Health Problems Associated with the Merle Allele

Both heterozygous merle (Mm) and homozygous double merle (MM) dogs may exhibit auditory and ophthalmic abnormalities including mild to severe deafness, increased intra ocular pressure, ametropia, microphthalmia and colobomas. The double merle genotype may also be associated with abnormalities of skeletal, cardiac and reproductive systems.

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Due to the amount of colors and patterns occurring in the Chihuahua breed the ethical breeding of the merle pattern can be much more difficult than in other breeds who limit the allowed colors and patterns.  The purpose of these guidelines is to give breeders the beginning tools to avoid or limit the production of hidden merles.

 

1. Merle should never be bred to another merle.  The purposeful production of double merle puppies is never ethical.
2. If you suspect a dog MAY be a hidden or cryptic merle, it is essential that you have the genetic testing performed to be certain if the pattern is present.  A hidden merle is a merle dog who does not exhibit the merle pattern because their coat color does not show the pattern.  Merling is not normally shown in red, gold, fawn, or cream coat colors.  Cryptic merles are dogs that are the appropriate coat color, but do not show any or very slight merling which causes them to appear as non merles.  Breeding a dog to test whether or not he/she is a hidden or cryptic merle is not sufficient, as
   Chihuahuas
   often have small litters and may carry colors that would hide the merle pattern in their offspring.
3. Merle should not be purposefully crossed with colors or patterns that will create hidden merle puppies.  This includes sable, cream, ee/recessive red, or brindle.  It is important to note that not all adult "hidden merles" will have visual merling as puppies. Intentional breeding of merles to colors and patterns which produce hidden merling greatly increases the risk of unintentional future merle to merle breedings, which is unethical.
4. Many merle breeds limit the amount of white markings merles exhibit, at this time CCA recommends that we follow this tradition.  In keeping with tradition a merle should not be bred into lines that carry piebald (white dogs with spots) or extreme white spotting patterns (dogs that are mostly white with minimal spotting).  Merles should be bred to dogs that have up to 33% (1/3) white coats, such as a typical irish markings pattern (typically a white collar, legs and tail tip).  This
   tradition arises from the recognition that the incidence of vision and hearing problems increase when white markings cover the eyes and/or ears.  Scientific research has not been completed regarding the merle pattern and white spotting present in a single individual although these patterns have been extensively studied separately.  This area of the guidelines may be revised when scientific research has been completed.   
5. The Merle pattern is best shown on a solid black or black and tan pointed dog.  Chocolate or blue, with or without tan points, are also another option to use in a breeding program.
6. When registering puppy colors/patterns be sure to always include the merle markings even if they are just slightly present or are no longer
   visible. If genetic testing indicates that a Chihuahua is a hidden merle, its registration should indicate "with merle markings", although the markings may have never been visible or have faded/muted at the age of registration. The identification of the gene's presence is the very purpose of testing.
7. All merle should be CERF and BAER tested before being bred.