Horse Sense

Using Equines to teach Genetics and Evolution for High School Biology

Rebecca AkinsEric Swanson

Chandra Nielson

Color Coat Genetics of Horses Color is one of the first things that you usually notice about a horse.

Through the centuries that man and horse have been together, colors have been associated with luck, speed, strength—even temperament.

The importance of color is apparent when a new foal is born. Almost always the first questions asks is “Is it a colt or filly?” and the second is “What color is it?”

Color can increase a horses value, especially palomino, pinto, and dun/buckskin.

Horses expressing these colors can be registered secondary to it’s breed registry and shown at venues that are for only certain colors of horses.

Color of horses can be determined by simple Mendelian Genetics.

Color Coat Genetics of Horses Colors of Horses White Gray Chestnut Bay Grulla Buckskin Palomino Cremello Black Pinto Roan And Many More!!

All can be explained by using Mendelian Genetics showing dominance, recessive, and codominace.

Students can be taught how to use pedigrees of horses to determine the possible coat colors of offspring.

The color coat of horses is determined many different genes.

White

Represented by W White is dominate White is not considered a

color, but actually a modifying gene that causes the the absence of color.

Skin color will be pink If the W gene is present, there

will be no other possibilities for any color but white.

Gray Represented by G Gray is dominate Gray is similar to white, a

modifying gene, but the change is gradual over time. A horse will be born with color, but will turn gray.

Skin will be black

Many horse colors are located on different 7 genes

Chestnut ee, aa, CC, dd, gg, ww, toto Bay E, A, CC, dd, gg, ww, toto Black E, aa, CC, dd, gg, ww, toto Grulla E, aa, CC, D, gg, ww, toto Buckskin E, A, CCcr, dd, gg, ww, toto Buckskin Dun E, A, CCcr, D, gg, ww, toto Red Dun E, aa, CC, D, gg, ww, toto Palomino- ee, aa, CCcr, dd, gg, ww, toto Cremello-ee, aa, CcrCcr, dd, gg, ww, toto Tobiano- any color but has TO Overo- ?

This is only a few of the hundreds of different coloration and shades of horses.

These colors are some of the most common.

Alleles and Actions of Horse Coat Color Genes

WW: Lethal Ww: Horse typically lacks pigment in skin, hair and eyes and appears to be white. ww: Horse is fully pigmented.

GG: Horse shows progressive slivering with age to white or flea-bitten, but is born any non-gray color. Pigment is always present in skin and eyes at all stages of silvering.

Gg: Same as GG. gg: Horse does not show progressive silvering with age.

EE: Horse has ability to form black pigment in skin and hair. Black pigment in hair may be either in a points pattern or distributed overall

Ee: Same as EE. ee: Horse has black pigment in skin, but hair pigment appears red.

AA: If horse has black hair (E), then that black hair is in points pattern. A has no effect on red (ee) pigment.

Aa: Same as AA. aa: If horse has black hair (E), then that black hair is uniformly distributed over body

and points. A has no effect on red (ee) pigment.

CC: Horse is fully pigmented. CCcr: Red pigment is diluted to yellow; black pigment is unaffected. CcrCcr: Both red and black pigments are diluted to pale cream. Skin and eye color are

also diluted.

DD: Horse shows a diluted body color to pinkish-red, yellow-red, yellow or mouse gray and has dark points including dorsal stripe, shoulder stripe and leg barring.

Dd: Same as DD. dd: Horse has undiluted coat color.

TOTO: Horse is characterized by white spotting pattern known as tobiano. Legs are usually white

Toto: Same as TOTO toto: No tobiano pattern present.

RN-RoanRNrn-Roanrnrn-no roaning

Chestnut

ee, aa, CC, dd, gg, ww, toto

There are many shades of chestnut. This would be considered sorrel because of the red hue to the coat.

Bay

E, A, CC, dd, gg, ww, toto

Brown body with black points

Points are legs, tail, mane, and tips of ears.

Black E, aa, CC, dd, gg, ww,

toto

Grulla E, aa, CC, D, gg, ww, toto The only difference

between a black horse and a grulla is the dun factor (D). Notice the obvious dorsal stripe and the mousy gray body.

Buckskin E, A, CCcr, dd, gg, ww, toto Note that this horse is

basically a bay, but the areas of red pigment have been diluted to yellow

Buckskin Dun E, A, CC, D, gg, ww, toto The buckskin dun is different

from the buckskin in that primitive markings can be seen such as striping on the legs and a dorsal strip down the back.

The D allele codes for the dun factor.

