Homozygous (or Double)
A dog that is of the merle pattern, that received two copies of the gene that codes for the merle
color, is said to be a double merle, or a homozygous merle (M^M)
A "M/M" (homozygous dominant) dog, known as a double merle (from a merle to merle mating), has
much more white than is normal for the breed and may have hearing loss, vision problems including
small or missing eyes, and possible infertility. The health effects seem worse if a gene for white
markings is also present. In Border Collies and Australian Shepherds, all of which normally have fairly
extensive white markings, the "M/M" white has a strong probability of being deaf or blind.  A double
merle mated to a non-merle (black in color), is the only way to produce 100% merles (except breeding
two double merles together and that is
STRONGLY discouraged).
This female has perfect
hearing and eyesight.
Some of you may have read the following on a particular breeder of "Australian Koolie's"

Having put all the facts out there it is best to write I own three deaf Koolies myself and have never
had a reason to complain.  Deaf Koolies will learn as well if not better because they are not
distracted as easily as a hearing pup.  They are totally devoted to you and follow you everywhere.  
There is more help available these days for owners with deaf dogs.  The upside of owning a deaf
Koolie, they won't bark at the dogs next door or join in when the doggie chorus begins at three in
the morning.

Even those with reduced eye sight adapt, one of my boys has only thirty percent of sight and yet
plays and navigates around our small property and knows every inch of it

To knowingly breed two dogs when a person knows there is a strong possibility of producing a
genetic defect (blindness or deafness) is unethical and irresponsible.   How cruel can a person
be to knowingly allow the breeding of dogs she knows will produce a blind or deaf, or blind/deaf
puppy or puppies.  

Deaf puppies bark MUCH louder and longer than a puppy with normal hearing, because he can
not hear himself bark.  A deaf puppy has a much lower survival rate than a hearing one.  Why?  
Because he can't hear a rattle snake rattle, he can't hear livestock running toward him (if he's
being raised on a farm/ranch), he can't hear farm implements coming his way, he can't hear
vehicles start, he can't hear preditors coming at him.  The response from most will be - "keep him
up".  That's fine, but that's not always possible.  A cougar or coyote can jump a fence.  

If he's visually impaired, he may become frightened and run toward the danger, instead of away.  
He can't see the preditors coming after him.  

The point being, do NOT breed dogs that could possibly produce a genetic defect.  Do NOT buy
or accept for free a blind/deaf puppy from a breeder, because this ONLY encourages them to
breed more!  They produced the genetic defect, now it's their responsibility to give that puppy
the best care for the rest of his/her life.

Accidents are one thing and are forgiven --- but to knowingly and willingly continue to breed the
same pair that produce defects over and over, is a totally different thing!     
This pup has perfect hearing
and eyesight.
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Coolie Stud/Ida Parmer