The Merle Gene: Beautiful and Dangerous

I have been breeding dogs for forty years. In that time, I have seen fashions come and go, watched breeds rise and fall in popularity, and witnessed the best and worst of what breeders can produce. Nothing has given me more sleepless nights than the merle gene.

The first time I saw a blue merle Australian Shepherd at Crufts in 1987, I was utterly captivated. That swirling pattern of diluted pigment against darker patches, those often striking blue eyes - it looked like someone had painted the dog with watercolours. I understand completely why puppy buyers queue up for these animals. The problem is, most of them have no idea what they are actually purchasing.

What Merle Actually Is

Let me be absolutely clear from the start: merle is not a colour. Merle is a gene that affects how colour is distributed in a dog's coat. More specifically, it is a dilution pattern caused by a mobile genetic element called a SINE insertion in the PMEL17 gene. This insertion causes random dilution of eumelanin, the pigment responsible for black and brown colouration.

The genetics in plain terms:

A dog with one copy of the merle gene (Mm) displays the merle pattern. A dog with no merle genes (mm) is non-merle. A dog with two copies (MM) is a double merle - and therein lies the tragedy we must prevent.

The merle gene does not just affect coat colour. It influences pigmentation throughout the body, including the eyes and inner ear. This is crucial to understanding why double merles so frequently suffer from blindness and deafness. The same mechanism that creates those beautiful blue eyes can, in excessive doses, destroy the structures needed for sight and hearing.

The Breeds Most Affected

When I started in dogs, merle was primarily seen in Collies, Shetland Sheepdogs, and Australian Shepherds. The gene had been in these populations for generations, and experienced breeders understood the risks. Today, merle has spread - often through deliberate crossbreeding - into dozens of breeds where it has no historical presence.

I have personally encountered merle in Bulldogs, Chihuahuas, Cocker Spaniels, Dachshunds, Great Danes, Poodles, and even Pomeranians. The French Bulldog breeding world has been particularly affected, with merle Frenchies commanding extraordinary prices despite the combination of brachycephalic issues and potential merle-related problems.

Why Merle Demands Respect

I use the word "dangerous" in this article's title deliberately. Not because merle dogs are aggressive - quite the opposite, many are wonderful companions. The danger lies in what happens when breeders treat merle as merely an aesthetic choice without understanding the underlying genetics.

Every merle dog carries within it the potential to produce double merle puppies. Every single one. It matters not whether the merle is subtle or dramatic, whether the dog's eyes are blue or brown, whether the pattern covers most of the body or just a small patch. The gene is present, and it will follow Mendelian inheritance patterns regardless of our wishes or our ignorance.

I learned this the hard way in 1994. A breeder I trusted assured me his merle Collie was actually a "cryptic" that barely showed the pattern. Visual inspection, he said, confirmed it was safe to breed to my merle bitch. He was wrong. That litter produced two double merles. I still remember the day we realised what had happened, watching those white puppies fail to respond to sound, their eyes milky and underdeveloped. It was the worst moment of my breeding career.

The SILV Gene and Its Variations

Modern genetic testing has revealed that the merle gene is more complex than we initially understood. The SINE insertion that causes merle varies in length, and this variation produces different expressions of the pattern. Researchers have identified several allelic variants:

• M (Classic Merle) - The full merle pattern, clearly visible
• Ma (Atypical Merle) - Modified merle, often with unusual patterns
• Ma+ (Atypical Plus) - Slightly higher expression than Ma
• Mc (Cryptic Merle) - Little to no visible pattern, but still genetically merle
• Mc+ (Cryptic Plus) - May show minimal merling

This complexity means that a dog can carry the merle gene without showing any obvious merle pattern. These cryptic merles are perhaps the most dangerous of all, because they slip under the radar of breeders who rely on visual assessment alone.

The Beauty We Must Preserve Carefully

I do not write this to condemn merle dogs or those who love them. A well-bred merle from health-tested parents can be a magnificent animal. The swirling patterns are genuinely beautiful, and the dogs themselves often possess wonderful temperaments. My objection is not to the gene, but to the ignorance and greed that surrounds its misuse.

When a breeder produces merle puppies without genetic testing, they are gambling. When they breed two merles together, they are creating predictable suffering. When they sell "rare" double merle puppies to unsuspecting buyers as "albino" or "white merle" dogs, they are committing an act I can only describe as morally bankrupt.

My Commitment to Education

I have spent the past three decades trying to undo the harm caused by that 1994 litter. Every puppy buyer I meet hears about merle genetics. Every breeder who asks my advice receives the same message: test, test, test. The reference resources on canine genetics are freely available. There is no excuse for ignorance in 2026.

The merle gene is neither good nor bad. It simply is. What matters is how we handle it. Read the other articles on this site. Understand how to breed responsibly. Learn to recognise the warning signs. And please, if you take nothing else from my forty years of experience, remember this: the most beautiful pattern in the world is not worth a single blind, deaf puppy.