The only way a merle colored pup can be produced is if at least one parent is merle. Some breeders are of the understanding that the merle gene is a recessive gene and is carried from generation to generation. This is not correct. The merle gene is not carried, meaning -- the dog is either a merle or is not a merle. There are no exceptions to this law of genetics (for now, at least, until further research
If someone tells you that they have a litter of merled colored pups and there are no merles for many generations in their bloodlines --- then these merled pups were not sired by the sire the owner thinks there were. In fact, he should look for the hole in the fence!
The merle gene is an incomplete dominant or a gene with intermediate expression and is another dilution gene. Instead of diluting the whole coat it causes a patchy dilution, with a black coat becoming gray patched with black. Brown becomes dilute brown patched with chocolate, sienna, brick, and various diluted brown colors. While sable merles can be distinguished from sables, this is sometimes
very difficult because the merle coloration looks like -- to just slightly different from -- the sable color. The merling is clearly visible at birth, but may fade to little more than mottling of the ear tips as an adult. Merling on the tan points of a merle black and tan is not immediately obvious, either, though it does show if the mask factor is present. Eyes of a merle dog are sometimes blue or marbled (brown
and blue segments in the eye).
A "m/m" (homozygous recessive) dog is normal color (no merling). A "M/m" (heterozygous) dog is a merle. A "M/M" (homozygous dominant) dog, known as a double merle (from a merle to merle mating), has much more white than is normal for the breed and may have hearing loss, vision problems including small or missing eyes, and possible infertility. The health effects seem worse if a gene for
white markings is also present. In Border Collies all of which normally have
fairly extensive white markings, the "M/M" white has a strong probability of being deaf or blind. A "M/M", double merle, to "mm", non-merle black in color breeding, is the only one that will produce 100% merles.
Cryptic or phantom (as it's sometimes called) merles are dogs which carry a merle gene but are phenotypically (look like) tri, bi or self colored. These dogs will have some small area of merling somewhere, usually a tiny patch of merle pattern on their ear, tail, top of head, etc. Keep in mind the tiny patch can be only one hair and it can be located anywhere on the body. Cryptic merles are very rare. AGAIN, a cryptic or visible merle can only be produced when one or both parents are merles.
GENOTYPES AND COLORS:
("-" is either the dominant or recessive allele)