completely
white. In either case, the various gray stages spark many different
terms: dappled gray, iron gray and porcelain gray are a few
examples. In addition, many gray horses have blood marks –
distinct, large patches of usually red color that tend to enlarge
with age and seem to have no relationship to the horse’s
underlying base color.
Gray is caused by a single dominant allele (GG). Thus a horse
that is heterozygous or homozygous dominant will be gray. The
speed of the graying process is also at least partially genetically
based but not due to whether the horse has one or both dominant
alleles.
Roan |
The
true definition of roan is any mixture of white and colored
hairs where the white is scattered (not in patches), a definition
which could technically be applied to gray horses. Traditionally,
however, a roan’s body displays intermingling of white
and colored hairs, while the head, mane, tail and lower legs
stay the base color. Roan is not progressive like gray is;
usually foals are either born roan or shed out to roan and
do not gradually lighten over time. However, this can vary
depending on the season or the horse’s age. Many roans
are lighter in the spring and darker in the winter, and some
will also darken with age. In addition, a horse’s base
color influences terminology; roans have specific names depending
on the base color (See Table 1). Roan occurs in most breeds
except those that specifically select for certain colors (i.e.
Cleveland
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A
traditional roan has white hairs mixed with colored hairs
over the entire body but maintains a solid colored head. |
Bay
or Suffolk Punch).
Roan is caused by a single dominant allele (RnRn), and there
is no visual difference between homozygotes and heterozygotes.
Either will be roan. However, some research has documented
the loss of homozygotes during early embryonic
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development,
but this has not been well-studied. Many lightly roaned horses
reproduce as though they were non-roan, which indicates other
factors may be involved in the genetic process besides the
roan gene. The roan locus is known to be linked to other loci
(i.e. Extension locus, Tobiano locus), and this linkage can
alter the normal predictability of roan versus non-roan offspring.
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Table
1: Roan Colors |
Base
Color |
Roan
Terminology |
Bay
Black
Brown
Chestnut |
Red
Roan
Blue Roan
Purple Roan
Strawberry Roan |
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Discrete, Asymmetrical White Patches
There are several groups of white patterns that fall within
another realm where all the white hairs group together as
separate entities from colored hair patches. These can be
either irregular or regular in nature. In the asymmetrical
category, there are four main groups, although in North America,
three of the four are lumped under the title “overo”.
However, each pattern has proven to be genetically unique
with distinct physical characteristics that set it apart
from the others. To complicate this, these patterns don’t
always affect horses to the same degree. In fact, each pattern
has groups classified as either maximally or minimally marked
by the pattern, and these horses can often be confused. Furthermore,
there are occasional cropouts (spotted foals from nonspotted
matings) and horses that have no physical manifestation of
spotting genes although they exist in their genetic codes
(termed suppression). Obviously, these four spotted patterns
need further study, but the basic genetic framework exists.
Tobiano
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Tobiano,
the first of our four categories, is a recognized pattern
world-wide. These horses are characterized by white body marks
that cross the topline somewhere between the ears and the
tail. These marks usually have defined edges and a vertical
character – as though the horse had been dripped with
white paint. In addition, these horses frequently have white
hooves and lower legs with minimally marked heads and dark
eyes. Their tails are usually mixed with both white and colored
hairs. A minimally marked tobiano will simply have four white
lower legs, a small spot of white somewhere on the topline
and a solid head, while a maximally marked tobiano can have
a completely white body and colored head (only sometimes preserving
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A
classic tobiano has white spotting that crosses the topline,
white legs and hooves and a minimally marked head.
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colored
patches on the chest or flanks). These extremes can make it
challenging to identify tobianos based on appearance alone.
The genetic control for tobianos is a single dominant allele
(ToT). In other white patterns, the homozygous dominant genotype
is lethal to the foal, but a tobiano is different. Homozygous
dominant tobianos do exist and are viable; by appearance alone,
they cannot be distinguished from heterozygotes. Minimally
marked tobianos sometimes are overlooked, which can lead to
progeny surprises. A helpful distinguishing feature is that
these horses usually possess the otherwise unlikely combination
of extensively white legs with a minimally marked head.
Frame
The first of three in the overo group is the frame pattern.
