This may be old news to those who are
up on the vision neuroscience literature, but I found it interesting and
surprising so I thought I would share. It appears that within the
human population, individuals vary in the number of red-cone genes
(from 1 to 4) and green-cone genes (from 1 to 7!) they carry. Neitz
& Neitz, (1995) found all of these variants in a sample of only
27 men. Further, these different genes are expressed as different
red and green opsins. So, there appear to be plenty of human
tetrachromats, pentachromats, hexachromats, etc. running around.
Neitz, M., & Neitz, J. (1995)
Numbers and ratios of Visual Pigment Genes for Normal Red-Green Color
Vision. Science, 267,
1013-1016.