The Merle gene is a much more common subject but still very little known by the general public. We see in the various forums of German Spitz that it is also very controversial. The Merle gene is attributed to a lot of flaws, rightly so, but sometimes also exaggerated.
Indeed, it is a colour that breeders know that must be bred with great caution. But do we really know how the Merle gene, at the origin of the colour and the typical traits, operates?
With this article, I will try to bring some elements in order to shed some lights. Please also note that some thoughts and conclusions expressed here are only my own.
Let’s start with a few definitions to better understand the rest of my article.
▪️ Genotype = genetic trait, inherited from parents
▪️ Phenotype = physical trait, visible to the naked eye
▪️ Allele = variant of a gene. For each gene type, each parent gives an allele to their offspring, which obtains a pair of alleles. A pair of alleles forms a genotype.
▪️ Locus (or plural Loci) = fixed position (of a gene or genetic marker) on a chromosome. We are talking about the M Locus (Merle) here.
[1] Complexity and subtlety of the Merle Gene
The Merle gene, also called M Locus, has multiple variants which are classified into two main categories:
✔️ m: normal variant, non Merle, recessive. Dogs with the genotype m/m are of normal colours without carrying a Merle allele.
✔️ Mx: Merle variant, dominant. The x means the length of the PolyT sequence, which determines how much the Merle pattern will express itself. The longer the PolyT sequence, the more visible to the naked eye the Merle phenotype will be. So a dog with a genotype Mx/m will have a Merle coat or subtle Merle coat or not at all, depending on the length of the x, but the dog carries the dominant Merle allele.
In addition to the coat, the Mx allele is expressed very frequently with light, blue or green eyes in dogs.
As a result, the Mx allele has multiple sub-variants, divided into 5 sub-categories, depending on the length of the PolyT sequence:
🔹 M(34T) and below: Mc or Cryptic Merle, also called Phantom Merle. The Merle pattern will not be visible. However, there will be signs in some dogs, such as light, blue or green eyes. Only a genetic test can confirm if the dog is Cryptic Merle.
🔹 M(35T) — M(62T): Mc or Cryptic Merle, also known as Phantom Merle. Dogs in this category will have a very subtle, barely visible expression of the Merle colour. Like the previous category, however, there will be signs in some dogs, such as light, blue or green eyes. Only a genetic test can confirm if the dog is Cryptic Merle. Because the Merle colour here expresses itself in subtle ways, it can be hidden by certain other colours, such as sable, white, cream and other light colours.
🔹 M(63T) — M(73T): Md or Dilution Merle. Dogs carrying the Dilution Merle allele will have parts of their coat of diluted colours.
🔹 M(74T) — M(85T): M or Merle. It is the typical Merle that we know and is visible to the naked eye.
🔹 M(86T) and beyond: Mh or Harlequin Merle. The patches of Merle colours are spread over a white coat. Some dogs will look like Particolor dogs.
Some Laboratories have introduced a little more granularity into the Merle allele spectrum, with a slightly different nomenclature. Some speak of Mc and Mc+ for the Cryptic Merle, Ma and Ma+ standing for Atypical Merle for the Dilution Merle.
Other Laboratories have even introduced an additional Locus: the H Locus to designate the Harlequin Merle (Mh). However, they also specify that the H Locus test must be carried out in combination with the M Locus (Merle). Why not, but you would then have to pay the price of two genetic tests instead of a single one.
Some scientific researches have revealed that not only can the Mx allele be of different length, but that a same dog can carry a mosaic of Mx alleles of different lengths. Because of this different mosaic of Mx alleles, the patches of Merle colours are very often irregular.
In addition, the length of the PolyT sequence of these Mx alleles can shorten or extend from one generation to the next. The mechanism that influences this phenomenon of shortening or extension is still unknown. Genetic tests will always indicate the Mx allele with the longest PolyT sequence because it shows the extent of the Merle colour expression and its maximum genetic influence in a dog.
[2] The Dos & Don’ts
A majority of people think that because a dog is Merle coloured, it necessarily has health impairments.
We now know that the Merle “gene” is indeed linked to a number of health impairments, such as blindness, deafness, increased sensitivity to the sun and a higher rate of skin cancer.
However, it is rare to observe these health impairments in heterozygous Merle dogs, i.e. those with a genotype Mx/m. On the other hand, these health impairments are very common in homozygous dogs Mx/Mx.
Remember that a pair of alleles forms a genotype and that a puppy receives an allele from each parent.
