Tan refers to German Shepherds that are tan in combination with another color. This can be black and tan, liver, and tan, blue and tan, and isabella and tan.
The tan parts are due to a slightly lightened red pheomelanin pigment giving it a brownish-red appearance due to the activities of the intensity gene. This intensity gene reduces the concentration of the red piments by causing less pigment to be produced. As such, parts of the coat that should have had a rich mahogany red color appear red.
No German Shepherds seem to be all tan. For a dog to be all tan, it would need to inherit two copies of the recessive red gene which is denoted as e/e. The red pigment would then need to be slightly diluted to appear pale red otherwise known as tan.
However, German Shepherds with the e/e genotype are creamy-yellow to almost white or ivory white. None of them have a pure red or tan coat. This is because the red and intensity genes which are inherited together (haplotype) cause all German Shepherds that should have been otherwise red to be cream to off-white in shade.
All cream to white GSDs has two copies of the recessive red and intensity genes. The intensity genes cause extensive dilution of all red pigment to very diluted shades. With that said, a lot is yet to be discovered about recessive red German Shepherds and how various genes interact to cause variations from blonde to almost white.
But for all tan German Shepherds, none seem to exist as explained above. Tan GSDs are a combination of tan and another darker color such as black, liver, blue, or lilac on sections of the coat.
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There you go, WOOF!