Genetics Crash Course

[Zsadist]

Greetings and welcome to Zsa's Genetics Crash Course! Here you can learn about the basics of equine genetics and how they work. When applying these to your characters, keep in mind what your breed standards do or do not allow. And of course, feel free to shoot me a PM if you have any questions!

Lineart/grayscale copyright EscyKane.deviantART.com

Common Terms
- Allele: "One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome." Basically what makes up the genetics.
- Heterozygous: Often shortened to "Hetero". Both alleles are different (i.e. Ee or Aa).
- Homozygous: Often shortened to "Homo". Both alleles are identical (i.e. EE or AA).
- Recessive Allele: The allele that is not expressed, often noted by a lower case letter (i.e. "e" or "a"). The presence of a recessive allele may be masked by a dominant allele, which leads to the expression "hidden recessive."
- Dominant Allele: The allele that is expressed, often noted by a upper case letter (i.e. "E" or "A"). Dominant alleles are never hidden by their related recessive alleles, if they have one.

Now, on to the good stuff!

[/i] I'll start with the Gray and Dominant White gene, since they both have the ability to completely cover the other genetics. If your horse isn't gray or white, feel free to skip this section.[/size][/blockquote][/blockquote]

From left to right: Steel gray, dapple gray, rose gray, fleabitten gray, white gray, dominant white.

Now, gray horses are rarely born "gray". Most horses are born a base color like black, bay, or chestnut (we'll talk about those later) and will lighten as they grow older. By the time they are 7-9, they will be completely "grayed out". Each horse will be different as well. A black horse may turn steel gray, then eventually turn white gray. A bay horse may begin to dapple, then turn to a rosy gray before it goes white gray. Or a chestnut may slowly turn into a fleabitten gray. Or of course any combination of the above. A gray horse will never turn white however, because a gray horse has black/gray skin. When you talk about a dominant white horse, you will notice that the skin is actually pink. This is what makes a true white horse (not all horses with pink skin are white however, and we will talk about that later). Most dominant white horses have brown eyes, though blue is possible.

A gray horse is represented by the letter "G". A white horse is represented by the letter "W".

gg - Recessive gray. The horses will not gray out, nor will it have gray foals.
Gg - Heterozygous gray. The horse will turn gray, 50% of its foals will be gray.
GG - Homozygous gray. The horse will turn gray, 100% of its foals will be gray.

ww - Recessive white. The horse will not be dominant white.
Ww - Heterozygous white. The horse will be white and has the chance of passing dominant white to its foals.
WW - Homozygous white. If this were to occur, it would be lethal. It is not known to exist in horses, however.

From left to right: Fading black, true black.

If your horse is not gray or dominant white, next we'll look at the black pigment. It may be localized into points, or may cover the whole body. Either way, we have to determine if your horse has the "E" gene, also called "Extension". Most horses will be "fading" black, meaning that their coats will fade over time/during the summer. Rarely will a horse be "true" black, unless genetics call for it (i.e. the Friesian breed is mostly true black).

Ee - Heterozygous black. Your horse will be black if the black is not localized into points (that will be talked about in just a moment), 50% of the foals will be black.
EE - Hemozygous black. Your horse will be black if the black is not localized into points, 100% of the foals will be black

From left to right: Black chestnut, liver chestnut, chestnut, flaxen chestnut.

You probably noticed that I didn't include the recessive black in the last group. Why? It's because the recessive black is in fact not black at all! It's chestnut! And of course, there are different shades of chestnut. Liver chestnut is that darker, burgundy version where the mane is often seen a lighter shade. A "black chestnut" may seem black, but can be determined as chestnut by the red on its lower legs. In reality, however, a black chestnut is just the darkest form of a liver chestnut. A flaxen chestnut carries the flaxen gene. It lightens the mane and tail considerably to a golden yellow color, or sometimes it may be a white-ish color. Also notice that its only the recessive form of flaxen that is visible.

ee - Recessive black. Your horse will be chestnut. All of its foals will be chestnut as it cannot produce black pigment.

Ff - No black visible. Flaxen carrier.
ff - No black visible. Flaxen visible. All offspring will carry flaxen.

From left to right: Seal bay/brown, dark bay, bay, light bay, wild bay.

This is the localization of black into points that I was talking about. Bay horses! When the "Agouti" gene is present, your horse will be a bay of some sort. Your most common bays are, of course, bay, light bay and dark bay. From there you have seal bay (A^t) and wild bay (A⁺). Seal bay, also called brown, is often confused with black. You can tell the difference between seal bay and black by the light brown/orange points on a seal bay horse. Wild bay is recognized by the black points that only extend up to the pastern or fetlock. There's also another version of bay called "blood bay" or "mahogany bay"which is noted by its more-red coat (not pictured).

aa - Non-agouti. The horse will be black or chestnut.
Aa - Heterozygous agouti. The horse will be bay, 50% of the foals may be bay.
AA - Homozygous agouti. The horse will be bay, 100% of the foals may be bay.

