Another site I recommend is http://www.animalgenetics.us/ In addition to genetic testing, they have a Coat Color Calculator on their site that is very benefitial in calculating the possible results when breeding various patterns and colors together.
I hope you will enjoy reading.
To begin to understand the discussions it is important to understand the appropriate terminology. Colors can be expressed as either PHENOTYPE or GENOTYPE.
PHENOTYPE is the color a horse appears to be. In other words, what it looks like (e.g. black, red, brown, gray, white, etc.). The casual observer usually describes a horse by its phenotype.
GENOTYPE is the genetic make-up of the horse that creates its distinct color.
Confusion begins when horses of a similar phenotype (appearance) have a totally different genetic make-up. Genotype can be particularly important to a breeder trying to produce or avoid a particular color.
Complicating matters even more is the fact that there are genetic tests for some colors and patterns and not for others. Some genes can only be proven by their physical appearance.
Two more important terms in understanding color genetics are HOMOZYGOUS and HETEROZYGOUS. Every horse has two base color genes, one received from each parent. If a horse receives two matching genes, they are homozygous and will pass that gene to all their offspring. If a horse receives a different color gene from each parent, they are heterozygous and can pass either gene to their offspring.
A commonly held theory is that there are only two genetic base colors of horses, BLACK and RED. [Some hold to the theory that there is also a genetically White horse, but I lack enough knowledge to discuss that topic and will leave it for others.] Horses that are Red are correctly referred to as Sorrel or Chestnut, depending on the breed they are from. Red is a recessive color and therefore, a horse must be homozygous for the Red gene to show red. Black is a dominant gene, which will always show whether it is homozygous or heterozygous. A genetic test can show if a Black horse is homozygous or heterozygous.
So, if there are only two genetic base colors, what causes all the many different phenotypes? The answer is other genes that either modify or dilute the base color. Common dilute or modifying genes include:
AGOUTI – This gene on a black horse modifies the body color to some shade of red to brown and restricts the black to the points (legs, mane and tail, and possibly the head) creating what is known as a BAY. Bays may be either homozygous or heterozygous for either the Black gene or the Agouti gene with no difference in appearance. Agouti will always show on a Black horse, but not on a Red horse (as a Red horse has no black to be modified). Therefore, a solid Black horse can be bred to a solid Red horse (which has a hidden agouti gene) and the resulting foal can be bay by inheriting the black gene from one parent and the Agouti gene from the other parent. The Agouti gene can be tested for.
This is a Black horse that carries Agouti.
CREAM – This gene dilutes the red color (whether genetically red or modified by Agouti). A Red horse with a single Cream gene becomes a PALOMINO. A Red horse that is homozygous for the Cream gene becomes a CREMELLO. Cremellos typically have blue eyes and pink skin. In the past, Cremellos were often incorrectly identified as ALBINOS (a genetic defect which results in no pigment and therefore red eyes). One Cream gene on a black horse is referred to as a SMOKEY BLACK, but does not necessarily appear Smokey in color. One Cream gene may be totally hidden in a Black horse. However, a Black horse that is also homozygous for the Cream gene becomes a SMOKEY CREAM, which greatly resembles a Cremello. The Cream gene on a Bay horse dilutes the modified color to create BUCKSKIN. Shades of Buckskin can vary just as shades of Bay do. A Bay that is homozygous for the Cream gene becomes a PERLINO, a color that is often difficult to distinguish from Cremello. The Cream gene can be genetically tested for but is usually unnecessary to do so except on a Black horse. Any color horse that is also homozygous for Cream can be very difficult to tell its base color just by looking.
This Palomino (Pinto) is an example of a Red horse with one Cream gene.
Buckskin is an example of a Bay horse with one Cream gene. (Black + Agouti + 1 Cream)
This Perlino colt is an example of a Bay horse with two Cream genes. (Black + Agouti + 2 Cream) Double Dilutes (Cremello, Perlino, and Smokey Cream) are often difficult to distinguish from each other without genetic testing.
