Merles and unrelated eye defects


In the previous post, defects in the eyes were used to help identify a homozygous merle. That is often a strong indicator, but there is one situation where that is not always helpful. Collies, and some of the closely related breeds, have a number of issues with their eyes that are unrelated to the merle gene. Compare the normal eyes of the merle Shetland Sheepdog, above, to the eyes on the merle Rough Collie below.


Like the Great Dane in the previous post, this dog has an eye that appears to be too small and set incorrectly. Her left eye, which is blue, also has a distorted pupil similar to the one seen on the Dane. It is more obvious when viewed from the front.


Although it might look like she is looking to the side in this shot, her pupil actually skewed over toward that corner, giving her a cross-eyed look.

She is not a double-merle. Her pattern is typical for a single merle with moderate white irish patterning. Whatever is wrong with her eyes, it is probably separate from her merle coloring. Her one blue eye makes the problem more noticeable, but chances are she would have had issues whatever color she happened to be.

And that is why distortions in the eyes on collie breeds are not necessarily proof that the dog is homozygous for merle. Fortunately for identification purposes, most double-merle Collies are quite dramatically white so they are unlikely to be mistaken for a heterozygous merle.

But dogs like this one also point to the reason why using double-merles in Collie breeding programs is a bad idea, even when someone else made the ethical compromises necessary to create the dog in the first place. Because two copies of the merle mutation damages the eyes, there is no way to know if a homozygous merle breeding animal had eye problems unrelated to the merle coloring. Dismissing eye problems with the assumption that the heterozygous offspring will not be affected could be a mistake, because there is no way to be sure that the homozygous parent has otherwise normal eyes.

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5 Responses to Merles and unrelated eye defects

  1. Kristina Pry February 19, 2012 at 6:34 pm #

    That Sheltie is gorgeous =) If you ever need more blue merle pics, let me know… or bi black, or sable… 😉 Sadly my blue has normal colored eyes.

  2. J Ferre February 19, 2012 at 6:49 pm #

    your last post on merle made me do a search … and they have what they call cryptic merles, ( Catahoula Cur). These were dogs that looked normal but when tested came back with one copy of merle. That would explain how they could get double merles even while trying their best to avoid them. Still it is all fascinating and even more confusing is how some double merles are both deaf and blind yet others have complete hearing and sight. Your blog is fantastic as always. 🙂 Do you know if heterozygous merles have a higher probability of sight and hearing issues than not merle dogs? Strange that it is not always one way or another.

    • The Equine Tapestry February 20, 2012 at 10:44 am #

      In the past, it was usually cryptic merles (or sables that made it hard to see the merle) that were behind most homozygous merles. Unfortunately the old taboo against breeding merle to merle has broken down, at least to some extent, among Collie and Sheltie breeders. That’s a shame because they really are the last ones that should be making an exception, with white patterns and existing eye defects.

      There was a study that showed a slight increase among the heterozygous merles, but there was a later study that showed a much lower number, if I remember correctly. I will have to see if I can find those two papers and link them. I wondered at the time if the dogs studied had white patterning, since some patterns have been linked to hearing issues in dogs, too.

    • The Equine Tapestry February 20, 2012 at 10:50 am #

      Here is the more recent study on deafness in merles.

  3. Onawa Rock February 8, 2013 at 9:02 pm # I happened upon your blog, and it made me think of my daughter, who has bilateral iris coloboma. I knew that it had also been seen in Australian shepherds. She/we are part of a study at NIH/NEI to try and find the genes for it. I can’t see if the dog above has a coloboma but it looks to have the microphthalmia. Are there any vets doing similar studies in animals?