Dogs with the merle gene typically have blue eyes and often exhibit a wide range of auditory and ophthalmologic abnormalities.
Merle, which is present in the Shetland Sheepdog, Australian Shepherd, Dachshund, Great Dane and others, is a dominant gene. Merle produces a color pattern where patches of color are diluted or absent (white), again by acting on melanocytes; animals homozygous with the recessive allele (mm) have solid color. Dogs with piebald must be homozygous to have areas of white, while erls can be either heterozygous (mM) or homozygous (MM).
Dogs carrying piebald or merle can be deaf in one or both ears, but in a given ear the deafness is nearly always complete. Deafness results when the pigment gene is strongly expressed, not only suppressing melanocytes in skin and hair follicles, but also in a specialized vascular bed in the cochlea known as the stria vascularis.
Dachshunds have an hereditary condition resulting in auditory dysfunction, ranging from mild to severe deafness. Merles and double merles had significantly greater frequencies of ocular abnormalities, including increased intraocular pressure and ametropic eyes.
Microphthalmia and colobomas are well described in merle and double merle Dachshunds and Australian Shepherds. In these breeds, the double merle genotype can be sublethal and is associated with multiple abnormalities of the skeletal, cardiac, and reproductive systems. For these reasons, merle-to-merle breedings are strongly discouraged.
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