The eyes of 18 adult Dachshunds of both sexes were examined histologically. Nine dogs were hornozygous for the Merle Factor (MM), five were heterozygotes (Mm) and four were normal (mm). The width of the ganglion cell layers as well as of the inner and outer nuclear layers was measured. There were statistically significant differences between the three groups of dogs. Half of the number of eyes from MM dogs showed severe pathological changes. The nuclear and ganglion cell layers were badly differentiated and retinal folds and rosettes were found. None of the homozygotes had a Tapetum lucidum (T. 1.) and only two of the heterozygotes had a T. 1. and this has in only one eye in each case. Retinal pigmentation was scanty in homozygotes as well as in heterozygotes. The remarkable resemblance of the Klein-Waardenburg syndrome of man and the Merle Factor of the dog is pointed out and the importance of the Merle Factor as an animal model is emphasized. For reasons of animal welfare it is necessary to restrict breeding with the Merle Factor to scientific purposes.
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