When 2 Merle dogs are bred together, so Mx/m with Mx/m, statistically, they will produce 50% of Merle Mx/m puppies, 25% of non-Merle m/m puppies, and 25% of Mx/Mx puppies.
These homozygous Mx/Mx puppies are those at high risk of having the health impairments mentioned above. It has also been observed that these puppies are very often white.
Yes 1/4 of the offspring is at risk! And that does not mean that the other puppies of that litter are without any of the health impairments either.
It is commonly known that it is therefore not advisable to breed 2 Merle dogs together, i.e. Mx/m with Mx/m. Personally, I would even say that it is not advisable either to breed 2 Cryptic Merle dogs (Phantom Merle) or Dilution Merle together, and I will tell you why later in this article.
As explained hereinbefore, it is not always easy to detect a Merle dog, and the “gene” has very complex and subtle patterns.
The Cryptic Merle or Phantom Merle dogs do not appear Merle, although some signs can be detected with the naked eye such as the eye colour that may be light, blue or green.
But because it is not the case for all dogs, a genetic test is needed to confirm. Even without a genetic test, when a parent is Merle and the puppy does not show the Merle traits, I think it is necessary to warn the buyer, because this puppy can be Cryptic Merle (Phantom Merle), and especially if this puppy is destined to breed.
I would advise that a genetic test on the M Locus be done on such a puppy, in order to avoid:
🔺 breeding it to another Merle dog (Cryptic Merle, Dilution Merle or Merle),
🔺 or introducing Cryptic Merle alleles in lineages that we want to keep in line with the FCI standards. Yes, remember that Merle colours are not permissible by the FCI.
I have previously mentioned that Harlequin Merle dogs can be confused with Particolor dogs. And yet, Particolor is a permissible colour by the FCI……. This can cause a lot of confusions and risks when breeding.
Personally, and this is my own opinion, the introduction of Particolor dogs in a lineage must be done with caution and never without genetic testing of the different genotypes, so to be sure to avoid:
🔺 breeding a Merle with a Particolor dog that is actually a Harlequin Merle,
🔺 or introducing Merle alleles of a Particolor dog that is actually Harlequin Merle, into lineages of colours that we want to keep in line with the FCI standards.
I am not a big fan of Particolor dogs anyway because I also think that, even if they are not related to the Harlequin Merle allele, their typical genotype which is quite dominant, can bring colour flaws (without other risks though) and make the offspring off-colour standard. But we deviate from the subject. Maybe this will be the subject of one of my future articles. 😊
The table on the picture on the heading here in my post has been circulating on the internet. I do not know itssources. It shows the possible combinations and those at risk between dogs carrying the different type of Merle alleles. It shows that dogs with Cryptic Merle alleles and Dilution Merle alleles can be bred with each other and even with typical Merle dogs too.
I do not agree at all with this part of the table: I circled that part in red and I added the words “Attention ⚠️”. Again, this is only my opinion.
When one knows the complexity and subtlety of how the Merle “gene” works, one cannot agree with that part of the table. Personally, I would not venture to make these kinds of combinations that seem “safe” according to the table.
Not only is it not scientifically proven or even observed by experience, but the authors of that table have not taken into account the fact that the Merle allele can have its PolyT sequence length extend from one generation to another, thus increasing the risk of health impairments for the dogs resulting from such combinations.
[3] Examples of 3 of my Poms: Blue Moon, Chanel and Scarlett
I had all my Poms tested on all Loci, including the Merle one, to be sure not to introduce Cryptic Merle alleles in my standard colour lineages and not to breed an Cryptic Merle dog with a Merle one, or 2 Cryptic Merle dogs together, and thus to avoid the aforementioned health impairments.
Here are 3 examples of my Poms that are with so-called “exotic” colours: Blue Moon, Chanel and Scarlett. They either carry the Cryptic Merle allele or have Merle ascendants.
✔️ Blue Moon is light blue and his genotype is Mc(29T)/m. He is therefore Cryptic Merle with a PolyT sequence of rather small length, according to the above sub-categories. I suspected he would be Cryptic Merle because he has got light greenish eyes. His dad is lilac (lavender) and his mom is white & blue Particolor.
✔️ Chanel is blue-Merle and her genotype is M(76T)/m. She is typical Merle. She has hazel-blue eyes. Her dad is blue Merle and her mom is cream-sable.
✔️ Scarlett is dark blue and her genotype is m/m. She is therefore not Merle at all. Her father is lilac (lavender) and yet her mother is Merle (typical).
It can be inferred from their genotypes of the M Locus as well as from the phenotypes of their parents, that the breeders did a proper job.
