Merle Ocular Dysgenesis (MOD)

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This chapter presents an overview of Merle ocular dysgenesis (MOD) that 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, pseudopolycoria, iridocorneal angle dysplasia, lenticular coloboma/cataract/subluxation, choroidal hypoplasia, optic nerve coloboma, retinal dysplasia, and/or retinal detachment. Commonly affected breeds include the Australian Shepherd, (merled) Collies, Shetland Sheepdog, Harlequin Great Dane and (merled) Long-Haired Dachshund. The diagnosis of MOD is made on the basis of a combination of signalment and clinical findings. Surgical intervention in order to address scleral, lenticular, and/or retinal pathology may also be indicated in select cases. Selective breeding reduces the incidence and severity of disease.

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Deafness in dogs is frequently associated with the pigment genes piebald and merle. Little is known about the prevalence of deafness in dogs carrying the merle allele. To determine the prevalence of deafness in dogs heterozygous and homozygous for the merle allele of the mouse Silver pigment locus homolog (SILV) gene. One hundred and fifty-three privately owned merle dogs of different breeds and both sexes. Hearing was tested by brainstem auditory-evoked response and classified as bilaterally hearing, unilaterally deaf, or bilaterally deaf. DNA from buccal cells was genotyped as either heterozygous or homozygous for the merle allele. Deafness association tests among merle genotype, eye color, and sex were performed by the chi(2) test. Deafness prevalence in merles overall was 4.6% unilaterally deaf and 4.6% bilaterally deaf. There was a significant association between hearing status and heterozygous versus homozygous merle genotype. For single merles (Mm), 2.7% were unilaterally deaf and 0.9% were bilaterally deaf. For double merles (MM), 10% were unilaterally deaf and 15% were bilaterally deaf. There was no significant association with eye color or sex. Deafness prevalence in merle dogs was greater than that in some dog breeds homozygous for the piebald gene, such as the English Cocker Spaniel, but comparable to, or lower than, that in the Dalmatian and white Bull Terrier. Dogs homozygous for the merle allele were significantly more likely to be deaf than heterozygotes.
Microphthalmia with coloboma behaves as an incompletely penetrant recessive trait in the merle Australian Shepherd dog. Microphthalmia and related anomalies occurred more often in merle dogs with predominate white than in merles with limited white hair coat. The study did not establish a genetic relationship between the amount of merling and microphthalmia. The inheritance of merling behaved as a dominant trait, but fewer non-merles occurred than were expected. Variations in white spotting were satisfactorily explained by several hypotheses involving 2 or 3 alleles at the S locus. Each requires some or all homozygous merles to be largely white and 1 or more of the S alleles to exhibit some extent of dominance over other alleles in the series.