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Originally Posted by BamaFan121s  They are implemented into a breeding program to bring richness in color back to lines that have become 'washed out' looking. When they are used in breeding, they are done so with the intent of still producing offspring that meet the standard. Also, this is a very recent change to what is accepted--it will take time but I would suspect that eventually you will see less and less of the black and tan yorkies. |
So here's is where an understanding of genetics comes into play - I have to think back to the equine genetics here ....
The gene that causes our yorkies to have steel blue coats is the grey dilution gene. Grey is a dominate gene, meaning a dog will be grey if they have one copy of the gene (heterozygous form). If mother and father are both homozygous for grey (each parent carries 2 copies of the grey gene) all their pups will be homozygous with 2 copies of the grey gene and grow up to be "blue" with no chances of being black (that's the goal, you want to insure that you will produce blue 100% of the time). By breeding a black coated yorkie (they have no copy of the grey gene) to a heterozygous steel blue coated yorkie (a yorkie who is blue but only has one copy of the grey gene), you are deliberately producing litters where some of the the pups will grow up to be grey and some of the pups will be black.
Black coated yorkies have been accepted in the breed and are used in many breeding programs. The reason why we have black coats in our yorkie blood lines, is the same reason that we have parti, golden and chocolate in our bloodlines - it's from the foundation stock that were a hodgpodge of unknown genes. We see many more black coated yorkies in our breed because they weren't culled the way the other off colors were but that doesn't mean that the colorful genes that are popping up now are new, it just means that these surprise colors are no longer culled as quickly, since they are now allowed to be registered with AKC.
If all yorkies were homozygous for the grey gene, there would never be another black coated yorkie produced and that obviously isn't the case as we know ... it's all in the genes (or lack of genes).