GRAY – A horse carrying the Gray gene can be born any color (though occasionally the graying process may begin before birth) and then begins to fade to Gray at varying rates of speed. The Gray hairs usually appear first on the face (often appearing like goggles around the eyes), and then spread to the rest of the body. A gray horse may fade to totally white by one to two years of age or may retain some degree of color for a much longer period. A small amount of color may be retained on the lower legs. Genetic testing for Gray is unnecessary as it is evident on both Black and Red. It can however make the base color extremely difficult to determine.
A Black horse turning Gray, as a 2 year old.
The same Gray mare, as a 5 year old.
PANGARE' – This gene causes the color of a horse to fade to a lighter shade on the lower legs, the flank area, the inner legs, and possible the muzzle. It can affect Bays as well as genetically Red horses. To my knowledge, there is no test for this gene.
Bay horse exhibiting Pangare'
SILVER – This gene affects the color of only black-based horses. Red-based horses can carry the gene but it will remain hidden. This gene dilutes the Black base color. The mane and tail becomes silver to flaxen in color and the body color may range from light silver-gray to a deep chocolate brown. Horses carrying the Silver gene often exhibit dapples on their body, but sometimes may not. The Silver gene on a bay horse will cause the mane and tail and lower legs to be bleached, but often not affect the body color. This often causes confusion between SILVER BAYS and Sorrels or Chestnuts. Therefore, if two silver bay horses are mated and at least one of them passes on the Black base color gene, but neither parent passes on either the Silver or Agouti genes, the resulting foal will be Black. This would be impossible from two horses which were genetically Red. By the same token, two Silver Bays who were heterozygous for Black could be mated and not pass on the Black gene and the foal would be Sorrel or Chestnut. The foal could carry both the Silver and Agouti genes and they would not show because of the Red base color. THERE IS NOW A GENETIC TEST FOR THE SILVER GENE.
Typical Silver Black (Dapple) in clipped coat.
Sorrel/Chestnut mare that carries a hidden Silver gene, and her Silver Black foal who is a minimaly marked Splash.
ROAN – This gene causes a blending of white hairs with the colored hairs throughout the body. The head, mane, tail, and lower legs remain darker color. [Roan is sometimes confused with Sabino, which will be discussed later.] Roans can vary from light to dark and affect every color. Roan is a dominant color/pattern and cannot skip generations. It can however be difficult to determine when accompanied by other modifiers/dilutions such as Cream, Silver, and Gray. Roans in the homozygous form are widely believed to contribute to absorption and miscarriages, an occurrence known as LETHAL ROAN. Recent studies indicate that there are SOME homozygous Roans, but the occurance is fairly rare.
The combination of multiple modifiers/dilutions greatly complicates the identification of a horse’s color. A Brown horse is genetically Black but, carries a gene similar to Agouti that causes it to appear brown. It should also be pointed out that Grey, Pinto, Silver, Appy, and Roan are simple Dominants- if one parent does not have the pattern/color then the foal cannot have it. However, Pinto may be so minimally expressed that the horse may be registered as a solid even though it carries the Pinto gene. Silver may be totally hidden in a Red horse (as is Agouti). And, Roan may be very hard to distinguish on a Cream dilute or Gray horse.
Appaloosas seem to defy all rules and make up their own as they go along. I will leave their description to others who have more experience with them.
Once again, terminology is important. PINTOS are horses with splotchy patterns of mixed color and white with varying degrees of white and color. PAINTS are horses registered with the American Paint Horse Association.
PINTO is not a color, but a pattern that can be expressed on any color. Many horses carry the genes that result in pinto markings but, are so minimally expressed that they are not considered Pintos by the registries. In the Unites States, there are four pattern genes that are often referred to as Pintos: TOBIANO, FRAME (LWO), SPLASHED WHITE, and SABINO. Frame, Splashed White and Sabino are often referred to as Overos, but each is the result of a different gene. In Europe, only the Frame gene is considered OVERO. The combination of these genes in a single horse can often enhance the possibility of loud or flashy markings. Pintos can be MINIMAL EXPRESSION or marked (with very little white exhibited on the body) or MAXIMUM EXPRESSION or marked (mostly white with very little or even no color). A pinto with approximately 50/50 white/color is often referred to as loud marked.
