Double Merle is the common term for homozygous merles (dogs with 2 copies of the merle gene). The abbreviation "MM" is also commonly used (genetic shorthand for 2 copies of the merle gene). These dogs are also sometimes (inaccurately) referred to as lethal whites (this is considered by many to be a derogatory term). Merle is a dominantG gene (technically, it's "incompletely dominantG"). A dog with one copy of the gene will have partial lightening (a normal merled coat). With two copies, the effect is doubled, resulting in some to most of the coat turning white.
Double Merle pups are born when both of the parents are merle. It doesn't matter what color merle, how much merle the parents have, how little (or how much) white trim, what breed they are, or even that the parents are the same breed. Statistically, 25% of the pups from two merle parents will be Double Merles.
(Remember, all pictures with blue borders are links.)
Is this a problem?
Often, but not always, Double Merles will have hearing deficiencies. As with Pattern White pups, if the excess white does not affect inner ear pigmentationG, they will be able to hear, or have only minimal hearing loss (or possibly only in one ear). It is not possible to tell if a Double Merle dog is deaf by looking at any visible hair on the ears (look through our Pictures page for examples). An interesting article is at Clan Duncan Shelties Growl Pages - Seeing Double.
Double Merle dogs may also have a variety of eye defects that can occur in any color eye (see Double Merle Eyes for some pictures), referred to as "Merle Ocular Dysgenesis." Many dogs will have multiple eye defects, and most of those will have some vision loss, sometimes to the point of blindness. Visible eye defects are often the easiest way to determine whether a dog is a Pattern White or a Double Merle.
Visible Eye Defects in Double Merles
Some dogs will have an irregularly shaped pupil. The pupil may have spiky projections (called "starburst") or jagged/irregular edges (called "eccentric"). This is not the same as a colobomaG, which can occur in the eye of a dog (or human) of any color or pattern. This can make the eye very light sensitive if the pupil doesn't react as well as it should.
Be aware that some dogs with no vision defects can have darker pigment in their iris which can resemble a misshapen pupil, so look closely to be sure what you are seeing. Often, it is best to get the opinion of a veterinary ophthalmologist.
Corectopia is the term used to describe a pupil that is subluxated (usually referred to as a "dropped pupil"). In this condition, the pupil is not centered in the iris. When the pupil is off just a bit, the dogs seem to cope well, but can have trouble when the condition is more pronounced.
Microphthalmia, or an abnormally small eye, is the most common eye defect seen in homozygous merles. This can vary from just noticeable to appearing to have no eye at all (anophthalmia). Usually, the smaller the eye, the more of the third eyelid that is permanently visible, and the more vision trouble they have.
These defects are generally stable, so if a pup has functional vision, that will probably not change as it matures, unless affected by another condition (see below).
"Invisible" Eye Defects in Double Merles
Double merle dogs are more likely to have defects within their eyes, which will require an exam by a veterinary ophthalmologist for proper diagnosis. None of these conditions are exclusive to double merles. Most of the time, their eyesight stays pretty much the same throughout their life (whether it starts out pretty good or very poor). In some cases, they will gradually lose their vision, but often they will cope so well that no one ever notices. If a dog seems to be having vision trouble, or if the appearance of their eye changes, a veterinary ophthalmologist should be consulted.
It is also important to remember that merle dogs with vision problems are not always Double Merles (especially if the dog has "gone blind," but does not show any of the above mentioned conditions). The same is true for deafness. As already stated, the visible eye defects common to Double Merle dogs are generally stable (what the pup is born with is what the adult dog retains). There are other eye conditions that can cause a dog to lose its vision that have nothing to do with whether it is a merle or not. See Can You See?: Inherited Eye Disease in Aussies for more information.
Remember though, that not all Double Merles are deaf, and not all are blind (and very few are both).
Are There Other Problems?
No. Although you may hear otherwise, these are the only known health problems proven to be associated with homozygous merles. Anecdotal stories abound about "internal organ" problems, or allergies, or immune system problems, or a shortened life span. The fact of the matter is that many of their normally pigmented kindred suffer from the same problems (and worse). See ASHGI - The Dirty Dozen Plus a Few: Frequency of Hereditary Diseases in Australian Shepherds for more information.
Think About It
Many Double Merle dogs come from disreputable sources, such as uninformed back-yard-breeders or puppy mills (none of which are concerned with the health of the dogs they breed or produce, or the emotional impact on the unsuspecting families purchasing those dogs). Health problems can (and do) crop up in the lines of the most conscientious breeders, who always have the best interests of the breed foremost in mind.
This being the case, how much more likely is it that dogs produced by unconcerned/unknowing breeders will suffer from these health problems with greater frequency? The conundrum is further complicated by the proliferation of Double Merles in rescue. It is simply unrealistic to generalize about the health of an entire population of dogs based on a few individuals who typically have completely unknown family histories.
The White Aussie Project 2003-2005