How are dog coat colors related the health problems?
Health Problems caused by coat color inheritance patterns in Olde English Bulldogges.
Most white spotting on dogs is determined by the genes on the S locus. When we use the term “white spotting” we simply mean white areas on the dog, not actually white spots. White spotting can occur on any color. So any dog can have white markings, whether they’re black, blue, liver, lilac, brindle, sable, tan-pointed, merle or whatever.
White hair occurs when the skin cells are unable to produce any pigment. The white spotting gene impairs the ability of cells on particular parts of the skin to make pigment, so the skin becomes pink and the fur white. Nails and paw pads will also become pink in areas where pigment is not produced.
So far, only two white alleles have been proven to exist on the S locus: S – no white sp – piebald. The genotypes are as follows SS, Ssp, spsp. S being slightly but not completely dominant to sp.
The white spotting alleles are thought to be examples of incomplete dominance. This means that a heterozygous dog (Ssp) will express its most dominant gene, but may also be affected by the more recessive one to a lesser extent. For example an Ssp dog may have some white spotting. However, the relationship between the alleles is complicated and can vary between breeds.
The M locus is the home of the merle allele. Merle is dominant, and so denoted by the capital letter M. Non-merle is recessive, and denoted by m. Merle is interesting because all normal merles are heterozygous (Mm). A homozygous merle is actually a double merle or homozygous dominant. The merle gene dilutes random sections of the coat to a lighter color (usually grey in a black-pigmented dog), leaving patches of the original color remaining. The patches can be any size and can be located anywhere on the dog, unlike the patches on a piebald dog which are generally confined to the body and head.
White coated dogs are often affected by deafness because they carry the piebald gene sp, which is demonstrated by their predominant white coat and often blue eyes. The piebald gene causes the absence of melanocytes, the cells which create pigment.
These melanocytes are the part a dog’s DNA that determines if he will have brown or black hair, blue or brown eyes, and so on and so forth. When a dog is born without the genetic material to create melanocyte cells, a white hair coat and sometimes blue eyes, are the result.
So, what does hair color have to do with hearing loss? Well, the ability to hear is made possible by a special layer of cells within the inner ear. This specialized layer of cells and the cells that determine hair color, come from the same stem cell source. Without this stem cell, the dog’s body won’t be able to make this specialized layer of hearing cells and will likely be white in coloration because it lacks the ability to produce melanocytes. Congenital deafness is also linked to the merle gene.
Dogs possessing the merle gene do not have the piebald gene, which allows them to have a colored hair coat but blue eyes. Blue eyes are not a true eye color, but rather a lack of color producing pigment within the iris. The lack of pigment on the iris suggests a lack of pigment cells throughout the body, including the inner ear which makes hearing possible.
At Olde South Bulldogges we refrain from breeding dogs that are merles. The color was once not accepted by the IOEBA registry and an Olde English Bulldog with the merle coloration could not be registered. IOEBA registration rules have changed and now merles can be registered. Due to the high occurrence of health problems with the double merles in other breeds we do not have or produce any dogs in our program with the dominant merle gene. Special care must be taken when breeding merles to avoid health problems, so merle is a color best left to experienced dog breeders.
The merle gene can also cause eye deformities when homozygous MM. 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 and other abnormalities causing blindness and bad vision.
There is a common misconception that dilutes (blues or lilacs) are in some way naturally sickly – this is not in fact the case. As with most recessives, the dilute allele is in some way “faulty”, but it is only faulty in its ability to distribute melanin evenly along the hair shaft (the uneven distribution causes the paler appearance).
This does not affect the health of the dog at all, simply its color.
I do not in any way condone the breeding of dogs deliberately for color, or breeding for colors or patterns that are not on the breed standard. The primary concerns for any dog breeder must be health, temperament and conformation. Color should only be a consideration where it is important for the breed standard. Fortunately the health problems that are caused by the piebald and merle genes are very very rare in the Olde English bulldogge breed. There are no documented cases currently of the piebald gene in OEB causing deafness.
https://blog.pawedin.com/dogs/why-are-white-coated-dogs-deaf/ by Mary Beth Miller
http://www.bulldogsworld.com/health-and-medical/deafness-hereditary-deafness-bulldogs