A^t - Seal bay, only visible in the homozygous form or when paired with a.
A⁺ - Wild bay, dominant.

Note: Wild bay is dominant to regular bay and seal bay, regular bay is dominant to seal bay.

From left to right: Smoky black, buckskin, palomino.

Now that we've determined whether your horse is black, chestnut, or bay (assuming your horse is not gray in this case), we'll move on to dilutes. This is the single cream gene, represented by "nCr". We'll talk about the double cream gene next. A single dose of the cream gene will dilute a black, chestnut, or bay coat as follows:

ee/__/nCr - Single cream over chestnut. Will turn the coat palomino. __ is any form of Agouti.
E_/A_/nCr - Single cream over bay. Will turn the coat buckskin. Both the second "e" and "a" allele can be dominant or recessive.
E_/aa/nCr - Single cream over black. Will turn the coat smoky black. The second "e" allele can be dominant or recessive.

From left to right: Smoky cream, perlino, cremello.

This is the double creme dilute, "CrCr". A double dose of the cream gene will change your black, chestnut, or bay coat accordingly. As talked about earlier, some of these dilutes may look white, but are in fact not. Just because they have pink skin does not mean they are white. A cremello, for instance, may be recognized by its slightly more gold/peach tint and/or its blue or "hazel" eyes.

ee/__/CrCr - Double cream over chestnut. Will turn the coat cremello. __ is any form of agouti.
E_/A_/CrCr - Double cream over bay. Will turn the coat perlino. Both the second "e" and "a" allele can be dominant or recessive.
E_/aa/CrCr - Double cream over black. Will turn the coat smoky cream. The second "e" allele can be dominant or recessive.

From left to right: Blue roan, bay roan, red roan.

Here we have the roan genes. This is the mixture of white hairs with the base coat, covering most of the body (usually stops at the base of the head and the upper legs). The bases are of course chestnut, bay, and black, though roan can also occur over coats such as the dilute palomino.

ee/__/R_: Roan over chestnut. The __ can be any form of agouti while the roan can be heterozygous or homozygous.
E_/A_/R_: Roan over bay. Extension (e), agouti (a), and roan (r) can be heterozygous or homozygous.
E_/aa/R_: Roan over black. Creates an almost blueish color. Extension (e) and roan (r) can be heterozygous or homozygous.

From left to right: Grullo, bay dun, red dun.

Dun is also a dilute, but unlike the cream gene, it does not dilute the black points (cream dilutes the whole coat). Also, the dun creates the presence of a particular pattern which includes dark points, dorsal stripe, shoulder stripe and leg barring. Dun doesn't have a single or double factor like the cream gene. Red body color is diluted to a pinky-red, yellowish-red or yellow; black body color is diluted to a mouse-gray.

ee/__/D_: Dun over chestnut. The __ can be any form of agouti while the dun can be heterozygous or homozygous.
E_/A_/D_: Dun over bay. Extension (e), agouti (a), and dun (d) can be heterozygous or homozygous.
E_/aa/D_: Dun over black. Creates an almost blueish color. Extension (e) and dun (d) can be heterozygous or homozygous.

From left to right: Classic champagne, sable champagne, amber champagne, gold champagne.

Champagne is yet another dilute. They are often confused with other colors, but can be determined otherwise by the mottling/freckles around the eyes and nose as well as having hazel eyes (though not all champagnes will have them). Having a homozygous champagne will not dilute the coat more than a heterozygous champagne, like a single vs. double cream dilute.

ee/__/Ch_: Champagne over chestnut. Produces gold champagne. Agouti can be any form.
E_/A_/Ch_: Champagne over bay. Produces amber champagne. Agouti can be hetero or homo.
E_/A^t_/Ch_: Champagne over seal bay. Produces sable champagne. Agouti can be hetero or homo.
E_/aa/Ch_: Champagne over black. Produces classic champagne. Extension can be hetero or homo.

Shown: Pearl.

Pearl, also called "Barlink factor", is a dilution often confuses with the cream and champagne genes. It's a recessive gene that only shows up when the horse has two copies of the trait. In other words, the horse must be homozygous. Pearl will have no effect over bay, black, or chestnut when there is only one copy. Two copies will turn any base coat a uniform apricot color.

__/__/PrlPrl: Pearl over any base coat. Must be homozygous to be expressed.

From left to right: Sooty (shown over bay), pangare (shown over flaxen chestnut).

