Merle Ocular Dysgenesis (MOD) is a hereditary, congenital condition, which may affect animals displaying merled coat coloration. MOD results from abnormal embryonic differentiation of tissues and may lead to multiple ocular abnormalities including any combination of microphthalmos, microcornea, scleral staphyloma, persistent pupillary membranes (PPMs), heterochromia iris, iridal hypoplasia/coloboma (with resultant dyscoria/chorectopia), pseudopolycoria, iridocorneal angle dysplasia, lenticular coloboma/cataract/subluxation, choroidal hypoplasia, optic nerve coloboma, retinal dysplasia and/or retinal detachment (with or without hyphema). Changes are bilateral but not always symmetrical. Affected dogs (notably homozygous “white” merles) may also be congenitally deaf. Commonly affected breeds include the Australian Shepherd, (merled) Collies, Shetland Sheepdog, Harlequin Great Dane & (merled) Long-haired Dachshund.