A Rare Case of Cryptic Merle

A tricolour Cardigan bitch (mm) was mated to a blue merle Cardigan dog (Mm). Both came from tricolour to blue merle matings, i.e. both had a tricolour sire and a blue merle dam.

As could be expected, the litter contained tricolour (mm) and blue merle (Mm) puppies. But to the breeder's astonishment there was also a so-called "whitely", a white puppy with blue eyes and only a few blue-gray patches. But how come? It couldn't very well be a double-merle (MM), or could it? But how was this possible when one parent was a tricolour? Not a single strand of "blue" hair and not even a tiny blue fleck in the iris of one eye could have given an indication that the dam might be a cryptic merle. (In this context cryptic means "hidden").

Double merle pup (MM)

An expert at a DNA testing laboratory was consulted. She was of the opinion that it was practically impossible that the bitch was a heterozygote merle (Mm). In 40 years she had never encountered such a case without a small patch of blue merle being visible or the presence of at least one blue eye. She even thought a DNA test would be a waste of money. However, the breeder wanted to make sure and ordered a test. And lo and behold, the bitch turned out to be a cryptic merle, i.e. phenotypically she is a tricolour, but genotypically she is a blue merle (Mm). So the white puppy is without any doubt a double merle (MM).

Bitch, phenotypically tricolour, but genotypically blue merle (Mm)

At 4.5 months old

This, however, put the breeder in a somewhat awkward position as the breed club does not allow blue merle to blue merle matings. This ban also applies to other breeds that have the merle factor. Eventually it was decided that the breeder at the time of the mating was in good faith and no further steps were taken. Because of the disqualifying white colour the puppy has a stamp in the pedigree that it may not be bred.

Should the bitch have another litter, she will be mated with a tricolour dog. While this will exclude the risk of a double merle puppy it is uncertain whether there will again be normal blue merle puppies and how many. The inheritance of the blue merle colour does not always follow the rule of ~50% tricolour and ~50% blue merle:

Dam, blue merle (Mm) or phenotypically tricolour, but genotypically blue merle (Mm) Sire, tricolour (mm)
Mm Mm
mm mm

A double merle (MM) as described above will only be possible with the following combination:

Dam, blue merle (Mm) or phenotypically tricolour, but genotypically blue merle (Mm) Sire, blue merle (Mm)
Mm mm