In its classic and probably best-known form, the Merle gene in (Classic) Merles (m/M) causes a brightening of the Tricolors black color to a light silver-blue (dark steel gray is undesirable). This is why they are also called Blue Merles. Due to its incomplete penetrance, areas of the original color (here black) always remain, in form of small or large patches, that often appear torn. Because only eumelanin appears to be affected, the pheomelanin of the reddish-brown tan markings is normally unaffected by such changes. In the Blue Merle, the brightened coat (sometimes it darkens a bit) with the dark spots remains for life. In the Sable Merles, on the other hand, which developed from the brightening of sable-colored coat, the darker sable mottling is usually only visible in very young puppies. Later, the spots usually disappear, so that an adult Sable Merle can often no longer be easily distinguished from a Sable without a Merle factor. They are then also called Hidden Merles, as well as the other variants where Merle is not or hardly visible (see below). Sable Merles that originate from Pure Sables are often very similar to Gold Sables when fully grown, although the color can often appear faded. Sable Merles that originate from Dark Sables can also be quite light, but they can also develop a fairly strong, often almost reddish-brown color. Very rarely, Classic Merles can have one or more lighter spots that tend towards White. This has nothing to do with the Harlequin/Fawnequin variant (see below), even if it looks that way at first glance. Such spots are called Dilute Spots. 

Cryptic Merles have the m/Mc or m/Mc+ gene variant. In terms of their phenotype, they look like normal Sables or Tricolors. Even if the gene section already shows an extension here, it is not long enough to produce pigment brightening anywhere. This is how the dogs grow up with their original intended coloring. There are reports here and there that the puppies may be a touch lighter at their hair-ends shortly after birth, but this is no longer visible some time later. The genetic change is so small that, based on current knowledge, one could theoretically even breed Cryptic Merles with the m/Mc, m/Mc+ and m/Ma variants without puppies being born with health problems. By the m/Mc variant (without a plus sign) even m/Ma+, m/M is added. In practice, however, it remains forbidden as long as the legislature fundamentally prohibits the mating of Merles, because he does not differentiate between the different Merle variants.

Atypical Merles have the m/Ma or m/Ma+ gene variant. While by the m/Ma variant a slight brightening of the base color is possible at the best, comes by the second variant m/Ma+ a slight Merle pattern usually through. The Tricolore's fur becomes silver-greyish, a faded merle pattern may or may not be visible. Sable can appear a little paler and no pattern will be visible here by adults. Atypical Merle can be seen as a kind of transitional form from Cryptic Merle to Classic Merle. The extended gene section is not as short as in the Cryptic Merle, but also not as long as in the Classic Merle. Similar to the Cryptic Merle, according to current knowledge, no damage to the health of the resulting offspring is to be expected from a theoretical mating of m/Ma with the variants m/Mc, m/Mc+ and m/Ma. With the m/Ma+ variant, mating with m/Mc would theoretically be possible. But in practice both remain prohibited, according to the requirements of the legislature.

At first glance, Harlequins (Harlekins) resemble classic Blue Merles, and Fawnequins (Fawnekins) resemble classic Sable Merles, depending on the intensity of their characteristics. Often, the brightened areas (such as the silver-blue of the Blue Merle) are even brighter. Furthermore, the desired fine spottings in the base color (Tricolor in the Blue Merle and Sable in the Sable Merle) is usually less pronounced and more likely to be found in patches. These are then also called Herding Merles. Very large patches are also called Tweed. Many are found more at the edges and less within the lightened areas. This is also referred to as a "loud Merle pattern." In Fawnequins, the color can deviate from the expected natural Sable color, sometimes appearing faded, ranging from a light gold tone to a reddish brown tone, the latter especially when a Tricolor allele is present. Particularly striking are the often newly added, more or less extensive white areas without pigmentation. These are usually the first obvious indication of Harlequins or Fawnequins. They are not caused by a white factor. The tan markings are usually normal, although sometimes somewhat paler. Since Harlequins/Fawnequins have the same genetic status as Minimal Merles (m/Mh), these can also arise from them (see below)! These color variations are not to be confused with the Harlequin of Great Danes, which have an additional genetic mutation at the so-called H locus.

