Understanding Merle Coats: Patterns & Potential Health Risks in Dogs

A merle coat is a distinctive mottled pattern seen in dogs, formed by patches of diluted colour intermixed with solid areas. Unlike a single coat colour, merle is a genetic pattern that modifies base colours, creating the appealing and often striking look admired by many potential pet owners.

This unique pattern can appear in various colours such as blue, chocolate, or red, and influences not only the coat but also underlying skin pigmentation. It often causes interesting eye colours including blue or heterochromia (one blue eye), making merle-coated dogs visually captivating.

The presence of the merle pattern arises from a well-researched gene mutation. DNA tests can confirm whether a dog carries the merle gene, though these tests only indicate genetic markers without predicting the exact colour variations.

However, the gene responsible for the merle coat isn't just about aesthetics. It can also impact a dog's health, especially if two merle genes are inherited, increasing risks for serious congenital conditions. This article explains the merle coat in detail, its genetics, the breeds it appears in, and the importance of responsible breeding for health and welfare.

What Does a Merle Coat Look Like?

The merle gene creates a mottled or patchy coat pattern by diluting parts of the dog's base colour. This results in irregular patches of lighter shades embedded within a generally solid or piebald coat.

Dogs may present with merle over much of their body or have small, cryptic patches of merle. The coat colours affected include numerous shades of blue, brown, or black. This genetic modifier also influences skin colouration on areas such as the nose and paw pads, as well as eye pigment, commonly producing blue hues or varying eye colours in affected dogs.

How Does a Merle Coat Occur Genetically?

The merle pattern is controlled by a mutation in the PMEL (SILV) gene, which affects pigment production leading to areas of diluted colour. The inheritance of this gene is incompletely dominant, meaning the expression depends on one or two copies being inherited:

1. Single merle (heterozygous): Dogs with one copy usually show the classic merle pattern with mottled patches.
2. Double merle (homozygous): Dogs inheriting two copies often have predominantly white coats and are at much higher risk of health problems.

Breeding two merle-coated dogs carries a 25% chance of producing double merle puppies, which can suffer from significant health defects. Cryptic merle is a variation with minimal visible patches, complicating detection without genetic testing.

Modifier genes can create other interesting variants like harlequin merle, famously seen in Great Danes.

Dog Breeds Commonly Exhibiting the Merle Pattern

Several dog breeds have the merle coat recognised as standard or acceptable and are sought after by enthusiasts. These include:

• Great Dane (harlequin type)
• Shetland Sheepdog
• Dachshund (known as "dapple")
• Australian Shepherd

Yet in some breeds, merle is considered a fault and may be prohibited for pedigrees registered with The Kennel Club. This restriction arises mainly to minimise the increased health risks associated with producing double merle offspring, promoting ethical breeding practices.

Health Issues Linked to the Merle Gene

The presence of the merle gene can carry health implications beyond coat pattern, especially when two merle copies are inherited (double merle). Common concerns include:

• Congenital Deafness: Can affect one or both ears, arising more frequently in double merles but less common among single merle dogs.
• Ocular Defects: Abnormal eye development, microphthalmia (small eyes), microcoria (small pupils), and in some cases, blindness, often associated with blue or partially pigmented eyes.
• Skin Pigmentation Irregularities: Affecting nose, lips, and paw pads, these features may accompany health concerns.

Research highlights that double merle combined with piebald markings poses the highest risk of severe health defects. Therefore, it is essential for prospective owners to verify whether a dog is single or double merle before purchase.

Responsible Ownership and Breeding Considerations

When considering a dog with a merle coat or merle parentage, it is crucial to:

• Check whether the merle pattern is acceptable within the breed standard, especially for Kennel Club registration.
• Request genetic testing to confirm single or double merle status.
• Avoid breeding two merle dogs together to prevent double merle offspring and the associated health problems.
• Work with reputable breeders who prioritise health and responsible breeding practices.

Responsible breeding and informed purchasing decisions help maintain the health and welfare of dogs with this striking but genetically complex coat pattern.