Silver Dapple Gene - Inheritance and Expression

Inheritance and Expression

The silver dapple trait is caused by a missense mutation (labeled Z) in the PMEL17 gene on horse chromosome 6. It is transmitted by autosomal dominant inheritance (simple dominance). PMEL17 is active from quite early in embryonic development through to the mature cell's melanosome and is involved with the production of the black pigment eumelanin.

Melanins, which provide color in the eyes, skin, and hair, are found in two types: eumelanin, which produces black to brown pigment, and phaeomelanin, which produces red to yellow pigment. Most horses can produce both types; the brown appearance of a bay horse's coat is caused by alternating bands of eumelanin and phaeomelanin, for which the agouti gene is responsible. Eumelanin predominates in the legs, mane and tail of bay horses. By contrast, horses which lack a functional agouti gene cannot produce such alternating bands, and thus have wholly black coats with no visible phaeomelanin. Chestnut horses lack the ability to manufacture eumelanin altogether, and so have wholly red coats devoid of true black pigment.

While the role of PMEL17 is not fully understood, the silver dapple gene exclusively produces dilution, or hypopigmentation, of eumelanin. The dilution changes black into various shades of platinum, silver and flat grey, though the original black-brown character of the color is usually preserved. The effects of the gene are more striking in the mane and tail. Horses with chestnut or chestnut-family coats - such as palomino, red roan, or red dun - are therefore unaffected by the gene and may silently carry it and pass it on to their offspring.

On the template of a black horse, which has a coat rich in eumelanin, the effect is that of complete conversion to varying shades of silver. Often the body remains quite dark while the mane and tail are strongly diluted. On the template of a bay horse, which has eumelanin largely restricted to the points, these points are converted to silver while the phaeomelanic body is mostly unaffected.