Sooty, the first modifier, can be seen on just about any coat, though most often bay-based horses. The coats exhibit pronounced counter-shading; the top of the horse is usually darker than the underside. It may sometimes be the opposite, however. It's not visible on black horses. Pangare is most often seen on breeds such as the Haflinger, Fjord, Belgian, and Exmoor Pony. It creates light points around the eyes, muzzle, and underside of the body. Depending on the base coat, the lighter hairs can vary from off-white to light tan. Pangare can also be called mealy or light-pointed. It's thought not to express on black coats.

__/__/Sty_: Sooty over any base coat. Extension and agouti can be any form. Sooty can be hetero or homo.
__/__/nPg: Pangare over any base coat, though not black. Extention can be any form, agouti can be anything but "aa" (unless combined with "ee").

From left to right, top to bottom: Tovero, tobiano, overo, sabino, splash white (all over chestnut). Solid-bred is not shown.

Paint patterns are always unique, but they can usually be grouped into one of these four categories.
Tobiano may be predominantly white or dark. The dark typically covers one or both flanks. All four legs are usually white at least below the hocks and knees. Spots are usually regular and distinct as ovals or round patterns that extend down over the neck and chest, giving the appearance of a shield. The head is typically dark with regular markings like a blaze or star. The tail is typically two colors.
Overo (or frame) can be either predominantly dark or white, with a solid tail. White, often irregular/scattered/splashy, usually will not cross the back of the horse between its withers and its tail. Head markings are distinctive, often bald-faced, apron-faced or bonnet-faced.
Tovero is like a combo of tobiano and overo. The dark is usually seen over the ears (which may expand to cover the forehead and/or eyes), around the mouth (which may extend up the sides of the face and form spots), over the chest (varies in size, may also extend up the neck), over the flanks (varies in size, often accompanied by smaller spots that extend forward across the barrel, and up over the loin), and at the base of the tail. One or both eyes may be blue.
Sabino can be anywhere from minimally white to completely white (dominant white can be confused with the more common maximum-expressed sabino). Sabino patterning is visually recognized by roaning at the edges of white markings, belly spots, irregular face markings, especially white extending past the eyes or onto the chin, white above the knees or hocks, and "splash" or "lacy" marks anywhere on the body, but particularly on the belly. Some sabinos have patches of roan patterning on part of the body, especially the barrel and flanks. Some sabinos may have a dark leg or two, but many have four white legs. Both blue and brown eyes are seen.
Splash white looks as though a horse was dipped in white, or the color was dripped over the top of the horse. The white can vary from minimally expressed (such as over the legs and face only) to extensively expressed (such as only small spots on the top of the horse are dark). Look here for an example.

__/__/T_: Tobiano over any base coat.
__/__/Oo: Overo over any base coat. This is the only way overo can be expressed. "OO" will result in a lethal white (LWO) foal.
__/__/T_/Oo: Tovero over any base coat. Note that this shows both tobiano and overo genetics.
__/__/Spl_: Splash white over any base coat. This is the most rare of all paint patterns. Homozygous splash is thought not to exist or be linked to any genetic defects.
__/__/SB1_: Sabino over any base coat. Maximum-expressed sabino horses are not associated with dominant white or LWO horses.

From left to right, top to bottom: Spots, blanket/snowcap, blanket with spots, leopard, few spot leopard, snowflake, appaloosa roan/marble/varnish, roan blanket/frost, roan blanket with spots. Mottled is not shown.

Appaloosas all have the same genetic code ("leopard complex"), but they come in many variations. Some appaloosa horses may not show any outward spotting, by may exhibit characteristics such as vertically striped hooves, white sclera of the eye, and mottled skin around the eyes, lips, and genitalia.
Spots is a general term that refers to a horse that has white or dark spots over all or a portion of its body.
Blanket or snowcap is a solid white area normally over, but not limited to, the hip area with a contrasting base color.
Blanket (snowcap) with spots is a white blanket which has dark spots within the white. The spots are usually the same color as the horse's base color.
Leopard is a white horse with dark spots that flow out over the entire body. Considered an extension of a blanket to cover the whole body.

[/i]Few-spot leopard[/i] is a mostly white horse with a bit of color remaining around the flank, neck and head.
Snowflake is a horse with white spots, flecks, on a dark body. Typically the white spots increase in number and size as the horse ages.
Appaloosa roan/marble/varnish is a distinct version of the leopard complex. Intermixed dark and light hairs with lighter colored area on the forehead, jowls and frontal bones of the face, over the back, loin and hips. Darker areas may appear along the edges of the frontal bones of the face as well and also on the legs, stifle, above the eye, point of the hip and behind the elbow. The dark points over bony areas are called "varnish marks" and distinguish this pattern from a traditional roan.
Roan blanket or frost is a horse with roaning over the croup and hips. The blanket normally occurs over, but is not limited to, the hip area.
Roan blanket/frost with spots is a horse with a roan blanket that has white and/or dark spots within the roan area.

__/__/Lp_: Appaloosa over any base coat.[/size][/blockquote][/blockquote]