A Digression on Dog Genetics, Part 1

You can see Carlton’s markings in this photo. The very pale markings around his ears are hard to see, but there.

My lovely puppy, Carlton, is 6.5 months old, as far as we can tell. He weigs 31 pounds, and is all legs and teeth at this point. He loves other dogs, warms up to people, and is generally the best puppy ever. He also has “weird eyes,” as one of our veterinarians put it. She advised that we check with a veterinary opthalmologist as soon as possible.

That visit came on Tuesday, and it’s sent me down a long path of figuring out exactly how Carlton got to be who he is, and why. I wrote up some of this on Facebook, but since then I’ve been doing a lot more research, and as a person who once considered majoring in biology, I found it really fascinating. In fact, writing up my findings is so complex that I am going to break it into more than one post.

Vet visit findings

The regular vet had diagnosed Carlton as having some kind of eye abnormality, in addition to being blue, so she sent me to the veterinary ophthalmologist to see what’s up. This is the same woman, Dr. Yu-Speight, I went to when my corgi, Gwynneth’s eyes went bad (she ended up having them removed and lived 4 more years). We had a wonderful visit.

First of all, Carlton was quite the little man through the whole appointment. He even jumped into the car on command, finally! I am so proud of this dog. He was incredibly well behaved until we got back home, when he went bonkers.

There was a great deal of eye prodding and dropping involved, but they tested everything from tears, to pressure in the eye to the insides. So, he dealt with many substances and implements. I was amazed at his patience, even though he was obviously not enjoying the process.

Sure enough, his eyes are not “normal,” which we knew. But he CAN see, better in darker light, which we also knew.

This extreme closeup shows the areas of color in his iris that should not be there, and you can sort of see that the iris is not round.

Here are the things that are odd about his eyes:

They are slightly malformed, from when the eye was developing in the womb, so his iris and optic nerve are both tear-shaped rather than round, and his iris doesn’t make a full circle. That is why Lee says he has “cat eyes.”

  • He has a few spots in each eye that are leftover blood vessels from when the eye was developing (these are called corneal endothelial deposits).
  • He has some “pits” or colobomas on his optic nerve head, but they aren’t too bad.
  • He also has a small cataract in the left eye (that is why he has trouble seeing in bright light, because when his pupils contract, vision is blocked).
  • And his retina has folds in it (dysplasia), something like a piece of cloth.
  • The good news is that his pupils DO dilate and contract fairly normally, which means if his cataract gets worse, he might be able to get a replacement lens.

How did he get this way?

There are two possible causes of most of these things: Collie Eye Anomaly (CEA) and Merle Ocular Dysgenesis (MOD). It turned out that Carlton did NOT have one of the things that signals CEA (besides not looking one bit like a collie), so the doctor diagnosed him with MOD. This is a genetic condition that occurs when both parents contribute the merle gene to a puppy.

We recently found out that his mother has blue eyes (Sandra Ritch has seen her), and the rumor is that his father was pit/cattle dog. If that cattle dog was, instead, an Australian shepherd, that would explain two merle carrying parents. (ACD have the roan gene, not merle, to explain their spotting.)

I’m going to stop here, and save more on genetics for the next post. The merle gene is just the beginning of Carlton’s genetic oddity!


Chapell, Jess, 2018: Dog Coat Color Genetics, http://www.doggenetics.co.uk/index.htm

Gelat, Gilger, and Kern, 2014: Veterinary Opthalmology, Volume 2, 5th ed., pp. 1311-1316.

Rich, Sandra Dee, July 2018, Facebook post, Cameron Texas.

Yu-Speight, Audrey, July 10, 2018: Diagnosis and Comments, Veterinary Eye Center, Austin, TX.


Author: Sue Ann (Suna) Kendall

The person behind The Hermits' Rest blog and many others. I'm a certified Texas Master Naturalist and love the nature of Milam County. I manage technical writers in Austin, help with Hearts Homes and Hands, a personal assistance service, in Cameron, and serve on three nonprofit boards. You may know me from La Leche League, knitting, iNaturalist, or Facebook. I'm interested in ALL of you!

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