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First Yorkies are almost
"unique" in the dog world. They carry a silk gene, and
if they are breed correctly carry 2 blue (bb) or 2 black
genes (BB) and 2 progressive graying genes.
In addition, they also have a dilute gene.....this is what determines
how light or dark the blue pigment color of the saddle is.
When all these genes are present in the correct combination in one dog
you have one beautiful animal. There's a lot more to this part
of the blue coat.....but you need to read Dr. Little's
"Inheritance of Coat Color in Dogs". He did much study
on many breeds and although written long ago is still considered the
bible of coat color in dogs.
The eye in all animals except pigs and
man have a reflective layer along the back of the retina called
the tapetum. This layer serves to reflect light to allow maximal
usage of the available light. In some animals the tapetum is
colored in others it is not.
The reddish color that you see in
human eyes is the reflection of the blood vessels in the back of the
eye because the tapetum is NOT colored. This is the
same in the "red eye" of the Yorkie.
The blue and green you see in the
Yorkie's are from a pigmentation in the tapetum -- how this relates to
coats I am not sure.....but could be that pigmentation of the eye
and coat texture are related genetically. Tapetums are
very beautiful -- metallic, brilliant colors in most animals.
You can watch Yorkies, clipped down or
in full coat, outside in the sunshine ... and you can easily separate
them into two groups - those with a "blue cast" to the coat,
and those with a "gold cast" to the coat.
Those with gold or bronze cast to the
coat have eyes that reflect a flash bulb or flashlight with the
"red eye" or "shiny" red reflection to the
pupil.......no pigment.
Those blue cast dogs always seem to
reflect blue or green from the pupil.....pigment.
The "red eye" dog has a
thicker, softer, more opaque (not shiny silky) coat ... silky
coated pups look kind of straggly.
What is going on is that those with
the gold/bronze cast has LESS eumelanin (black) and more phaomelanin
(yellow). Typically phaomelanin (yellow) does NOT get put
in the tapetum. So NO tapetal pigment results in red color from
the blood vessels. Whereas the blue cast dogs have more
eumelanin resulting in a pigmented tapetum and the reflection that you
see is bluish or green. It's very similar in chocolate and black
Labs-- chocolate labs tend to have pale gold or red tapetums while the
black labs have brilliant green and blue tapetums.
Again it has to do with overall production of melanin (pigment) and
where it is put. It also has to do with whether the dog carries
the genes for true blue, clerical blue or black AND whether they also
carry the gene for progressive graying (dilute). That is my
explanation I have learned from study of the breed and genes and
pigment.
Is it any wonder that Yorkies are
difficult to breed to the standard......<g>
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