Health problems related to colour genes- Double merle problems and CDA


Health Problems Linked to Colour

There are a few colour genes which can occasionally cause health problems in dogs, most notably merle.

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 homozygous piebald allele (sp), 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!


There is a common misconception that dilutes are in some way naturally sickly – this is not in fact the case. The dilution gene does impair the ability of the cells to make pigment, but only in that it causes the pigment that is made to be less intense. As with most recessives, the dilute allele is in some way “faulty”, but it is only faulty in its ability to produce full-strength eumelanin.
The ability or inability of the cells to produce full-strength eumelanin does not affect the health of the dog, simply its colour.

That said, the idea of dilutes as unhealthy most likely has its foundations in Colour Dilution Alopecia. This is an apparently genetic disease causing 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). Any colour can carry CDA or be homozygous for it, but only blues and isabellas will have symptoms.

CDA does not occur on all dilutes and its frequency varies between breeds. It is particularly common in Dobermanns, occuring in up to 80% of dilute dogs. Dilutes in other species such as mice are caused by the same gene, and yet CDA is not known in these, implying it is not an unavoidable consequence of dilution. It is thought that CDA may be caused by a specific dilution gene – labelled dl. Just as there are various different b alleles that all cause the liver colour (phenotypically the same, so only distinguishable through genetic testing), it is probable that there are a number of different d alleles as well, and only one of these causes CDA.
What this means is that CDA is most likely caused by a recessive allele but could theoretically be bred out of most lines by careful breeding and genetic testing.

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.

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, although these conditions do not generally cause the dog to be itchy or uncomfortable and so are mostly harmless.

“Dog Coat Colour Genetics.” Dog Coat Colour Genetics. N.p., n.d. Web. 24 June 2014.