WhoDunIt.pdf

By Irene Stamatelakys

The dun dilution gene can

fool you unless you know the clues

hat dilutes coat colors to shades that range from

apricot to peanut butter to olive? Here’s a clue.

These horses also sport dorsal stripes and other primitive

markings.

The solution to this mystery may be the dun gene. But

before you jump to conclusions, let’s examine the genetic

evidence. When is it dun? And when is it a look-a-like

impostor?

“There are a variety of horse colors that sometimes ap-

pear to be very similar to some of the dun colors,” said

Nancy Castle of Paradise, Texas, founder of duncentral-

station.com , an educational Web site. “The most com-

monly confused ‘look-a-like’ color is buckskin, but buck-

skins are not the only color that can be confused with

the dun colors.”

How can you tell the dif-

ference between a dun and a

similar shade? If a horse has a dor-

sal stripe, doesn’t that make it a dun?

Can a buckskin have a dorsal stripe?

Can dun markings fade with time? These

are just a few of the questions our experts

have fielded over the years.

The dun dilution gene creates three main colors: grullo,

Quanah Little T's color (above); red dun, as shown by

Painted Red Wren (right); and dun, the color of Gay Bar

Drummer (below).

Like the tobiano

gene, a foal only needs

to receive one dun gene

from its parents to ex-

press the dun pheno-

type on any base color.

“Because it is domi-

nant, it cannot skip

generations,” said

Lord. “A dun horse

must have at least one

dun parent.”

This allele, named

D, is located at the

dun locus.The dun al-

lele lightens both red

and black pigment

equally, except for

black points on the

legs, mane and tail. It

has no effect on skin

pigment or eye color.

Visually, heterozygous and homozygous duns look identi-

cal.

In his book Equine Color Genetics , D. Phillip Sponen-

berg, DVM, PhD, wrote, “the dun allele causes black on

the body to be lightened to a slate blue grey or beige, and

red on the body to be lightened to tan (on bays) or light

red (on chestnut). It tends to leave points unaffected, as

well as leaving the head darker than the body.”

There’s more to dun than just diluted body hair, says

Cecilia Penedo, PhD, who is actively researching the dun

gene at the Veterinary Genetics Laboratory at the Univer-

sity of California-Davis.

“The display of the darker

‘primitive marks’ is an associated

characteristicofdun,”saidPenedo.

What confuses people is that

some duns have very minimal

primitive marks, but some non-

dun can have them too, although

the marks in those cases are usu-

ally fainter.

Genetic Evidence

“Dun is a dominant gene in the dilution category,”

explained Julia Lord, a Paint breeder in North Liberty,

Indiana.

A variety of colors can resemble dun, making it one of the

most frequently misidentified coat colors.

The dun palette

Surprisingly, dun dilutes make

up only 4.5 percent of all regis-

tered Paints. Of the nearly 1 mil-

lion horses registered by the

American Paint Horse Associa-

tion (APHA), there are approxi-

mately 20,600 red duns, 18,000

duns and 5,200 grullos in the

stud book. Not only are the colors rare, they are also chal-

lenging to identify.

All it takes is one dun allele to dilute a sorrel or chestnut

base coat into a red dun.They can range from a distinctive

light peach or apricot tone to a darker shade that could

pass for a sun-bleached chestnut. Points and markings are

generally a darker red but in some cases are very light and

difficult to see.

Add a dun dilution gene to a bay base coat and you have

a dun, also called bay dun, zebra dun or yellow dun.

Breeders say the shade is similar to peanut butter and that

duns tend to be more earth-toned than buckskins.

PAINT HORSE JOURNAL

APRIL 2009

“This basic dun is the one that can be confused with a

buckskin,” said Barbara Kostelnik of Cincinnati, Ohio,

who created dungenes.org to educate horse owners about

dun characteristics. “It will be a tan color with a black

mane and tail, and the markings may be anywhere from

dark red to black in color.”

The dun dilution gene lightens the black base coat to a

grayish color known as grullo (pronounced “grew-yo”) or

grulla (pronounced “grew-ya”). Grulla is the Spanish word

for crane and recalls the bird’s slate-gray color. Keeping

with the name’s Spanish origins, grullo is traditionally used

for stallions and geldings while grulla is used for mares.

Unlike a gray, which when examined closely is a mix-

ture of dark and white hairs, a grulla’s hairs are all the same

grayish color. It can tend toward a tannish shade is some

horses but is usually a “cool” tone tending more to bluish.

