This is information I had found on an old Geocities page and found it to be an extremely useful reference in my model horse pedigree assignments. When Geocities announced they were closing I decided to save the information and graphics so it would still be available online for others. I lay no claim to have compiled this information or to have created the wonderful graphics. If you are the original creator, please email me and let me know so I can properly credit you.
All horses are either black or red (chestnut). All other colors are caused by modifying genes. A horse can be any number of modifying genes. For instance, a black horse can carry agouti, dun, cream, frame overo, and tobiano to end up being a dunskin tovero.
Add A, or agouti, to black and it restricts the black color to the points, (mane, tail, legs), giving you bay. Red is not changed by the agouti gene, although a red horse can carry it, and therefore produce bay foals from black horses.
Cream, Cr, is an incomplete dominant. This means that one Cr gene dilutes the color of the horse, while two Cr genes dilute it further. Cream dilutes red while black can remain relatively unaffected, often making it hard to identify whether a black horse carries the cream gene.
Two cream genes result in a white or cream colored horse with pink skin and blue eyes. These are not albinos, there has never been a documented case of albinism in horses. Neither are they lethal white horses, which are homozygous for the frame overo gene and always die soon after birth. Two cream genes do succeed in diluting black hair, but still less so than red, so that double dilute bays, or perlinos, often have noticeable points.
Dun is another dilution gene. It dilutes black and red. Dun also results in the horse having dun factor, which can include leg barring and dorsal striping, among others. However, dorsal striping does not guarantee that a horse carries dun, as the countershading gene can also produce dorsal striping in breeds such as Arabians or Thoroughbreds, where the dun dilution gene doesn’t exist in the genepool.
A third dilution gene, recently discovered, is Champagne. Champagne dilutes are very often mistaken for creme dilutes. Champagne doesn’t appear to be an incomplete dominant, so that two genes don’t affect the coat any more than one gene. Champagne dilutes usually have lighter, “Amber “eyes, and pink skin that is freckled.
Any of the above shades of Champagne can be combined with cream to make an ivory.
Silver is also a dilution gene, and is also known as silver dapple. It dilutes in an opposite manner of cream, diluting black while not affecting red for the most part. So a chestnut horse can carry silver but not express it, hiding it unless it’s passed on to a black based offspring.
Roan is a mixture of hairs, those of the basecolor and white. This gives the body of the horse a lightened appearance, while the head and legs remain the original color. Roan is a lethal gene, meaning that a foal that has two copies of the gene will die, usually early in the pregnancy.
Grey does just as it’s name suggests, gradually lightening the coat until the horse can appear to be pure white. Greying can progress very quickly on one horse and very slowly on another. A grey horse turned white will still have the normal skin and eye color, unless there are other modifying genes, such as Champagne or double Cream dilutes.
Pinto can occur on any base color. There are four pinto genes, Tobiano, Sabino, Frame, and Splash White. The last three are often categorized together as Overo. Tovero horses carry Tobiano and one of the overo genes. The amount of white in the coat can vary from very little to almost or entirely white. Horses can carry anywhere from one to all four of the pinto genes.
Like pinto, Appaloosa patterns can occur on any basecolor. Appaloosa patterns can vary tremendously, from leopard to blanket to horses that don’t fit any specific description.