There are a lot of resources out there that go into more detail as to why the white dog is deaf, so if you want an in-depth explanation of why the white dog is deaf, I urge you to look at some of these sites. Google will also help you to find information on the genetics of deaf dogs. I’m going to give a simplified explanation here for the layman, just so that it’s recorded in this blog for people reading through. The same genetics can be used to explain deafness in cats, as well as blindness in both cats and dogs.
With dogs, there are a couple of genes that can be responsible for deafness. There are the merle gene and the piebald gene. The merle gene is seen in collies, Harlequin great danes (like Castiel!), old English sheep dogs and other breeds. The piebald gene is seen in dalmatians, Samoyeds and greyhounds, among others. Basically, some of these animals have two copies of these genes and they don’t develop enough pigment (the substance that makes the black spots on Castiel’s face and tail or the spots on a dalmatian–basically, pigment is colour) in their inner ears or, in some cases, their eyes while the ears and eyes are developing long before the animal is even born.
There’s some debate on what happens and why, but the most common reason I’ve found for what happens is that there is something about the lack of pigment that makes the veins and arteries to the inner ear and/or the eyes that makes them smaller and therefore not put as much blood through to the inner ear and/or the eyes. After the animal is born, before its ears and eyes open, the nerves in the inner ear and, in some cases, the eyes, die off from lack of oxygen, rendering the baby blind or deaf or, in some cases, both.
The result? A cat or a dog who is beautiful and white, but who is also either blind or deaf. Blindness is a rarer result of the double merle/double piebald, but it does happen. We’re fortunate in that Castiel is already motoring around with his belly off the ground like other puppies do when their eyes are freshly open, so he’s very unlikely to be blind. Even if he was, we would still take him and love him. He’s a beautiful animal and we can’t wait for him to come home in February.