i can't remember if this is still the q tax but: i learned that horses are more horrifying on a biological level than i thought! so i know that in quite a few dog breeds, partially or all white coats are linked with deafness/blindess. why is this only in certain breeds (dachshunds, dalmatians, etc) and not others? is this seen in other species?
Simply put, there are multiple different genetic mechanisms that can result in ‘white’ dogs. Some of these genes result in a lack of pigment in the fur only, but other genes are associated with issues like undersized eyeballs, or lack of pigment in the inner ear.
We don’t know all the genes that cause whiteness in dogs, but if it helps to simplify it think of white dogs like this:
- Dogs that have colour, but have white on top
- Dogs that have no colour (or patches of no colour)
Pigment isn’t just for decoration. An animal needs pigment in areas like the eye and inner ear to function. On the skin areas that are devoid of pigment are just white, but if you happen to have a white patch in your retina or inner ear you might be partially deaf or blind. This is why sometimes, some white dogs are associated with deafness and blindness. Some individual white genes are also associated with deformities of particular organs (eg eye), or a potential increased cancer risk.
Dogs that are actually coloured dogs with white over the top (eg White shepherds, Samoyeds, Maltese) typically have black (or brown) noses and pigment around their eyelids.
Dogs which genuinely lack pigment (either completely or in patches), often don’t. Their eyelids and nose are more likely to be pink.
These two pictured dogs are both white, but genetically very different and consequently have different risk factors.
Problems associated with lack of pigment are also seen in other species to varying extents. Lethal White Foals is probably the best known example.