merle, it turns out, has more going on that just a simple dominant/recessive (like a lot of other genes!). every M allele has a certain number of base pairs (which i am not going to explain, for economy) and the number of base pairs effects the expression of the gene.
i'm going to use the base pair outline provided by paw print genetics (since that's who Dune is tested through). none of these numbers are hard and fast. different labs will have different bp scales.
M (typical merle) - 265-269 bp
M (tweed/harlequin) - 270-280 bp
Dune is probably patterned the way he is because he is at the high end of "typical" merling—his M allele has a length of 269 bp. so what about allele lengths that don't fall into the ranges i've shared here? they're the lower ones.
a merle with an M allele length of 247-264 results in a pattern we call "atypical merle". these dogs are muted with fewer defined patches and often a "washed-out" sort of look that is different from true dilution.
(you may recognize this pattern if you're a follower of fellow dogblr @theborzoiarebackintown and have seen her aussie, Georgia)
the atypicals are neat and a whole other can of worms in terms of misidentification but we're interested in what happens when we drop the length of that allele by even more base pairs—because when we get down to an allele length of 200-246 bp we finally get to see a true cryptic merle.
this dog is Mm, just like Dune, but you can see that his M allele is written "Mc". The c stands for cryptic! That M allele is only 210 bp long, resulting in a dog who not only does not look like a merle... but whose genetics won't behave like a merle's, either. check this out:
this dog has not one but TWO copies of the merle gene and they're even longer than the dog above. and yet he's not merle, nor does he look like he's got a ton of extra white or any physical defects, so what gives? didn't i just tell you one (1) post ago that that's the fate of many (even most) MM puppies?
with true cryptic merles, the allele length is so short that the gene does not express like regular merle genetically OR phenotypically! for this reason, there are cryptic merles who can safely be bred to typical (or atypical) merles.
but do you remember what i said in the last post about the merle gene being highly mutable? that means likely to mutate. and sometimes the merle gene's instability means that its allele length will change from cell-to-cell (resulting in mosaicism) or one generation to the next! it's also more likely that the allele length will increase than decrease so generations of cryptic dogs could, conceivably, start to produce more typical patterns—and there is a slim chance that certain cryptic merles can produce traditional double merle puppies when bred to a typical merle stud.
all of this is why colour testing is so important in any breed that comes in merle, and especially in aussies where we see such massive pattern variation! it is also why people will fight each other to the death about what is and isn't safe to breed, especially if they don't have all the facts.
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