How Genetics Cause Blindness in Dogs
Blindness in dogs can be caused by many reasons: accidents, disease, and genetics. Genetics play a factor when you look at hereditary characteristics.
Inherited characteristics can be good and desired: strength, agility, and coloring. The American Kennel Club defines breed specifics that are inherited in puppies from their parents. Read more here. This is the process that allows us to recognize a German Shepherd Dog from a poodle and what separates a beagle from a collie.
Other inherited characteristics can be unwanted or even unhealthy: physical ailments, behavioral issues, and birth defects. We’ve worked with several dogs who had a condition caused by genetics. Hope and Iris are two representatives of the most common genetic affect: Blind / Deaf due to Double Merle.
What is Double Merle?
First, ‘merle’ is a color of a dog’s coat. Brown or black are easily understood. Merle is a combination of colors. The most defining characteristic of a merle dog is a mottled coat of black, tan, or white colors. Odd or blue-colored eyes also define these breeds. We have two main categories depending on the coat colors-red merle and blue merle. This color is carried on a piece the dog’s DNA.
Second, ‘double’ means that the dog carries two merle pieces of DNA.
Some people call ‘merle’ by the name ‘dapple’. This chart does a good job of showing the color combinations that appear when a dog has a merle genetic marker. In the explanations here that marker is defined by a capital M. (photo chart credit)
Genetics have a way of appearing in a bloodline when you plan for the desired characteristic and as an unplanned, unwanted characteristic. Many people want the coat colors that are shown in this chart, but at times, the results include blindness.
Iris, a smooth coated collie puppy, is a representative of this situation. She is a double merle color, and she is both blind and deaf. She is also adorable, intelligent, and happy.
Iris came to Rescued Rollers as a 14 week old puppy. She has limited sight and hearing ability. Her coat is a classic color for double merle dogs – primarily white with patches of gray. You can see that her markings are similar to Hope’s, another Rescued Rollers dog.
Gregor Mendel
Any student of biology has been asked to study Gregor Mendel. Best known for his studies of peas and their genetics, his findings can be applied to genetics in any species.
He focused on dominant and recessive traits and developed a method for identifying the statistical probability of each trait appearing in the offspring of a mating. For a detailed explanation of his studies, read more here in this excellent article.
Mendel used a chart to show one parent’s genes across the top and the second parent’s genes down the left column. The intersection of those created the probability of the gene being received by the offspring.
Charts
A simple explanation is broken down in the next three charts. The first example shows both parents carrying two genes named X and Y. When you track the first square of the intersections at row one and column one, you have an offspring with XX. When you follow the first row (X) over to the intersection with the second column (Y) you find an offspring who matches the parents with XY. The same result appears at row two, column one (XY). The fourth offspring will have YY genes as shown in row two, column two.
The probability of the offspring matching the parents genetically in this scenario is 50%. If you associate the coat color black with the X and white with the Y, then you will have two dogs carrying both a black and white gene from this match. 25% of the time you will have a dog with two black genes and two white genes.
The next scenario is when you have a match with one dog having two X genes (XX in the column) and a dog with one each (XY) in the top row. Puppies from this match will be equally divided between having XX or XY combinations. 50% of the offspring will carry a double black gene and we know what color their coat will be! 50% of the offspring will have both black and white coat combinations.
The results of a match between both parents having XX genes will be that all offspring have double X genes as a result. Using the black and white coat colors, this litter of puppies will be all black.
If we substitute the ‘merle’ gene for the ‘black’ or ‘white’ color combination you can see the probability that your puppy will have a merle colored fur coat. This same equation works with other genetic traits – blindness for example. Hope pictured above with Hobbes and below here, is an Australian Shepherd / Great Pyrenees mix. She is blind and deaf.
This chart from Deaf Dogs Rock clearly illustrates the outcome of breeding two merle dogs.
Blindness
Blindness is closely associated with coat colors as the markers on the DNA for coat color tied to the markers for sight. The same genetic markers lead to hearing impairment and deafness. The two most common unwanted outcomes for breeding double merle dogs.
Blind dogs are different, not less. They will develop their other senses more fully in order to offset their lack of vision. They are full of life and love, just like their seeing litter mates.
Bringing a blind dog into your home will be a fulfilling and heartwarming experience. If you would like to foster a Rescued Rollers dog, complete an application.
Additional Resources
For more reading on double merle dogs:
Animal Genetics / Canine Color
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