Red Dun ee, aa, CC, D, gg, ww, toto Like most colors, red dun comes in a

variety of shades. The horse above is pretty light (a shade often called orange dun), whereas the horse at right is rather dark. Some chestnuts are so light that they are lighter than some shades of red dun. The difference, of course, is that the points of the red dun are darker than its body color, and the red dun, like all duns, has primitive markings (usually hard to see in photos!).

This is basically a chestnut horse with Dun factor (D)

Palomino ee, aa, CCcr, dd, gg, ww,

toto CCcr is the dilution of the

red coat color of the chestnut to yellow.

Cremello ee, aa, CcrCcr dd, gg, ww, toto The codominate dillution factor is

homozygous in a Cremello. Eyes are always blue with pink skin and very light cream colored coat.

Offspring of cremello bred to any color will produce desirable colored offspring including

Palamino-from chestnut or black Buckskin-from bay or black Smokey black-from black Horses possessing this coloration are

highly prized as breeding stock.

Bay Tobiano Pinto E, A, CC, dd, gg, ww, TO This is a bay horse with the pinto

Tobiano gene. Tobiano horses generally have

large, distinct round patterns of white that extend down over the neck, withers and chest, and the flanks are usually the dark color. Head markings are like those of solid horses and generally, all four legs will be white, at least below the hocks and knees.

.

Overo Overo horses generally have white under their bellies

and on the sides of their abdomens and necks. The white usually does not cross the back of the horse. At least one leg, and often all four, will be the dark color. Head markings are often bald-, apron- or bonnet-faced and the white markings on the body tend to be scattered or splashy. The tail is usually one color.

Currently there is still work being done on determining the color patterns of pinto horses. It is not yet known how to determine the color pattern of a horse if it’s not tobiano.

There has been shown that overos can carry a recessive lethal white gene. There is a ¼ chance when two overos that are carriers of lethal white are crossed, the foal will die within a few hours of birth due to a neuromuscular dysfunction.

Roan Roan(RN) is similar to how

tobiano works. A horse can have all the genes for a color, but roan is the mix of color and white hairs throughout the body.

The naming of Roan horses is either Red or Blue roan. A red roan is any base color but black, which is called blue roan.

Punnett Square Since the only difference

genetically in coat color is the the chestnut dilution factor, C, you may show a punnett square only concerning the C allele for dilution factor.

Chestnut is ee, aa, CC, dd, gg, ww, toto

Cremello is ee, aa, CcrCcr dd, gg, ww, toto

Palamino is ee, aa CCcr, dd, gg, ww, toto

C C

Ccr CCcr CCcr

Ccr CCcr CCcr

Genotype-100% CCcr

Phenotype- 100% Palomino

Breeding Questions You have a chestnut mare, but

you have always wanted a “Black Beauty”.

You have two stallions what you like, a homozygous bay, Ben’s Great Bay, or a homozygous black, Midnight. What stallion should you breed your mare to have a chance to get a black foal.

Chestnut-ee, aa, CC, dd, gg, ww, toto

Bay- E, A, CC, dd, gg, ww, toto

Black- E, aa, CC, dd, gg, ww, toto

All are the same except for the alleles for E(e) and A(a)

You can use a dihybrid cross

Punnett Square of Ben’s Great Bay X Your Mare

Ben’s Great Bay Homozygous Bay- EE,

AA, CC, dd, gg, ww, toto

Your Mare Chestnut-ee, aa, CC,

dd, gg, ww, toto

EA EA

ea EeAa EeAa

ea EeAa EeAaGenotype-100% EeAa

Phenotype-100% Bay

Midnight X Your Mare Midnight Homozygous black- EE,

aa, CC, dd, gg, ww, toto Your Mare Chestnut-ee, aa, CC,

dd, gg, ww, toto

Ea Ea

ea Eeaa Eeaa

ea Eeaa EeaaGenotype-Eeaa

Phenotype-100% black

You choose Midnight

Many more examples

We have written may handouts where students can practice determining the outcomes of color coat genetics of horses.

Horse Evolution

How the Horse EvolvedEohippus aka Hyracotherium

The first horse evolved about 50 million years ago. It was called Eohippus ‘Eo’ is Greek for ‘dawn’ ‘hippus’ is Greek for ‘horse’

How the Horse EvolvedEohippus

Eohippus was the size of a small dog of fox.

Horses today have one toe, it’s hoof on each leg Eohippus had four toes and each

of its hind legs had three toes as well as two vestigial ones.

