Merle, and the litter that should never be bred

What merle does

Merle randomly dilutes patches of dark pigment across the coat, leaving a mottled or marbled pattern of lighter areas over a base colour. In a blue merle the black base is broken up with grey; in a red merle the liver base is broken with lighter tan.

It often lightens the eyes too, producing blue eyes or an iris that is partly blue and partly brown. In some breeds that is prized; in the Catahoula it is called a cracked or glass eye and is entirely normal.

The gene is incompletely dominant, which is the key to everything else. One copy produces a merle dog. Two copies produce something quite different.

Two copies

A dog with two copies of merle — a double merle, called double dapple in dachshunds — is usually predominantly white, because the dilution has been applied twice and very little pigment survives.

That lack of pigment is the problem, and not for cosmetic reasons. The inner ear depends on pigment-producing cells to function; without them the structures degenerate in the first weeks of life and the dog is deaf. Rates in double merles are high, and deafness may be in one ear or both.

The eyes are affected by the same mechanism during development. Double merles commonly have microphthalmia — abnormally small eyes — along with colobomas, misshapen pupils, and varying degrees of blindness.

Breeding two merle dogs together gives, on average, one double merle in every four puppies. That is the arithmetic, and it is why no reputable breeder does it.

The cryptic merle problem

There is a complication that makes this less obvious than it sounds. The merle gene varies in length, and shorter versions produce little or no visible merling — a dog that looks solid but carries merle and passes it on exactly as a visibly merle dog does.

These are called cryptic or phantom merles. Bred to a visible merle, a cryptic merle can produce double merle puppies from a pairing that appeared perfectly safe.

This is why merle DNA testing exists and reports the length of the allele rather than a simple yes or no. In any breed where merle occurs, a test result is better evidence than a photograph of the parents.

It matters most in breeds where merle has been introduced recently or is not traditional, because there the assumption that a solid-coloured dog is not a merle is least safe.

Which breeds carry it

Merle occurs in the Australian Shepherd and Miniature American Shepherd, Border Collie, Shetland Sheepdog, Cardigan Welsh Corgi, Catahoula Leopard Dog, American Leopard Hound, Koolie, Dachshund, Chihuahua, Pomeranian, Great Dane and Alapaha Blue Blood Bulldog, among others. It has also appeared more recently in breeds where it is not traditional, including some bully lines, which is where the cryptic merle risk is highest.

The harlequin Great Dane is a special case: harlequin is a separate gene acting on a merle base, and Dane breeders follow specific pairing rules because of it.

In the Australian Shepherd and other collie-lineage breeds, merle is worth thinking about alongside the MDR1 drug-sensitivity mutation, which is common in the same populations and is a separate test.

What to ask, and what to do

Ask the colour of both parents and ask to see them. Two merle parents is the answer that ends the conversation.

In breeds where cryptic merle is plausible, ask for merle DNA test results on both parents rather than relying on how they look.

Be wary of a predominantly white puppy in a merle breed, particularly if it is being sold at a premium as rare or unusual. Rare is often the marketing word for a dog that should not have been bred.

None of this is an argument against a double merle dog that already exists. Deaf and partially sighted dogs live full lives with reasonable adaptation, and rescues place them successfully. The argument is only ever against deliberately producing more of them.