With all the controversy surrounding the double-merle sire of the Westminster Best in Breed Collie, I thought it might be timely to finally get this post up about homozygous merles.

The motivation for posting this was a Great Dane I encountered at a dog fair this past fall. The dog was a homozygous merle, but the rescue worker was unaware because she believed that double-merles were “all white or nearly all white.” The dog at the top of this post is a good example of what people expect when someone says double-merle. That is often what they look like, especially in breeds that also have white patterning in addition to the merle.

This was the dog at the dog fair.

His handler did not believe he was a homozygous merle because he had more colored areas than white areas. She insisted that he was just a merle dog “marked with white.” She was unaware that some homozygous merles actually have a fair bit of coloring on them. In my experience, breeds that have solid merles (that is, merles without any white patterning) tend to produce homozygous merles with more color.

A close look at the placement of the white on this dog will show why it comes from a doubling up of the merle, rather than a white pattern. Here I’ve filled in his merled areas so that he looks like a black dog with white patterning.

Here are some white patterned dogs to compare, starting with a Shetland Sheepdog. He has the irish spotting commonly seen in herding breeds.

Here is a very similar kind of white spotting found in Boxers. (This was the pattern discussed indirectly in this post.)

And here is an Ibizan Hound with the pattern sometimes called extreme piebald.

Even though the edges on the last example are more ragged and irregular, it still looks quite different from the tinted image of the merle Dane. The placement of the color is wrong for any of these patterns. Here is what it does resemble.

These two patterns look much the same. That is because the only real difference is that the dog above has white areas where the Harlequin gene came and stripped the gray coloring away, leaving those areas white. The dog below (whose patches are actually gray) had this color stripped away by a second dose of merling. The white areas on both dogs have an outline and placement consistent with merle, not white patterning.

The white areas on this Dane do not make sense for any of the common white patterns found in dogs. Look at the difference between his two front legs, where one is white well up to the body and one is dark down to the end of his foot. His hind legs are similarly patched and uneven, just as might be expected with a merle, but not with an irish or piebald dog.

The other giveaway are his eyes. His handler believed his eyes were fine, and that they just looked odd because his one eye was particolored. Particolor eyes can make it hard to assess eyes, but the problems with his eyes can be seen despite their coloring. This first photo shows how the black pupil of the right eye has “bled” down into the lower part of the eye. This is common among homozygous merles. His left eye, meanwhile has an unusually small pupil, which is why the eye appears so very blue in this picture.

If you look carefully at that first eye picture, you might notice an odd angle to his eye. I suspect from this shot, it is something more likely to jump out at those of us who sculpt animals. These next pictures show it more clearly.

If the nature of his pattern was not enough of a clue that he was a double-merle, his eyes would give him away. Defects like this are typical.

This is also one of the reasons why the production of homozygous merles tends to generate some very emotional reactions. This particular dog has eyes that certainly look “wrong”, but his appearance – as double-merles go – is actually pretty mild. The eye deformities in many other homozygous merles are quite frankly disturbing to see. It is not just that homozygous merles are often blind and deaf, but that fact that hey look maimed is particularly upsetting. This probably contributed to rules in many countries that merle to merle breeding is abusive and therefor not permitted. Unfortunately, in the United States the Rough Collie registry has no such rule.