White and Deafness
There is a definite connection between White and Deafness in Great Danes. The melanocytes, the cells that produce the eumelenain and phaeomelanin pigment in dogs are derived from the neural crest cells. Early in embryonic development, a group of cells differentiate to form the neural crest cells. The majority of these cells for the brain and nervous system. A few "leftover" neural crest cells go on to form the melanocytes. These cells start of in specific pairs of areas on the head and along the back, and migrate down over the dog as it develops. It is, perhaps, easiest to imagine this in terms of paired areas where paint is poured, running down over the head and back. With the S-series, the SS dogs have paint that gets (almost) everywhere. From the starting points, the furthest reaches are, of course, the chest and toes, where these SS dogs may have some white.
The si allele limits the amount of paint, leaving much more white at the feet, belly, chest, tail tip, and the white facial markings.
The sp allele tends to produce less symmetrical effects than si or sw, perhaps acting by turning off half of one or more pairs of sites.
And the sw allele allows almost no paint, leaving color only on the head and possibly a round patch or two on the back or rump.
The M gene seems to act more by affecting the survival of these neural crest cells. The migration pattern of these cells is still determined by the S-series, but in the gray (or in Harlequins, white) areas have limited melanocytes. Most of the dog has no melanocytes in the MM double merle Whites.
The inner ear has small hairs in it. If these hairs are not pigmented, the dog is deaf. It is unclear whether pigment itself is the issue, or whether lack of pigmentation merely indicates that no neural crest cells are present. In any event, the lack of pigmentation of these hairs is linked to deafness, regardless of the actual cause and effect relationship. Both the MM genotype and the swsw genotype can and do produce deafness.
So, the lack of pigment in White Danes is indeed linked to deafness. The old "rule" that if a dog's ears are colored, it can hear, white ears, it's deaf, while not a hard and fast rule, does have some merit. Color on the ears certainly indicates that surviving neural crest cells are present in the ear area, making it more likely that the dog can hear.
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