Unwanted Blue Eyes in Corgis
The breed standard for the eye colour of the Cardigan Welsh Corgi says:
KC/FCI: Preferably dark, to blend with coat. One or both eyes pale blue, blue or blue flecked, permissible only in blue merles.
AKC: Clear and dark in harmony with coat color. Blue eyes (including partially blue eyes), or one dark and one blue eye permissible in blue merles, and in any other coat color than blue merle are a disqualification.
Eyes of blue merle Cardigans
And for the Pembroke Welsh Corgi:
KC/FCI: brown, blending with colour of coat.
AKC: Variations of brown in harmony with coat color.While dark eyes enhance the expression, true black eyes are most undesirable, as are yellow or bluish eyes.
It occasionally happens that a tricolor, red, sable or brindle Cardigan has one, very rarely two blue eyes. These so-called "wall eyes" do not automatically indicate that the dog carries the merle gene. There is a type of blue eye that follows a different pattern of inheritance. It is genetically the same sort of blue eye found, e.g. in the Siberian Husky. This type of blue eye is also seen in the Pembroke Welsh Corgi, albeit equally rarely.
The following is partly a quote from an old article Genetics of the Blue Eye written by Libby Babin (Babinette Shelties, USA):
"Research shows that this sort of blue eye occurs in the approximate proportion of one out of sixteen from parents that carry the recessive genes for this blue eye. Those two genes are inherited in an entirely different manner, the merle gene being an autosomal dominant and the blue eye being a polygenic recessive.
This blue eye does not appear as frequently as one could expect of the usual recessive quality because it depends on a polygenic mode of inheritance; that is, a blue eye does not appear unless there are more than one pair of recessive genes in a homozygous or pure state.
The diagram shows the recombinations possible of two pairs of genes in the mating of individuals that are heterozygous for both genes. The genes are labelled Aa and Bb. The capital letters represent the dominant gene for dark eyes and the lower-case letter is the recessive for the blue eye. The puppy must have four genes for the recessive quality in order to have a blue eye. The number of genes necessary for a blue eye to occur is theoretical at this point of research. We know that it takes at least two pairs.
Sire (AaBb) x Dam (AaBb)
A and B - the presence of either of these genes assures brown eyes.
a and b - four of these genes are necessary to produce a blue eye.
Notice that out of the sixteen there is only one that is homozygous (pure) for the dominant quality (AABB), thus cannot produce or pass on genes for anything but brown-eyed offspring and only one that is homozygous (pure) for the recessive quality (aabb), thus expresses the blue eye. The possibility of this particular pair coming up with that one-out-of-sixteen puppy is there, but rather slim.
The diagram also shows all the other possible combinations of these two pairs of genes. We can see the reasons why one cannot declare an individual free of the recessives by breeding results in just one generation as is possible for a simple recessive. For example, any dog with the pairing of either dominant such as AAbb or aaBB will be incapable of producing a blue-eyed puppy, yet they are passing along recessives for the quality - recessives that may be just the key to bring out the blue eye in the next generation. In the mating of AAbb to aaBB the offspring would all be the same, AaBb. These, of course, take us right back to the examples in the diagram.
All this adds up to the fact that this unwanted blue eye is genetically well-established in the breed; even a line that never produces the blue eye can still be carrying the genes for this quality; that it can and does reappear in the most unexpected places.
It would be impossible to eliminate these recessives without "sweeping" out our best studs and bitches, and we still could not be sure of any degree of success. The best course for breeding success still remains in the careful selection of our breeding stock for the desired qualities of the whole dog and in not getting lost in any one detail which is purely cosmetic and in no way detrimental to the dog's health."
Why do blue eyes reflect red?
The pupils of dogs with brown eyes reflect either green or yellowish when photographed with flashlight whereas dogs with blue eyes reflect red.
Red Cardigan with brown eyes
Dogs (and cats) have an iridescent layer behind the retina, the so-called tapetum lucidum, which gives a shining appearance to the eyes when illuminated in the dark. This layer acts like a mirror and reflects light back through the retina thus improving vision in low-light conditions.
The red reflection is caused by the lack of pigmentation or lack of tapetum lucidum whereby the underlying choroidea shines through just like in the eyes of humans who don't have a tapetum lucidum. The colour of the iris itself is of virtually no importance for the red-eye effect.
The cells of the tapetum are modified melanocytes but do not produce pigment (in contrast to genuine melanocytes). Some genes responsible for the general pigmentation, such as the merle gene (M) and the extreme-white piebald gene, also adversely affect the development of the tapetum, its structure and/or function. In certain cases the tapetum lucidum may even be absent altogether. So far, there is no clinical evidence that the lack of tapetum impairs the dog's vision at night.
In blue merles even the brown eyes reflect red, whereas in Corgis who do not carry the merle gene only the blue eye reflects red as is beautifully illustrated by the photo of the Pembroke puppy with one brown and one blue eye.
I wish to thank all those who have sent me photos of their blue-eyed dogs. Special thanks go to ophtalmologist Dr.med.vet. Marianne Richter, Dipl.ECVO, http://www.eyevet.ch, for assistance regarding the "red" eye effect.