TOBIANO – is probably the most common and recognized form of the pinto pattern. Tobianos have colored heads, usually a large spot of color on their chest (referred to as a shield) and color in their flank areas. White usually (but not always) crosses the back at some point between the mane and tail. Many contend that ANY white on the face is an indication of the presence of one of the so-called Overo genes. It is rather unusual to find a horse that is Tobiano only with no other patterns added in. White markings may be minimal or maximum. Generally, the addition of other patterns tends to increase body white.
Although this mare also has some Sabino characteristics, her general pattern is Tobiano.
This mare is a minimally marked Tobiano. Even though she is registered as a Solid, she is lab tested to carry the Tobiano gene. She also has some minimal Sabino characteristics.
FRAME OVERO – This color gene is so named because in its most desirable form the white on the horse appears to be framed by color. This gene (also identified as LWO) however affects far more than color. A horse may exhibit only a bald face or possibly so little white that it is un-noticeable, and still carry the Frame gene. One danger of breeding Frame Overos is the possibility of producing one homozygous for the Frame gene. These are known as LETHAL WHITE OVEROS. Their intestinal tract is not fully developed and they will die, usually within hours of birth. The LWO gene can be genetically tested for, and it is recommended if there is any suspicion that a horse carries the gene. Frame Overos often (but not always) exhibit blue eyes. Frame is often very difficult to detect, especially when mixed with other patterns.
This horse is minimally marked, but is lab tested positive for the Frame Overo gene (LWO +). He also carries some minimal Sabino characteristics.
SPLASHED WHITE (Overo) – NEWS FLASH! Veterinary Genetics Laboratory at UC Davis has developed a test for the Splashed White gene. More information on this link: http://www.vgl.ucdavis.edu/services/coatcolorhorse.php
The test for the Splashed White gene (SW1, SW2, and SW3) is now available from most genetic testing labs.
This gene is so named because horses carrying it look as though they may have run through white paint and splashed it up on their bodies. In its heterozygous form, Splashed White may result in only white socks and white in the face. If homozygous, the gene may result in a much greater amount of white on the body, even to the point of being mostly white. This is the cause of the so-called OUTCROP PINTOS in the Quarter Horse and other breeds. Splashed Whites also often exhibit blue eyes. White in the face is often (but not always) off center and very irregular.
Two minimally marked Splash mares. Both lab tested negative for Tobiano and Frame. The mare on the bottom has tested heterozygous for SW1.
SABINO – The last of the so-called Overo patterns can express itself in different ways. It may produce leg and face white similar to that of the Splashed Whites or it may produce roaning throughout the body. It can be distinguished from a true roan by the fact that the mane, tail, legs, and head are not necessarily darker as they are in a true roan. This pattern can also be maximum expression or minimal expression. Maximum expression of this gene can result in a totally white horse that still carries the Black gene. Evidence indicates that Maximum expression Sabinos are homozygous for the gene. One common characteristic of this gene (though not always present) is a white spot on the chin or jaw. It also has a tendency to produce jagged edges on Pinto patterns. It is now believed that there are several forms of Sabino that result in similar appearance. At this time, only one form, SB1, can be tested for.
Minimally marked Sabino mare, also exhibiting Pangare'.
Similarly marked Minimally marked Sabino filly.
Minimal Sabino Roaning on a Black horse.
TOVERO – This is the result of the combination of the Tobiano gene and one or more of the Overo genes. The result is often a very loud marked Pinto, which is very desirable among those who breed for color. Evidence of this combination is the presence of face white and/or roaning throughout the body.
Tobiano + Splashed White + Sabino. LWO negative.
Tobiano + Frame. Sabino and Splash likely.
Tobiano + Splash and likely Sabino. Negative for Frame.
Tobiano + Splash and likely Sabino. Negative for Frame.
Tobiano + Sabino. Negative for Frame.
Tobiano + Splash + Sabino. Negative for Frame.