Essentially, Harlequins and Fawnequins, as well as the other Merle carrier variants (m/M...), are, according to current knowledge, completely normal, healthy dogs. Accordingly, the changes associated with Merle relate solely to pigment brightening or loss, and not to other illnesses or behavioral effects. Pigment loss and the formation of white areas can only occur in the m/Mh breed due to Merle. This, too, has no harmful effects. Only in the quite unlikely event that the inner ear or the inner eye also suffers pigment loss, is there a risk that puppies could be born with varying degrees of hearing or vision impairment. Because Collies are specifically bred for a colored head, the risk of health impairments is extremely low, in contrast to Dalmatians (hearing) and completely white breeds based on pigment loss, such as White American Bulldogs. To ensure this remains the case, Collies with m/Mh may not be bred with those with a high proportion of white, such as a white factor or a blaze extending high on the forehead, and of course, not with other Merles, as required by law. In my many years of breeding Collies and researching Collies, I have yet to encounter a Collie with the genetic status m/Mh that has any hearing or vision impairments. If anyone ever finds such a case, please let me know so I can correct this.

Minimal Merles, also called Phantom Merles, often look like normal Sables or Tricolors (phenotype) at first glance. If you look more closely, you will often find one or sometimes several small, sometimes tiny spots where the dog has a normal, classic merle color, especially at the edges. Sometimes you will just find a colored dot in the eye. Sometimes nothing is visible at all if the changes are too small or hidden by long fur or color overlaps. This includes White, for example. The Collie does not even need to have a white factor for this to happen; the pronounced collar (Irish Pattern) or white on the legs is enough. If the Merle would be located exactly in this area, (we remember, it is often found in the edges), then it would be covered by White. The same would happen with very rare and undesirable phenomena, such as the e/e at the E locus and the d/d at the D locus (see below). All of these are also called Hidden Merles because the merle is not visible (hidden).

It was a great surprise when it was discovered some years ago that the allele variant of Minimal Merles (m/Mh) corresponds to that of Harlequins/Fawnequins! It was previously thought that they belonged to the Cryptic Merles, a transitional form to Classic Merles, with the first possible visible Merle areas. Due to the actual allele variant, Minimal Merles should not be bred with other Merles, even if the law were relaxed. It was previously thought that the genetic expression of Harlequins/Fawnequins & Minimal Merles was very rare. Today it is assumed that it was probably the first mutation that led to the later expression of the Merle variants known to us. Because here we have the longest A-tail appendage. Changes generally lead primarily to further shortening, which explains the other Merle variants. As Minimal Merles, this genetic variant managed to spread unnoticed due to the hidden appearance. Often, owners of Minimal Merles aren't even aware they have a Merle. They are often astonished when such a dog suddenly produces Harlequin or Fawnequin offspring, due to the same genetic status (m/Mh). Undetected Merle also increases the risk of Double Merles being born with health problems due to unknowingness (see below) after a wrong mating. Only a genetic test for Merle, cleverly using the currently common allele itemisation, can shed light on the mystery, as our breeding regulations require for every breeding dog: https://laboklin.com/en/products/genetics/coat-colour-coat-structure-coat-length/dog/m-locus-merle-alleles-mh-m-ma-ma-mc-mc-m-and-mosaics/ .

Mosaic Merle is a type of mixed form of various Merle alleles. Variations in the Merle alleles are relatively unstable. During embryonic development, which is characterized by numerous cell divisions until the final individual develops, minor variations can occur. Rarely, the A-tail becomes longer. It seems to be quite common for something to break off there, resulting in the A-tail shortening. This can lead to a proliferation of a different Merle variant alongside the original variant. Genetic testing then reveals three or more alleles in different cells instead of two alleles. Typically, the original variant remains the most prevalent, and is then referred to as a major allele. Added variants are referred to as minor alleles; to make them easier to identify, they are placed in square brackets. If, for example, a Classic Merle (m/M) has a shortening in the Merle allele, the Merle allele (M) changes into an Atypical Merle allele (Ma), and the result would be: m/[Ma]/M. Rarely, there is another additional shortening, for example a Cryptic Merle allele (Mc), in which would be also added, also in square brackets, m/[Mc]/[Ma]/M. If there is a plus sign somewhere, it will of course be added. If there is an extension of the original Classic Merle allele, an (Mh) allele is also possible, also in square brackets. Shortening and lengthening can also occur at the same time. Sometimes the respective number of base pairs (bp) is also added, e.g. in parentheses.