Its head was more like that of a reptile than a horse Its teeth were for browsing on

lush leaves and plants, not for grazing

How the Horse EvolvedMesohippus

The ‘second horse’ in the evolutionary chain was Mesohippus.

Twice the size as Eohippus Appeared about 35 million years ago Skull was larger, face appeared

longer, eyes set further back and changes in teeth were apparent.

Had three toes on each foot The middle toes was a primitive

hoof was larger than the lateral ones.

How the Horse EvolvedMesohippus

How was Mesohippus intermediate between the ancient Eohippus

horses and more modern forms by looking at foot structure? MesohippusEohippus

How the Horse EvolvedMesohippus

The Eohippus predecessors of Mesohippus had four toes on their front feet, but Mesohippus lost the fourth toe, therefore, having three toes on the front foot.

MesohippusEohippus

Tooth Structure of Mesohippus and Eohippus

Mesohippus premolar teeth became more like molars.  These premolars are said to be "molariform." The primitive triangular premolar pulps food, while the squared molariform teeth crush and grind food. 

This might reflect a shift from a more diverse diet including fruit to a more limited diet of leaves and possibly grass.

The first upper premolar is never molarized.  It is only occasionally present in modern horses.  It is popularly called the "wolf-tooth" by horse-breeders.

Eohippus Mesohippuspremolars

molars

premolars

molars

How the Horse EvolvedMiohippus

Miohippus 30 million years ago

Larger in size

How the Horse EvolvedMerychippus

20 million years ago Because of the abundance of grass lands 25 million

years ago, ‘horses’, including Merychippus, gradually became grazing animals

There were changes in jaw movement and tooth structure

Stood about 1 meter in height Skull and face became more horselike Front legs modified

Fusion of the radius and ulna into one bone Fusion of tibia and fibula fused into one bone The center toe of the leg became longer and

stronger and the two side toes became smaller

Though it retained the primitive character of 3 toes, it looked like a modern horse.   Merychippus had a long face.  

Its long legs allowed it to escape from predators and migrate long distances to feed.   It had high-crowned cheek

teeth, making it the first known grazing horse and the ancestor of all later horse lineages.

A thought to chew on....

Did the “ruminant horse” ruminate?

The strong crests of the teeth of Merychippus reminded Professor Leidy (the scientist who named this genus) of the teeth of ruminants.

The ruminant digestive system is a slow, but highly efficient method of processing vegetation.

So far as we know, no living horse, rhino, or tapir has ever had such a system, so it is unlikely that the “ruminant horse” ruminated.

Cows, sheep, and deer are ruminate animals. Ruminants swallow vegetation that is then processed in one or more of the “foregut” chambers (so-called because they occur before the “true” stomach [abomasum]).

Horses are not ruminate animals. Horses are called “hind-gut fermenters” because they have a digestive pouch in the intestine, or caecum, behind the stomach.   Microbes in the caecum break down the vegetation so that energy and nutrients can be obtained by the horse.

Paleontologists almost never find fossilized digestive tracks and so can only make educated guesses about the digestive physiology of extinct animals.

How the Horse EvolvedPliohippus

Appeared 5 million years ago Resemble today’s horse Originating in North

America, it emigrated to Asia, Europe, Africa, and South America by means of connecting land masses, Pangia.

How the Horse EvolvedPliohippus

Side toes disappeared to leave the vetiges that are the long splint bones on either side of the cannon bone in the present day horse.

Change in leg and food structure during this period were due to environmentSpeed was necessary for survival

How the Horse EvolvedEquus

Today’s domestic horses, ass, and zebra Domestic horses- Equus caballus The sole species of wild horse surviving

is Equus przewalskii. Found in its natural state in mountainous country on the boarder of China or in the zoos of Europe and North America

All other wild horses including Mustangs are actually feral horses whose

ancestry can be traced back to the domestic

horse.

Equus prewalskii

How the Horse EvolvedEquus

Prewalskii’s horse and Tarpan are said to be the predecessor of the Oriental or Eastern breed, including the Arabian.

The Arabian is thought to be the foundation of all domestic breeds.

Of all the domestic animals, the horse was the last to be tamed. In the early days, the wild horse was a convenient source of food for primitive peoples. The horse was probably first tamed in Eurasia about 5,000 to 6,000 years ago.

The Arabian The Arabian is the oldest and

purest of all breeds. It’s unique to other breeds in that

it has one less vertebra than other breeds of horses.

Arab-13 vertebra Others-14 vertebra Arabs are noted for having spirit,

endurance, beautiful dished heads, and high intelligence, sometimes outsmarting their trainers.

The Arabian