Points and markings are black.

Although not recognized as an official APHA color,

brown dun is the result of dun diluting a seal brown base

coat. Lighter brown duns may resemble dark bay duns;

darker brown duns may look like grullos.

“In the past, these would have been hard to identify, but

now that there is a test for seal brown [available through

Pet DNA Services of Arizona], they can be distinguished

from other duns,” said Lord.

Just like a horse can carry multiple coat pattern genes, a

horse can carry multiple dilute genes. For example, add

dun to a palomino—a sorrel or chestnut with a cream

dilution gene—and get a palomino dun or dunalino. Add

dun to a buckskin—a bay base with a cream dilution

gene—and get a buckskin dun or dunskin. And add dun

to a smoky black—a black base with a cream dilution

gene—and get a smoky grullo.These horses are registered

as the color they most closely resemble.

color hairs mixed in or dividing it into two or more [par-

allel] stripes.”

Just because your horse has a dorsal stripe doesn’t nec-

essarily mean it is a dun.

Experts generally agree that all duns will have a dorsal

stripe, if you can see it. White Paint markings can hide the

dorsal stripe, leg barring, or any other dun factor. Also, the

addition of other dilution genes, for example cream, could

possibly lighten the dun markings. Cremello and perlinos

have carried the dun dilution gene yet have not exhibited

dun markings. If the horse also carries the gray gene, the

dun markings will fade as the horse’s coat gets progressively

whiter.

The other markings vary from one individual to an-

other. Not all duns will have all of the markings, and non-

duns can have some of them. This is an important point,

explains Penedo.

“The primitive marks do not, by themselves, define

dun,” she said. “These marks can be found in horses that

show no dilution of color, as we have observed in many

Iberian horses. Foal coat colors are especially confusing, as

many are born with a dorsal stripe that fades or disappears

over time.”

Not only can markings vary with the horse’s age, but

seasons and condition can also bring changes. They could

be there but simply hard to see. For example, dun mark-

ings don’t really show up very clearly in winter coats.

Horses with a dun dilution, like this grullo, may have

shoulder stripes.

Other dun factors include ear markings, a face mask and

frosting on the mane and tail.

“Horses of various colors have been known to have a

visible dorsal,” said Castle. “But without the dun gene

diluting the coat, the horse is not a dun.”

The challenge comes in identifying the true dun dorsal

stripe. (See sidebar “The Great Imposter,” on page 78)

Mystery solved?

“The vast majority of people are confused about dun,

red dun and grulla,” said Lord. “For some reason, dun is

one of the most frequently misidentified colors.”

Castle agrees that dun dilutes can be challenging to

identify.

“Some of the other colors are definitely much easier by

comparison,” she said. “However, with practice and train-

While the dun dorsal has a sharp edge to it, the non-

dun dorsal has more of a tone-on-tone coloring, with the

stripe’s edge gradually fading into the body coat.

The next big dun clue is leg markings, also called leg

barring or zebra stripes. A wide variety of dun leg markings

are possible.

“Found primarily on the rear of the upper forelegs, these

may vary greatly from barring (like the rungs of a ladder),

to striping (like zebra stripes) to mottling (like sponge

painting),” said Kostel-

nik. “Other places to

look for these mark-

ings are inside the

upper forelegs, on or

behind the knees,

around the hocks, or

anywhere on the legs.”

The third dun clue

is shoulder markings,

also called withers

stripes, shoulder

stripes, or a shoulder

shadow. This can be a clearly defined stripe, like the type

seen on donkeys, or a larger shaded area descending from

the mane.

Concentric marks, also termed cobwebbing or spider-

webbing, are the fourth major dun clue. These are con-

centric circles of dark hairs on the forehead.

Duns can have other markings such as ear markings

(dark ear rims, bars on the ears or light tips), frosting on

the outer edges of the mane and tail, dark shading or strip-

ing on the neck, and a dark face mask.

Dun markings

Also called primitive marks or dun factors, these char-

acteristics are darker than the body color and essential to

distinguish duns from similar hues. The dorsal stripe, leg

markings, shoulder markings and concentric marks on the

forehead are four prime examples.

The dorsal stripe is

probably the best

known of the dun

factors. Kostelnik de-

scribes the dorsal

stripe as a clear,

sharp-edged line, not

an area of blurry

shading, running

along the spine.

“While the stripe

needs to have sharp

edges, it does not

need to be unbro-

ken,” she said. “Also,

it can have body-

W HAT D O Y OU G ET I F Y OU

C ROSS A D UN W ITH A B AY ?

