HEALTH PROBLEMS LINKED TO COLOUR
There are a few colour genes which can occasionally cause health problems in dogs. The main problematic colour genes are the merle and dilution genes.
DOUBLE MERLES AND HIGH WHITES
A homozygous (or "double") merle is one with two copies of the merle gene, and this severely impairs its ability
to make pigment, leaving large areas of the dog pigmentless (white). Pigment is actually necessary for certain parts of the body to function correctly, so lack of pigment
can cause health problems.
Dogs with large amounts of white caused by the extreme white gene (sw), such as Bull Terriers, Boxers and Dalmatians, can
also have some of the same health problems as double merles, particularly deafness (which is a huge problem in Dalmatians).
Lack of pigment in particular parts of the inner ear can cause deafness, which can be unilateral (just one ear) or bilateral (both ears). It is commonly claimed that dogs with white ears are always deaf, but in fact it's been shown that whether or not pigment is visible on the outer ear does not affect whether or not the dog can hear. In other words, a dog may have coloured ears but still be deaf, and a dog with white ears will not necessarily have any problems.
The double merle gene can also cause eye deformities. This is because the location of the eye cells in an embryo happens to be the same place that pigment starts to appear. If there is a problem with the pigment, this can therefore affect the development of the eyes. Problems include irregularly-shaped pupils, subluxated pupils (not positioned in the right place), microphthalmia (abnormally small eyes, usually with impaired vision), and other, less visible abnormalities causing blindness and bad vision.
Lack of pigment anywhere on the dog can make the skin much more sensitive to the sun. This is a particular problem on the nose, as it is so exposed, but any area of pink skin is susceptible to sunburn and skin cancer. The same problem occurs with any animal which has little or no pigment. White cats are probably the most well-known example. Skin cancer rates in white cats are extremely high and a surprising proportion of cats with white ears end up having their eartips amputated to stop the spread of cancer. The main way to prevent sunburn in animals is the same as with humans - apply suncream!
DILUTES
The main problem associated with dogs with the dilute gene (dd) is known as Colour Dilution Alopecia. It is also known to occur in mice, and causes hair loss and skin problems. A dog with this disorder will typically appear "mangy" and have partial hair loss. It is usually reported from blue dogs, particularly Dobermanns, but presumably it affects isabella dogs too (diluted livers).
This blue German Pinscher appears to have mild alopecia. Its coat is dull rather than having a healthy shine, and it seems thin and patchy.
CDA is recessive. That means that both parents must be carriers in order to pass it on, and only homozygous puppies will have it (they need two copies of the gene which causes it).
It's a bit like Down's syndrome in humans - children with it are born to two parents who carry the gene. Most
people carry it without knowing it, which is why the syndrome appears to occur randomly, but if one child is born with it, it is possible (but not certain)
that any other children the couple has will also have it.
Any colour can carry CDA or be homozygous for it, but only
blues and isabellas will have symptoms. There are now tests available for the CDA gene, which will hopefully help breeders to avoid breeding carriers.
The same problem can also occur (albeit rarely) on black or liver dogs, and is known as Black Hair Follicular Dysplasia. It affects black/liver hairs only, leaving all other hairs as normal. Because this condition is so rare, it often goes undiagnosed. I used to know a Jack Russell Terrier mix who was white except for a black patch on his back, which was hairless. His condition puzzled a whole string of vets and skin specialists, who suggested various types of mange and allergies, and he was never properly diagnosed as having Black Hair Follicular Dysplasia. Unfortunately for dogs with genetic hair loss conditions, there is no cure.