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. An undesirable effect associated with the gene is eye problems, notably multiple congenital ocular anomalies (MCOA). MCOA has multiple clinical signs including a bulbous bulging of the eye, cornea globosa, severe iridal hypoplasia, uveal cysts, cataracts, and in a few cases, retinal detachment. These conditions can be detected via ultrasound examination of the eye. In a 2013 study of Comtois horses and Rocky Mountain Horses, all animals carrying the mutated form of PMEL17 had some eye disorder, though milder problems in animals heterozygous for the allele versus those who were homozygous. None of the control horses of these breeds who lacked the mutated form of PMEL17 had any eye disorder. Similar mutations in other species provide insight into the roles played by the PMEL17 gene. Relatively few such mutations are known, which suggests that the gene is involved in processes critical to survival. Several of the mutations known are related to pigmentation: premature silvering in mice, diluted and white plumage in chickens, and the widely known merle dilution in dogs. The merle coat in dogs is associated with auditory and ophthalmologic disorders, such as deafness and microphthalmia. In Rocky Mountain Horses, the silver dapple color is sometimes associated with Anterior Segment Dysgenesis (ASD) which affects the structures in the face and the front of the eye. Most often, the syndrome presents as