|Barked: Thu Dec 14, '06 2:09pm PST |
|Actually Tohbi, I think you got merle a little bit confused. While yes, breeding two merle dogs will yeild some solid pups, it is impossible to get a merle pup from the breeding of two solid parents, unless one of those parents is a cryptic merle (that is to say, their merle coloration is hidden beneath a white spot or is so small as to undetectable unless you search really hard). Even if the solid pup is from two merle parents, there is NO way that it can pass on merle if it is bred again. Solid dogs have NO way of making a merle pup (unless of course it is bred to a merle dog, but then the merle dog is the cause for the merle pups).
There is only ONE gene that causes the merle phenotype, or appearance. When that gene is in the homozygous recessive form (that is to say, that both copies of the gene that the dog has are of the same form) and that form is lacking in mutation, then the dog will be solid colored. When the merle gene is present in the heterozygous form (that is to say, that one copy of the gene is recessive and the other copy is dominant, or has the mutation) then the dog will exhibit the merle phenotype. When the dog is "double merled" or has both copies of the gene in the dominant form, then you have a predominantly white dog with the possibility of vision and/or hearing problems discussed previously.
There seems to be a misconception floating around that merle is due to multiple genes. This is false. Only one gene and one mutation within that gene causes the merle coat color we are familiar with. Solid dogs do NOT have a copy of that particular forum of the gene, or to put it another way, a solid dog has two copies of the merle gene that are "normal". A merle dog has one copy of the gene that is "normal" and one copy of the gene that has a slight change in it.
The gene responsible for merle actually encodes for a protein that aids in the passing of pigment vacuoles up the hair shafts of the dog. When one copy of the gene is "messed up" then some of the proteins do not function properly so the full amount of pigment is not passed up the hair shaft. This results in some hairs being solid colored and some hairs appearing dilute (either black, red, or brown depending on the base color of the dog).
There was also mentioned (I think by Tohbi) that white headed dogs are more prone to deafness or eye problems. This is actually true, though is not always the case. The reason behind this trend is because, in fetal development, the precursor cells for nerve and for pigment cells are one in the same. As the fetus developes, the precursor cells migrate from the spinal column of the embryo in the direction of the head and then down the body. Once the precursor cells migrate to different areas of the body, they then differentiate to be either nerve cells or pigment cells. So that leads to say that a dog with fewer pigment cells on it's head (ie. white headed dogs) will have a greater likelyhood of having fewer nerve cells associated with the sensory organs of the head (eyes and ears). The migration of the precursor cells is regulated by many many factors, most of which are hereditary, but some of which are due to "dumb luck".
Another interesting side note is that the LAST area of the dog that the precursor cells migrate to is the "tip" of the chest. That is why, even in some purebred dogs, you will get a tiny white spot on the chest. Labs are very common for this, but they are not the only breed. Many people used to (and unfortunately, still do) believe that the white chest spot is due to crossbreeding. This is false!! It is just because the precursor pigment cells did not migrate all the way to the tip of the chest in that particular dog's fetal development. They are in no way any less "purebred" than the next dog! Just an interesting tidbit of info
Edited by author Thu Dec 14, '06 2:21pm PST
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