In the late 19th century, one dog authority of the day considered the color to be an oddity of nature, so uncommon was it in Collies.  Described as “mirled” or “marbled,” what we now call “blue merle” in the breed was originally known then as “tortoise shell.” 

The word, “merle,” likely comes from the fact that the main hair colors are diluted and leave “swirls” of colors to run in non uniformed streaks and patches. Actually, the merle gene isn’t even a color gene because it produces no pigment at all. It simply causes some parts of a black coat to go grey by preventing the black color to come through entirely. The merle gene has precisely the same impact on a Collie with two brown genes: It turns some parts of a brown coat lighter brown by allowing less color to come through.

It takes a single merle gene from a merle parent to produce a normal blue merle or sable merle. In more technical terms, the merle pattern is produced when one parent has a single copy of the M< allele. All merles have the genotype Mm, which simply means they have one allele for merle and one allele for non-merle. All non-merles are mm. If one breeds a merle (Mm) to a non-merle (mm),  on average, one will get a litter in which a half of the puppies get the M allele and are Mm (merle), and half get the non-merle allele, and they are mm.

When two merle-patterned dogs are bred together, each puppy in the litter has a 25% chance of inheriting that gene from both parents. Resulting offspring are referred to as double dilutes, homozygous merles, or white merles. Breeding an Mm and an Mm together can cause issues, namely puppies with visual and auditory problems, but if we are correct in our understanding, the issues aren’t passed on to the offspring of double merles provided they’re only bred to solid color Collies (sable or tri color)  with no merle gene. Resulting puppies will be normal merles.

Those interested in this topic will want to read the work of Sarah Murphy & Leigh Anne Clark.

Image: Blue Merle Smooth Collie by LA Shepherd
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