Elementary merle genetics for newcomers
This is intended to be a very basic explanation of the workings of the merle gene. If you
already know about double merles and sable merles and such, you may not find anything new here. If you're not sure why people get upset about sable merles and double merles, stick around and learn.
There is no such thing as a sable merle gene or blue merle gene. There is only a merle gene. Merle is a dilution gene, that is, it lightens whatever the coat color would otherwise have been. The lightening is not spread evenly over the coat, but leaves patches of undiluted color scattered over the dog's body. Also, the lightening seems to work primarily on the black pigment in the coat, so any tan on the face stays even. Note that "black" as used here includes liver or chocolate. These colors are rare color faults in Shelties, but everything written here applies also to other breeds such as the pomeranian with the merle gene. A red merle in the breed is produced by the merle gene acting on a liver (solid red-brown, not the same as sable) coat.
One dose of the merle gene on an otherwise black dog produces a blue merle-a more or less bluish gray dog dappled with black spots. Tan points - the tan spots over the eyes, on the sides of the muzzle, on the legs and under the tail of a tricolor dog - will still be there in the merled tricolor. If the tan spots would not be present in a black dog, giving what is usually called a bi-black in Shelties, tan will not be present in the merled black either, and the dog will blue merle and white without tan: a bi-blue. One dose of the merle gene on a otherwise sable dog produces a sable merle. Sable merles are less predictable in color than blue merles, and may range anywhere from an apparent sable, often with a pinkish or orange cast to its coat, to something that looks like a very rusty blue merle. White markings remain on the merled dog, and may even be slightly more prominant.
Merle acts on the black pigment in the iris of the eye just as it does on the coat, so merle dogs often have part or all of the eye blue. (This does not affect their vision, though since it happens to some extent in the retina as well it may make it harder to diagnose certain eye problems.) The Sheltie breed standard allows blue or merle eyes in blue merles, but not in sables. Thus a sable merle with blue or merle eyes such as the pomeranian, will not do well in the show ring.
Note: A single dose of the merle gene: There are always two copies of a gene, alike or different, in any dog. If we call the merle gene M and the non-merle gene m, any given dog can be mm, Mm or MM. The mm dog is the normal, full-colored tri, bi-black or sable in Shelties or Poms or liver red in Australian Shepherds. The Mm Sheltie is a blue merle or sable merle, depending on what color it would have been without the merling gene. An MM dog, often called a double merle or a homozygous merle, can be mostly white and may be def or blind and have other physical problems. Some MM puppies are born completely without eyes.
On average over a large number of litters, breeding merle to merle will produce one fourth full colored dogs, one half merles and one fourth defective whites. Breeding merle to full color will produce one half full color and one half merles but no defective whites. The merle to full color breeding, then produces just as many merles as does the merle to merle breeding, and without the danger of defective puppies. The safe breeding for a merle, then is to a non-merle mate. This breeding should produce all healthy puppies, and about half will be merles.
To breed in this way, it is important to know which dogs are merles. This is one of the reasons experienced breeders rarely breed merles to sables, as this mating may produce sable merles.
Sable merles are no more likely to have health problems than any other color, and they are equally good companions. Many do have colors that are not accepted in the show ring, either because they have blue or merle eyes or because the mottling produced by the merle gene is too obvious. The real argument against sable merles is that they may be mistaken for brindles or normal sables. If two such sable merles were mated together, the resulting litter could contain defective whites. What a shock for the breeder expecting normal, healthy puppies!
There is one kind of breeding that can produce all or almost all merles, and that is breeding of a tricolor or a bi-black to a double merle - but remember that the double merlehas a high probability of being blind or deaf. A very few breeders have been lucky enough to get high quality homozygous merles that are not too severely affected to breed - but it definitely takes a lot of luck and really top quality blue merles to start with. Merle to merle breedings are only for the very experienced breeder who knows her lines and what they will produce - and it has probably produced more heartbreaks than good homozygous merles, even for them. A blue merle from black to homozygous merle breeding is just as healthy as one from a more normal black to blue merle breeding. There are only three homozygous merles on the Register of Merit list.
Unless you have done a lot of merle breeding and really know what you are getting into, the safe rule is still that a blue merle should be bred only to a black, tri or bi. In Poms, black, black & tan, chocolate, blue's, blue parti, black parti, and chocolate parti.