Find the answer for this or

any other color combina-

tion with My Color Calcu-

lator, available with your

APHA Plus subscription at

aphaonline.com.

Leg markings–including barring, striping and mottling–

are dun factors typically found on upper legs and around

the knees and hocks.

A dun’s dorsal stripe is a clear,

sharp-edged line of color down

the horse’s spine.

“It certainly can be hard to tell a non-dun dorsal stripe

apart from a real dun dorsal stripe,” said Lord. “A non-

dun dorsal strip has a different ‘look’ and ‘character’ to it

than a dun dorsal stripe.”

Foals’ coats can make true dun markings hard to identify.

76 PAINT HORSE JOURNAL APRIL 2009

PAINT HORSE JOURNAL APRIL 2009 77

ing one’s eye to recognize the sometimes subtle differences

between a dun and a non-dun, it gets much easier.”

Some dun mysteries are getting solved the high-tech

way—via genetic testing for dun markers. UC-Davis began

offering a dun zygosity test last year. In cases where it’s not

clear if the horse is a dun or if a breeder wants to know if a

horse is homozygous, genetic testing is certainly useful.

Dun or not dun? Kostelnik gives this bit of advice.

“Never decide whether a particular horse is a dun or

not until you research it thoroughly,” she said. “There is

a great range and variety in dun coloration and markings.”

Breeders and enthusiasts alike are looking forward to

the discovery of the actual mutation that is responsible

for dun dilution and the day they can say with 100 per-

cent certainty, “Case closed. Mystery solved. My horse is

a dun.” n

Interview with Cecilia Penedo, PhD

Associate Director, Service and Genomic Research and Development, Veterinary Genetics Laboratory,

University of California, Davis

PHJ: How and when did you and your team at UC-Davis

develop the dun zygosity test?

Dr. Penedo: We began offering the dun zygosity test early

in 2008. The research that led to the development of the

test has been going on for several years. Mapping the dun

gene was the Masters project of my graduate student

Stephanie Bricker. We located dun on one of the horse

chromosomes in 2003. Using all horse genome resources

at the time, the status of the map was the equivalent of

having a street name and a five-block span where the “dun

house” was located.

We have continued to refine the location of the gene and

in the process identified several DNA markers very closely

associated with dun. Continuing with the “street address”

analogy, we know that dun resides in the middle of a block,

but we can only

identify the neigh-

boring houses, that

is, the DNA mark-

ers. We studied the

association of these

markers with dun

in many different

breeds and noticed

a common pattern.

Using letters to

designate the

DNA variants for

the critical mark-

ers, the prevalent

pattern becomes

“MMUOQ.” In

homozygous dun

horses, which have

two copies of dun and transmit dun to 100 percent of

foals, the genetic formula is MMUOQ/MMUOQ. In het-

erozygous dun horses, which have one copy of dun and

transmit dun to 50 percent of foals, it is

MMUOQ/“another pattern.”

the breeds in which it was validated, such as Quarter

Horses, Paints, Appaloosas, Icelandic Horses, Norwegian

Fjords and several of the pony breeds. We have not iden-

tified the same pattern in Iberian breeds such as Pure Span-

ish Horse (PRE), Andalusian and Lusitano, for example.

For this reason, we have restricted the dun zygosity test

only to validated breeds. We continue to investigate the

question about dun in Iberian breeds, among which we

have seen many examples of presence of striping pattern

without dilution of body hair.

Special thanks to Dr. Cecilia Penedo, Nancy Castle, Barbara Kostelnik, Julia Lord and Carolyn Shepard for their

invaluable assistance with this article.

T HE G REAT

or season to season in an individual horse,” he wrote.

When differences between a dun and a non-dun can be

subtle and confusing, even for the experienced breeder,

what’s the best way to tell them apart?

Start with the horse’s pedigree. A dun horse must have

a dun parent, unless the parent was mistakenly registered

as a non-dun.

Tracing a horse’s pedigree back to the dun ancestors can

be useful. In Quarter Horses, the majority of duns trace

back to Yellow Jacket, Blackburn, Joak and Hollywood

Gold. In Paints, one of the prominent ancestors was Yel-

low Mount, who has Blackburn and Yellow Jacket in his

pedigree.

Next, inspect the horse.

