![]() Severely affected dogs may be blind at birth. Microphthalmia is a congenital defect characterized by a small eye. There are other, more serious ocular problems associated with the Merle gene. The most severe abnormalities occur in homozygous merles with an excessive white hair coat involving the head region. Double merle animals may also have varying degrees of congenital deafness. Since a recessive trait is expressed only when homozygous, this means that affected dogs must be a double Merle (mm). Autosomal implies that this is not a sex-linked condition. In the Australian Shepherd dog, multiple ocular abnormalities due to the Merle gene occur secondarily to an autosomal recessive trait. A dog that is homozygous for non-merle (MM) is a normal, full-colored dog. One Merle gene copy is dominant over the non-Merle gene in that just one copy (Mm) will produce dilution of the coat and potentially different colored eyes, which is considered desirable in many breeds. If one copy is Merle and one is not, they are called heterozygous (Mm). ![]() A double Merle will be a predominantly white dog. If both copies are the same for Merle, they are termed homozygous (mm) or a double merle. For the purpose of this discussion, the Merle gene will be termed “m” and the non-merle gene will be called “M”. In any dog, two copies of a gene are present, one from each parent. Since it is understood that multiple congenital ocular abnormalities in the dog may be inherited, a brief review of basic genetics is in order. When the whole eye is affected, the condition has been referred to as Merle Ocular Dysgenesis. The abnormalities affect either the front or back part of the eye or a combination of both. Conversely, the other effects of the Merle gene may result in devastating blindness. There is no adverse consequence of the presence of the blue iris alone. A blue iris does not absolutely indicate the presence of the Merle gene it may also be expressed in dogs carrying the piebald gene, such as the Dalmatian. ![]() The blue appearance may also be as an ‘inclusion’ or as a partial segment of another wise brown eye (heterochromia iridis). With respect to ocular effects, the Merle gene’s most minor manifestation is a blue iris (or irides). Breeds that have been identified as having the Merle gene include the Australian Shepherd, Rough and Smooth Collies, Shetland Sheepdog, Dachshund, Great Dane, Old English Sheepdog, American Foxhound and the Catahoula Leopard dog among others. Unfortunately, the same gene that is responsible for the desirable coat and eye appearance is often responsible for many developmental eye defects. Colors expressed may range from a pale, light blue to greenish to amber. A combination of colors may be found in one or both eyes. The dappling effect it creates is not evenly spread rather, it is responsible for spotting of the coat and variations of the iris or colored part of the eye. This dilution gene acts to lighten the coat color. The Merle gene is responsible for a wide variety of beautiful coat and iris colors in the dog.
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