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Australian Shepherd Colors: The Dangers of Double Merle

Unfortunately, the merle gene can be pretty dangerous and cause some serious health issues such as eye deformities and deafness.

The Genetics of Merle

The color merle is a dominant trait, which means that it if the merle gene is present in a dog’s genome, it will always be expressed (aside from some rare cases, which we will talk about in a moment).  Genes are the areas of DNA that give an organism its specific characteristics. They always come in pairs. Aussies with one copy of the merle gene are called heterozygous merles. (This name tells scientists that the Merle gene is paired with a non-merle gene.)

These dogs are usually healthy, though they are sometimes born with merle-related deformities. Aussies with two copies of the merle gene (a matching pair) are called homozygous merles, or double merles. These dogs have a high risk of eye deformities or deafness because of the way the merle gene also affects development of the eyes and ears.

What Do Double Merles Look Like?

Before we go into detail about the defects caused by double merle, let’s talk about what a double merle Australian Shepherd might look like.  Many double merle Australian Shepherds appear depigmented, meaning their coats have large areas of white, and areas that are typically dark, such as noses and eye rims, will either be completely pink or spotted with pink.  Double merles frequently have pale blue eyes, but it is possible for Aussies to have this eye color and not have any genes for merle at all. It is also possible for double merles to look just like a healthy, heterozygous merle.

A Merle Quiz

On her website all about the genetics of color in dogs, Dr. Sheila Shmutz has put together a little quiz about merles.

Can you guess which dog out of all the pictures is heterozygous instead of homozygous? Me neither.  In other cases, it is extremely easy to tell a homozygous merle from a heterozygous one. This is because of the health defects we mentioned briefly before. Some of these defects, especially those of the eye, are quite visible.

Health Defects Caused by Double Merle

Unfortunately, double merles often suffer from defects of the eye and ear. The eye can be deformed in many different ways, and just about any part of it may be affected. The iris may look deformed, the pupil could be off center, or the lens of the eye could be out of place.  There is also a potential for the retina to be abnormal and for the optic nerve to be developed improperly.  Some double merles may also suffer from microphthalmia, which is when the entire globe of the eye is abnormally small.  Some dogs with this condition have even had to have one or both of their eyeballs removed.

Often times, double merles will have a combination of the aforementioned defects in each eye, and because of these defects, many of them are blind.  Aussies that are homozygous for merle are also frequently deaf. It is believed that this deafness is caused by a lack of pigment cells in the inner ear.  The inner ear pigment cells are very important because they help translate sounds into electrical impulses that are then sent to and read by the brain. Without these cells, the translation cannot be done properly, and thus the dog is deaf.

Preventing Merle Defects

Needless to say, none of these conditions are pleasant for an Aussie to live with, and all of them compromise quality of life.

To protect new generations of Aussies, responsible breeders do not mate two dogs who both have merle coloring.

This means that their puppies cannot inherit two copies of the merle gene.

Yellow and Sable Aussies and the Merle Gene

Remember those sable and yellow Australian Shepherds we mentioned?

These are pretty colors, but the AKC does not recognize them in its breed standard and there’s a prudent animal welfare reason for that.The genes which give Aussies yellow or sable coats also mask the presence of the merle gene.

Which is to say, a yellow or sable Aussie can carry the merle gene without expressing merle coloring in their coat.

That makes it impossible to make accurate judgements about which two yellow Aussies can safely mate together.

The risk of accidentally breeding from two dogs who both carry the merle gene and producing sick puppies is very high.

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