How do breeders use genetics to avoid double merles?

Breeders use an understanding of genetics and modern DNA testing to carefully plan litters and avoid producing double merles (MM), which are at a high risk for severe vision and hearing impairments.

Here is how breeders apply genetic principles to prevent double merles:

1. Recognizing That Harlequins Are Merles. A crucial genetic discovery was that the Harlequin gene is a modifier that only works in the presence of the Merle gene. Therefore, every single true Harlequin dog genetically possesses at least one Merle allele. By understanding this, breeders know that a Harlequin x Harlequin or Harlequin x Merle mating is genetically a Merle x Merle mating, which carries a 25% risk of producing double merle puppies. To avoid this, responsible breeders will not breed two of these dogs together.

2. Utilizing DNA Testing for “Cryptic” Merles. While a typical merle or harlequin dog is easy to identify visually, the merle mutation comes in different lengths. Dogs with a shorter insertion of the mutation are known as “cryptic merles”. These dogs carry the merle gene but show no obvious merle pattern on their coats, appearing solid-colored. Breeders use DNA testing at the M-locus to uncover these hidden cryptic merles. This testing ensures that a dog who looks solid black isn’t accidentally bred to a visual merle, which could unintentionally produce a double merle.

3. Planning “Safe” Breeding Combinations Because merle and harlequin are highly desirable colors, breeders use genetics to produce them safely without risking double merles. They do this by ensuring only one parent possesses the merle gene.

• Safe Pairings: A visual Harlequin or Merle will typically be bred to a dog that is genetically clear of the merle gene, such as a solid black or a Mantle (a black dog with Irish white spotting).

• Using Hidden Harlequin Carriers: Because a dog must have the merle gene to physically express the harlequin pattern, a non-merle dog (like a solid black) can safely “carry” the harlequin gene without showing it. Breeders can DNA-test a solid black dog to see whether it carries harlequin; if it does, breed it to a standard blue merle. This mating can successfully produce harlequin puppies with zero risk of producing double merles.