“I look for evidence of more than one dun trait to help

me determine if a horse is or is not dun dilute,” said Cas-

tle. “First, I look for the tone of the color. On bay duns

and red duns, I look for the body coat to be diluted, but

with some signs of red tones left in the coat. The cream

gene turns the coat more yellow-gold, while the dun gene

leaves some residual red tones in

the coat, so that the tone is more

yellow-tan, sometimes resem-

bling peanut butter or a shade of

red clay, like the stain of red clay

soil. On lighter shades of dun,

usually dunskins and dunalinos,

the body coat often has more of

a flat peachy-yellow tone.

“Then I look for the primitive

markings: their color, intensity of

color and shape and definition.

It does take some practice,

though. You have to train your

eye to recognize these traits and how they differ from

colors and markings that are similar.”

If you are registering a foal and are not sure if the young-

ster is dun or not, it’s helpful to send additional photos

with the registration application that clearly show the

markings you believe qualify the horse as a dun. Because

a dun foal must have at least one dun parent, the APHA

may require new photos of the parents or even parentage

verification to solve your dun mystery.

PHJ: Can you tell us a bit about current research on the

dun gene?

Dr. Penedo: In recent months, there has been a concerted

effort to find the specific genetic change that causes dun.

We are collaborat-

ing with Drs. Leif

Andersson and

Gabriella Lindgren

from [Dept. of

Medical Biochem-

istry and Microbiol-

ogy, Uppsala Uni-

versity] Sweden,

who are also inter-

ested in the genetics

of dun gene and

primitive marks,

and with scientists

at The Broad Insti-

tute in Boston,

where the horse

genome sequence

work was done.

This collaboration has made it possible to apply the latest

technologies in genome research to find what causes dun.

This research will generate DNA sequence from two ho-

mozygous dun horses in the genome region where the gene

is located. These sequences will be compared with those

of not-dun horses. Sequence differences found in these

comparisons will then be investigated for association with

the presence of dun across breeds.

I am very hopeful that we will find the dun mutation

and that a specific test for it will be available in the near fu-

ture. From a science perspective, the goal is to learn how the

dun mutation affects hair pigmentation to cause the dilu-

tion effect, as well as the appearance of the primitive marks.

Can a horse have a dark dorsal stripe and not be a

dun? Yes, it’s possible, says Allyson Pennington, APHA

Registrar.

“So many people are misled into thinking they have a

dun or red dun when they do not,” said Pennington. “Peo-

ple see any stripe down the spine and automatically call it

a dun and do not understand why we won’t register it with

that color.”

One common misconception is that a buckskin cannot

have a dorsal stripe.

“People believe that if the horse does not have a dorsal

it is called buckskin, but if it has a dorsal it is called dun,”

said Nancy Castle of Paradise,Texas. “Many buckskins do

have a dorsal stripe, which is generally referred to as a

countershaded dorsal.”

In fact, some dorsal stripes are caused by countershad-

ing, also called sootiness or smutti-

ness. Countershading modifies the

body color by adding black hairs,

usually over the top of the horse

and down the back, shoulder and

croup. In some cases, the counter-

shading is minimal and mimics a

dun’s dorsal stripe. It can also create

shading on the shoulder, which

could easily be confused with the

dun shoulder stripe or shading. But

duns with countershading are

rarely seen.

“Dun pretty much cancels out

‘sooty’ since it dilutes black hair,” said Paint breeder Julia

Lord from North Liberty, Indiana.

The genetics of countershading are still unknown.

According to genetics expert and author D. Phillip Spo-

nenberg, “the genetic control of the sooty effect is not sim-

ple, nor is it well documented.” He theorizes that more

than one gene may be at work and there is an environ-

mental link—that horses fed rich diets express sootiness to

a greater degree. As a result, “sootiness can vary year to year

This bay foal exhibits countershading.

PHJ: Can you explain how it works? How accurate is the test?

Dr. Penedo: The basis of the current DNA test is to de-

termine if the horse has the “dun” marker pattern and if it

is present in a single copy (heterozygous) or two copies

(homozygous). There are many different combinations of

DNA variants. To ensure that the correct DNA patterns

(dun and not-dun) are identified, most of the time we test

at least one of the parents to determine which variants are

being transmitted by that parent.

I have to stress, though, that this is what we call an “in-

direct test.” We are not testing for the actual mutation that

is responsible for dun dilution.

The test is highly accurate (greater than 95 percent) for

DUN RESOURCES

duncentralstation.com

dungenes.org

vgl.ucdavis.edu.

78 PAINT HORSE JOURNAL APRIL 2009

PAINT HORSE JOURNAL APRIL 2009 79

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: