Merle in Dogs: The Gene, Double Merle and Hearing Risks

Merle is a coat pattern of diluted, marbled patches over a dog's base colour, seen in breeds such as the Australian Shepherd and the Shetland Sheepdog. It's caused by an insertion in a gene called PMEL, also known as SILV. One copy gives a merle coat. Two copies make a "double merle", and research has found double merles are much more likely to be deaf than dogs with one copy.

The short version

• One gene: merle comes from an insertion in the PMEL (SILV) gene.
• One copy is enough to make a merle coat; the trait is incompletely dominant.
• Blue merle and red merle are the same pattern on a black or a brown base coat.
• Cryptic merle: some dogs carry a shortened version of the insertion and look solid.
• Double merle (two copies) was linked to much higher rates of deafness in a 2009 study.
• Pairing two merle dogs means about one puppy in four is expected to be a double merle.

The merle gene

A 2006 study traced merle to a short DNA insertion in the SILV gene, now usually called PMEL, which is important for pigmentation. The researchers first linked the pattern to that region in Shetland Sheepdogs and then found the same insertion segregating with merle in several breeds. They described merle as autosomal and incompletely dominant: one copy gives a merle coat, and two copies give a stronger effect.

The same study found something else. Some dogs carried the insertion with deletions in one part of it, and those deletions allowed normal pigmentation. That's the basis of what breeders sometimes call cryptic merle: a dog that carries a version of the merle insertion but has a coat that looks solid, or nearly so.

Blue merle, red merle and other colours

Merle dilutes pigment in patches, so how it looks depends on the base coat underneath:

• Blue merle: merle on a black coat. The diluted patches look grey or bluish against solid black.
• Red merle: merle on a brown (liver) coat. The diluted patches look lighter red or beige.
• Merle with white or tan: merle dogs can also carry white markings and tan points from other genes, giving tricolour merles.

Merle can also reach the eyes. When diluted areas extend across the face, a merle dog can have one or two blue eyes, or blue patches within an eye. Our guide to blue eyes in dogs covers the other routes to blue eyes too, and our coat colour hub explains how merle fits with the other colour genes.

Double merle and hearing

The 2006 study reported that dogs with one or two copies of merle can show a range of hearing and eye abnormalities. A 2009 study of 153 merle dogs measured hearing directly and compared single and double merles:

Double merles were significantly more likely to be deaf than single merles. A double merle usually comes from pairing two merle parents: on average, about one puppy in four from that pairing inherits two copies. If you're considering a merle puppy, ask the breeder whether either parent was merle, and talk to your vet about hearing and eye checks.

Breeds with merle

The merle insertion turns up in merle dogs of several breeds. The Shetland Sheepdog and the Australian Shepherd are two of the best known, and merle also appears in some mixed breeds and designer crosses, including some Cockapoo lines. Our guide to the Cockapoo covers merle in that cross.

Can a DNA test show merle?

The PuppyGenes coat colour section reports merle where the sample supports interpretation, along with colour, pattern and white markings. In some samples merle can't be determined. In Juno, one of our public example reports, the merle result reads "could not be determined", because the sample had too little information to rule it out. So a DNA report isn't a substitute for asking a breeder about the parents.