Frame horses are characterized by white spotting that splashes
the body horizontally across the horse’s sides and rarely
crosses the topline. They commonly have extensively white
heads (regularly with blue eyes), dark hooves and light to
non-existent white marks on the legs. Minimally marked frame
horses sometimes don’t have any body spotting, but they
often are still baldfaced. Maximally marked frame horses usually
have significantly white side spotting that can cross over
the topline. They also have nearly white heads and colored
lower legs and feet – an unusual combination in non-frame
horses.
The genetic control for this pattern is caused by a single
dominant allele (FrF), although this simple genetic explanation
is a bit problematic. This pattern has occasionally appeared
surprisingly in new breeds, which is probably due to new mutations
of this allele. There is also the possibility of occasional
masking by some unknown genetic mechanism, which would explain
the existence of frame horses that show no physical signs
of possessing the frame gene. The frame allele is also responsible
for lethal white syndrome in foals. No viable homozygotes
have ever been documented, and DNA testing is now available
to help breeders avoid mating two carriers of the dominant
frame gene, which can result in a lethal white foal.
Sabino
Sabinos are the second group in the broad overo category,
and this can be an ambiguous and confusing pattern to understand.
The strict Spanish definition of the word sabino translates
to “pale red” or “roan”, which muddies
the waters of what physical classification these horses should
have. General convention classifies these horses as having
body spots usually on the belly. However, these body marks
are rarely discrete white patches as in other patterns. More
commonly they are roaned or speckled areas – combined
with extensive leg and facial white. In fact, most sabinos
are flecked or roaned somewhere on their bodies, and in almost
solid white sabinos the only visible color can be roaned or
flecked areas. Minimally marked sabinos have extensive white
marks that tend to narrowly extend down the legs and throat
like “lightning strikes”; they can also be very
lightly roaned. Maximally marked sabinos vary. Some have small
flecks of color scattered throughout; some are white with
colored ears, and some are white with roaned or speckled patches
on the ears, chest, tail base and along the back.
Genetic control for this pattern is possibly due to a single
allele or could be a result of polygenic factors, a theory
supported by the fact that this color tends to produce better
by horses with more white on their bodies. Some lethal whites
have occurred, which is probably due to sabino and frame combinations
because viable white foals also exist within this group. Some
obviously marked sabinos have plain parents and produce the
pattern, which is a bit of mystery and probably due to a masking
genetic mechanism. The confusion surrounding sabino genetics
could possibly be due to subcategories existing within this
pattern that have yet to be classified.
Splashed
White
Splashed white horses, the third overo pattern, are characterized
by white spotting on the lower half of their bodies –
white on the legs that extends up over the belly and sides
with defined edges, often appearing as though they were
dipped in white. Frequently their heads are extensively
white with blue eyes. Minimally marked horses are solid
colored with extensive facial and leg white. Maximally marked
horses are white with colored patches on the ears and topline
and are often confused with minimally roaned/speckled sabinos.
The
genetic control for this pattern is due to a single dominant
allele (SpaS). In North America, splashed white horses are
a bit rare, thus the genetic control needs further study
as the pattern gains significance. Homozygotes are not well-documented
but could exist without lethal white syndrome. Some splashed
white horses are deaf, and the genetic relationship between
these two conditions needs research.
Discrete, Symmetrical Spotting (Leopard
Complex)
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The
other main group of horses that possess discrete white patches
tends to contain patterns symmetrical in nature, collectively
known as the leopard complex. Symmetrical spotting is characteristic
of the Appaloosa and the Pony of the America breeds. It is
a collection of different but related patterns that can occur
in combinations: mottled, frost, snowflake, varnish roan,
speckled, blanket, snow cap blanket, leopard and few spot
leopard. Some of these patterns, like blanket and leopard,
are present at birth, while others, like varnish roans, speckled
and snowflake, develop later in life. Often horses with symmetrical
spotting also have skimpy mane and tails – termed “rat
tailed” as a result.
Blanket
A horse with this pattern has white covering the croup and
hips and can have dark leopard spots dotting the blanket.
If there are no spots, it is referred to as a snowcap blanket.
Leopard
Leopard horses are white with dark spots scattered across
the white surface. The color of the leopard
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A
horse with leopard spotting has a white body with symmetrical
spots dotting the surface; the spot color is thought to be
what the horse's base color would have been.
