Biologists at the University of Utah showed how three pigment genes interact to cause a variety of colors among rock pigeons. From left to right: The first feather is blue-black because all three genes are normal. The second feather is dilute blue because the Tyrp1 and Sox10 genes are normal but the Slc45a2 gene is mutant, which dilutes the intensity of any color. The third feather from the left is ash-red because of a mutant Tyrp1 gene and normal Sox10 and Slc45a2. But if Slc45a2 also is mutant, the feather color is diluted to ash-yellow (fourth from left). The fifth feather from the left is red because the Sox10 gene is mutant, which overrides whatever color Tyrp1 normally would dictate. In the sixth feather, Slc45a2 also is mutant, making the feather yellow, which is the diluted form of red.