Merle

Description

The M-Locus test looks for the variants that cause the merle coat pattern in dogs. Merle is found in Australian Shepherds, Collies, Shelties and a number of other dog breeds. This phenotype is inherited as an autosomal, incompletely dominant trait. The merle gene creates mottled patches of color in a solid or piebald coat, blue or odd-colored eyes, and affects skin pigment.

Dogs that are “double merle,” a common term for dogs that are homozygous for merle (M/M, two copies), are predominantly white and prone to several health issues, including hearing and vision problems. The chance of producing puppies with these health issues increases when two merles are bred together. It is recommended that a merle dog only be bred to a dog that is neither merle nor cryptic merle.

Merle is caused by an insertion in the PMEL gene (also called SILV). The length of this insertion varies, and shorter versions have less effect on the coat. Animal Genetics reports three versions of the merle allele: merle (M), atypical merle (Ma) and cryptic merle (Mc). Cryptic merle dogs do not appear to be merle but carry the merle gene. Many solid-colored dogs are actually cryptic merles, also known as phantom merles, and can produce both merle and double merle puppies if they are not bred carefully.

What an atypical merle looks like

An atypical merle dog (Ma) has a coat pattern that differs from the high-contrast, sharply defined patches of a classic merle. The atypical merle allele has a slightly shorter insertion than the classic merle allele, so it affects pigment production differently. Atypical merle dogs typically show the following:

• Muted or muddy contrast: instead of crisp dark patches against a lighter gray or silver background, the contrast is often low, soft or washed out. The coat can look dusty or dull compared to a standard merle.
• Unusual undertones: the coat may have brownish, grayish or muddy tones rather than the clear black and silver of a blue merle or the rich red and liver of a red merle.
• Subtle or patchy dilution: depending on whether the dog has one or two copies, or a combination with a cryptic or classic merle allele, the coat can range from almost completely solid with minor shading to an odd, hazy pattern.
• Variable appearance: two dogs with similar atypical merle genetics can look entirely different. Some dogs with a single copy look almost solid, while others show unusual spotting or fading.

Appearance alone can be deceiving. Atypical merles are frequently mistaken for solid-colored dogs, so a DNA test that identifies the merle allele type is needed to confirm an atypical merle.

Merle and other colors

Merle can affect all coat colors. Recessive red dogs can also be affected by merle, but the patches are either hardly seen or, if the dog is a clear recessive red, not visible at all. Combinations such as brindle merle exist but are not typically accepted in breed standards.

In addition to altering the base coat color, merle also modifies eye color and the coloring of the nose and paw pads. The merle gene modifies the dark pigment in the eyes, occasionally changing dark eyes to blue, or only part of the eye to blue. Since merle causes random modifications, dark eyes, blue eyes and odd-colored eyes are all possible. Color on the paw pads and nose may be mottled pink and black.

Possible results

Reference

Retrotransposon insertion in SILV is responsible for merle patterning of the domestic dog.

[PMID: 16407134]

Genetics of pigmentation in dogs and cats.