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spots
generally indicates what a horse’s background color
would have been. Few spot leopards are mostly white, maintaining
bits of color on the flanks and head.
Mottled
Mottling is a pattern that often coexists with other leopard
patterns. It results in small points of color on the skin
of the anus, genitalia, mouth and eyelids. If the background
skin is pigmented, the dots are pink or white; on pink skin,
the dots are pigmented. Mottled skin, together with white
sclera and striped hooves, accompanies many leopard patterns.
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Frost
Frosted horses have the roan pattern (white hairs intermixed
with colored hairs) stretching over their croup and hips.
Snowflake
Snowflakes are white patches, approximately 3 cm in diameter,
sprinkled over the coat. Sometimes, as a horse ages, these
spots widen until the horse technically becomes speckled,
flecks of color on a predominantly white surface.
Varnish
Roan
A varnish roan has a mixture of dark and white hairs throughout
the body much like a classic roan. However, over bony areas,
the darker hairs cluster together, making a varnish roan visually
distinct from a typical roan.
Genetic
control for all these patterns is caused by a single dominant
allele (LpLp), while separate modifiers fashion each distinct
design. Heterozygotes tend to be darker than homozygotes (i.e.
few spot leopards are often homozygotes), although this is
not universally true. An important sidenote is that many homozygotes
tend to be night blind. |
LEG
AND FACIAL MARKINGS |
| Leg
and facial markings are the most widely distributed white
pattern. These marks are controlled by several different
genes existing at separate loci, and these genes work
together to create the unique combination of leg and facial
markings on an individual horse. There seems to also be
a genetic interaction between base hair color and white
markings that hasn’t been fully studied. One observed
phenomenon is that chestnuts are generally more extensively
marked with white than bays. Other white patterns can
also influence and merge with facial/leg markings. |
Facial
Markings |
Leg
Markings |
Star
Strip
Snip
Stripe
Blaze
Bald Faced |
White
Coronet
Half Pastern
White Pastern
White Fetlock
Sock
Stocking |
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White
White
horses are rare and are characterized by a solid white hair
coat, pink skin and dark eyes. Some of the other white patterns
(like gray, sabino, splashed white and few spot leopards)
can be confused with true white horses. The color is caused
by a single dominant allele (WhW); a homozygous dominant
genotype at this locus produces lethal white syndrome. Thus,
all white horses are heterozygous for the trait and are
capable of producing colored foals.
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We
have come to the end of our color genetics journey. We began
with the simple base colors, and from this beginning we
have broadened our scope to include the wide varying shades
and patterns existing in horses throughout the world. The
possibilities are nearly endless, created from gene combinations
and other known and unknown modifiers that can even include
the environment the horse lives in. When trying to decipher
what your horse’s true color is, take every detail
into consideration – hair coat, point color, white
pattern, offspring, etc, and be aware that things are often
not as simple as them seem.
Links to parts 1, 2 & 3 of our series on equine color
genetics:
Part I: Equine Color Genetics
Part
II: Modifications to Dark Colors
Part
III: Donkey Color Genetics
Part IV: Patterns of White
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Overo Lethal White Syndrome |
| Overo
Lethal White Syndrome is so named in North America for
being associated with overo coloring. (Remember that in
North America, frame, sabino and splashed white patterns
are lumped together as overo.) Typically, this syndrome
is a result of a horse being homozygous dominant at the
frame locus. This genetic disease affects newborn foals.
The first warning sign is that OLWS foals are born completely
white with blue eyes. The trouble begins when these foals
show signs of severe constipation, a result of an underdeveloped
intestine – a genetic problem that stems from cells
associated with both gastrointestinal development and
coat color. These foals always colic and die; there is
no treatment, and euthanasia is recommended. Because this
disease is always fatal, it becomes vital for breeders
to DNA test their horses to avoid mating two carriers
of this lethal gene. |
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References:
Sponenberg, D. Phillip. Equine Color Genetics. Blackwell Publishing.
2003.
Corum,
Stephanie J. “A Horse of a Different Color”. The
Horse. May 2003.
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Tracy
Williams is a graduate of Colorado State University
with degrees in Equine Science and Journalism. She is
a freelance writer and photographer living in New Mexico.
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