Even if three or more alleles are named in mosaicism, this does not mean that they are all found in one cell. It remains a double set of chromosomes (diploid), with two alleles in each body cell, just in a different composition. Thus, in the examples mentioned, in addition to the original M/m variant, cells with m/Ma, m/Mc, or m/Mh can also be found. To avoid unnecessary complications, the result is summarized by listing the named alleles one after the other, placing the minor alleles in square brackets. During the formation of eggs and sperm, the chromosome set in each cell is halved as usual (haploid), thus resulting in sex cells (gametes) with all possible variants. In these examples, in addition to cells with an "m" or "M", one would also find cells with "Ma" and/or "Mc," or possibly also with an "Mh." For male dogs, it may also make sense to submit a semen sample for testing instead of a cheek swab. So Mosaic Merles are completely normal, healthy dogs. The same rules apply to mating as before, namely that they should only be mated with Merle-free dogs. This also complies with legal requirements.

Double Merles are also called "White Tigers" because they like to have large areas of White, including areas of the head. This makes it easy to distinguish them from the White Collies, which always must have a colored head, this is why they are also called Color Headed White (CHW). The White coloring of White Collies is not due to Merle (see above)!

Affected Double Merles not only exhibit pigment loss (leucism) in their coat, but can also exhibit it in their skin, inner ear, and eyes. Where the skin is affected, it appears rosier. If the nose is affected, it also turns pink, as do affected lips and eyelid margins, which are then all more sensitive to UV light when exposed to sunlight. In the inner ear, the lack of melanocytes disrupts the development of the normally well-perfused vascular layer (stria vascularis) on the outside of the cochlea. This prevents the sensory hair cells normally present there, which in healthy dogs receive and transmit sounds like antennas. Depending on the severity, this can lead to inner ear hearing loss or deafness. Affected eyes often turn an unnaturally light blue (significantly lighter than the blue found in Blue Merles) with a dark pupil sometimes. Normally, the eye is protected from excessive light and glare by the pigment deposited in the iris. A loss of pigment there can lead to light sensitivity. The iris can become so light that it becomes transparent, making the eye appear pink, allowing one to see directly through it to the blood vessels of the retina at the back of the eye. Lack of pigment in the choroid causes (like the vascular layer in the inner ear) to become significantly thinner. According to Olivia Schatz (Schwab & Haas 2018, University of Graz), angiogenesis (the formation of new blood vessels from existing ones) appears to be significantly reduced. Affected eyes and ears are often surrounded by white fur.

The problem of the missing pigment-producing melanocytes (leucism) probably lies in a disruption in the transmission of their precursor cells (pluripotent melanoblasts) from the embryonic neural crest, along the nerve sheaths to the target location, such as the eye and inner ear. (In albinism, on the other hand, the melanocytes are present, but there is a dysfunction that prevents them from producing melanin.) In both cases, a lack of melanin can lead to more or less severe visual impairment. In addition, affected Double Merles often have eye deformities in form of microphthalmia. Then the eye is too small or not developed at all. Depending on the severity, this can lead to functional restrictions and even complete loss of vision. 

It is therefore essential to avoid mating two Merles (even across breeds) if this could result in puppies with limitations or a complete lack of hearing and vision. It is all the more important to know the Merle Status of each dog so that its responsible owners can take early influence to avoid matings that are harmful to the health of their offspring (whether planned or not). Special attention is also paid to the Hidden Merles (especially Minimal Merles & Sable Merles (see above)), dogs whose Merle factor cannot be easily seen. Responsibly managed breeding clubs, like ours, ensure that breeding dogs are tested for Merle (a newer test with allele breakdown) before mating, as well as for all other common testable genetic abnormalities, such as those that can occur in MDR1, DM, PRA, GCS, etc. This allows matings to be chosen appropriately without producing affected puppies. Buyers are also informed accordingly. Test: https://laboklin.com/en/products/genetics/coat-colour-coat-structure-coat-length/dog/m-locus-merle-alleles-mh-m-ma-ma-mc-mc-m-and-mosaics/