Double Merle Australian Shepherd

What are the possible outcomes?

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What is a Double Merle?

The concept of a double merle shepherd really is just that, a shepherd from two merle parents. It has become fairly common knowledge amongst Australian Shepherd breeders that this combination of double merles is best avoided as it can result in many genetic disorders and sometimes even death.

Coloration

The typical merle shepherd will have a single Merle gene (genotype: Mm) this will result in a slight lightening of the coat resulting in the commonly strived for "merle" coloration. When you have a double merle shepherd (genotype: MM) you can achieve an almost white coloring.

The partial and full coloring results from the Merle gene being what is considered an incomplete dominant gene. While it will show a partial effect when only one allele for merle is present, both alleles have to be a "merle positive" allele to show the full effect.

Sight & Hearing

Often double merle shepherds, or any merle dog regardless of breed, will suffer from a variety of vision or hearing impairments. While it is highly unusual to see a merle shepherd with both complete blindness and deafness it does occur.

Hearing deficiencies can range anywhere from partial hearing loss (just one ear) to complete deafness in both ears. This is most affected by the coloring of the inner ear pigmentation, if the traditional darker coloring remains in that location then the hearing will likely not be affected.

In regards to vision impairments there are many of these that are visible and some that are not. Of those that are visible, there may be an irregular shape or location of the pupil in the iris, or sometimes the eye itself will be smaller. While these can all result in the eye being more sensitive to light, often times these dogs will still have functional vision.

Genetic Pathway

Often when it comes to genes we assume things are fairly black & white. However, there are many situations such as this that aren't so cut & dry.

The merle gene itself is, as mention earlier, incompletely dominant. By being labeled as such this particular trait will only be shown to a partial extent when only one is present (Mm). In order to be exhibited to it's full capability, the given dog would have to be a homozygous merle (MM), which is when we are more likely to see the excessive white coloration.

While this particular trait may go by different names, it can be seen amongst a broad range of breeds: Dachshunds, Border Collies, Great Danes, and Australian Shepherds just to name a few.

Genotype to Phenotype Translation