Sarah Hartwell, 2016
Photos copyright of the individual contributors.

The terms “pseudo-merle" or "merle effect” are only used here because there is no current term for this phenomenon in cats. I've used the terms descriptively, not genetically. I've used these terms to refer to non-dilute patches on a dilute colour, or to non-silver patches on a smoke, tipped/chinchilla or shaded silver cat.

True merle is a pattern in a dog's coat where a dilute or roan colour is patched with the corresponding dense colour e.g. black patches on blue/grey, red/brown patches on red roan. The canine merle gene also causes blue eye pigment. In dogs, it’s a dominant mutation and homozygous merles - homozygous recessives -, if they survive, are white-coated and may have eye defects and deafness. This is not the case in the cats displaying this pattern

Visual Effect

In cats, there is no clearly identified gene and the effect it appears to be cosmetic only. Observation finds it to occur in smoke, tipped and shaded silver cats where some patches lack the silver undercoat. It appears to be most common on red-silver (chinchilla or shaded silver) cats that have non-silvered patches. It can occur in any breed, for example Janet Poulsen had a red-silver Norwegian Forest Cat with solid red patches. This cat’s grandson, Watzi, had a mix of red-silver areas and some non-silver (red-tabby) areas which made him look like a tabby-tortie cat. Watzi had two significantly large areas on his back where the coat was red all the way to the skin. His father was a very pale red silver that looked cream, but was not genetically dilute. Watzi’s grandson, Buster, is a black silver mackerel tabby with white. He has patches of brown black.

In 2009, on a mailing list, Bobbie Tullo reported some “red merle” (her term) cats that were red silver, with or without white, and had solid patches of red amongst the red silver part of the pattern. They had copper, gold or occasionally and green eyes and she had observed them most often in Maine Coons and in a few household pets.

Below is an Oriental Shorthair, genetically a chestnut silver spotted tabby and white, but with with non-silver patches resulting in the "merle effect". Kotchera Trail Blazer of San-Toi was bred by Wendy Heidt, a Siberian breeder and is being investigated by Pat Decano (CasaDecano Orientals, Siamese and Colorpoint Shorthairs). Pat has also considered chimerism as a possible explanation.

Lesley Morgan provided photos of a South African Exotic Shorthair (registered as fs 09 = black smoke with white), below. Her black areas are definitely smoke, not solid. She was owned by Kaai Du Pleases and his partner, Dirk. Janet Poulsen's Watzi is also shown below.

Possible Genetic Factors

The underlying genetics are not yet known. No single mutation has been identified so genetic explanations are based on observation and on “what hasn’t been found.”

Leslie Lyons has theorized that "silver" (white undercoat) may not be an exome mutation and that the visual effect is due to clonal differences in the regulation of pigment production or pigment transport. Others suggest that silver is a bit unstable and tends to revert to wild-type. Clonal differences means a change occurs in a skin cell and when this cell divides and multiplies as the embryo or kitten grows, the daughter cells all inherit the change (the change might be subtle or might be dramatic). A single skin cell that somehow suppresses the silver gene could give rise to a distinct patch on non-silver fur. A single patch may break up and move apart as the skin surface increases, resulting in several coloured patches with the gaps filled in by the base colour.

If silver was an exome mutation, it would already have been found as the feline genome is well studied. The exome is the part of the genome formed by exons. Exons are DNA sequences which, when transcribed, remain within the mature RNA in cells of any type and not just transcribed in specific cells as part of cell specialization. Exomes account for a very small percentage of the genome, but exome mutations can have dramatic effects.

The red-silver dapple pattern also seems more common in combination with white spotting, which suggests a link. Unlike true merle, this pattern in cats is not linked to recessive white. Because pedigree cats with this pattern don’t meet exhibition standards, there hasn’t been selective breeding for the trait so data is limited.