The silver dapple, chocolate flax or Taffy gene
silver dapple, chocolate flax and Taffy are all caused by the same geneSilver dapple is the common term used for horses with a dilution gene that works to dilute black pigment. The gene usually causes a partial lightening of black pigment in the body coat and a more extreme lightening (or “silvering”) of the mane and tail. The dilution effect is often, but not necessarily, accompanied by dappling.
The name silver dapple should probably be reserved for horses that have both dapples and a silver mane and tail. Not all horses with the dilution gene look like that. Similarly the term "chocolate" is often used in the Rocky Mountain Horse breed, but doesn’t describe the color of all horses with the dilution gene. Taffy is a more general term that’s sometimes used and is probably a better one since the dilution gene can occur in horses of any color of base coat, giving different visual affects accordingly. The silver dapple or Taffy gene is denoted by the symbol “Z” and the dilution allele is dominant over non-dilution.
The Taffy or silver dapple gene occurs in several breeds, including Connemaras, Saddlebreds and Mustangs. In the United States it's found most commonly in the gaited Mountain Horse breeds, including the Rocky Mountain Horse, the Kentucky Mountain horse and the Missouri Fox Trotter. It also occurs in Icelandic Ponies, Shetland Ponies, the Welsh Pony and some miniature horse breeds.
The Taffy or silver dapple gene is said not to affect chestnut horses so that from time to time a chestnut horse carrying Taffy can have an unexpected color of foal. However I know of some miniature horses breeders who believe Taffy does have a subtle affect, and that they can tell which of their chestnut horses carries Taffy on the basis of what they look like.
Taffy changes black body coat pigment to a color that ranges from a creamy chocolate to a deep bluish gray to a deep orangey red. Manes and tails are often, but not always, turned to white or very nearly white, often said to be “silvery”. Such manes and tails are often mistaken for flaxen, but unlike with flaxen the roots are dark. Taffy horses may often have a dark mask on the face, similar to that which occurs in dun horses. They may also have striped hooves, and often have light blonde or white eyelashes, especially as foals.
Bay horses with the Taffy gene are turned to a bright reddish orange with rich chocolate or tan points. The mane and tail are blonde, sometimes streaked with silvery black hairs. They may look like flaxen chestnut but their manes and tails have dark rather than flxen roots.
Brown horses with Taffy are unusual and quite stunning. Their coats are a dark bluish gray, like slate, but with an orangish highlights. The mane and tail contains an admixture of black and blonde hairs, making them appear silvery.
Black horses with Taffy can either be Silver dapple itself or chocolate (called chocolate flax in some breeds). Both are most attractive colors. Silver dapple horses have a sepia brown colored body with cream or flaxen dapples, which are especially striking on horses with a darker coats. Their manes and tails are flaxen or ivory. Dark taffies without dapples are called chocolate or choclate flax, and are especially common among the Rocky Mountain breed of gaited horses. Their manes and tails may be flaxen or silver, but can also be somewhat darker chocolatey color.
Anterior segment dysgenesis in silver dapple horsesAnterior segment dysgenesis is a genetic eye defect of the horse. Affected horses may have a variety of abnormalities of the eyes, including lesions and cysts, with defects in the iris, cornea, lens, and eyelids (Ramsey et al, 1999, Ewart et al, 2000). They do not respond normally to light, but affected horses don’t usually seem to have significantly impaired vision. Vets had noticed that anterior segment dysgenesis is more common in horses with the silver dapple gene than it is in other horses.
Ewart et al (2000) performed a study to determine wether anterior segment dysgenesis was inherited (Ewart et al., 2000). They studied the eye phenotypes and pedigree information from 516 horses in an extended Rocky Mountain Horse family. They found that anterior segment dysgenesis was inherited in a codominant manner in these horses. They concluded that it was being caused by a single gene, with cysts expressed in heterozygous horses and complex lesions expressed in the homozygous horses. Only in homozoygous horses was the disorder potentially a problem. Such horses should be bred to non Taffys to avoid passing the condition on to their foals. Nonpenetrance of the cyst phenotype appeared to occur in a small number of heterozygotes, and was associated with a particular lineage (i.e. not all heterozygotes showed eye abnormalities).
It would appear that either the Taffy gene can cause eye abnormalities (i.e. it has pleiotropic affects), or that (in some Rocky Mountain Horses) it is tightly linked to another gene that causes them. Horses with the palest manes and tails were most affected, possibly indicating that homozygous horses are also more diluted in color than heterozygous ones. Anterior segment dysgenesis does occassionally occur in horses of other colors. This might well mean that the gene for it is separate from the silver dapple gene (but closely linked to it). However it is also possible that another gene is involved in these cases.
References
Ramsey DT, Ewart SL, Render JA, Cook CS, and Latimer CA. 1999. Anterior megalopthalmos and megalocornea of Rocky Mountain Horses. Vet Opthalmol 2, 47-59. Ewart SL, Ramsey DT, Xu J-F, and Meyers DA. 2000. The horse homologue of congenital aniridia conforms to semidominant inheritance. Journal of Heredity 91, 93-98.
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