Chapter 5, Problem 16Q

The amino acid sequences encoded by the red and green visual pigment genes found in humans are 96% identical (Nathans et al. 1986). These two genes are found close together on the X chromosome, while the gene for the blue pigment is located on chromosome 7. Among primates, only Old World monkeys, the great apes, and humans have a third pigment gene—New World monkeys have only one X-linked pigment gene. Comment on the following three hypotheses:

• One of the two visual pigment loci on the X chromosome originated in a gene duplication event.
• The gene duplication event occurred after New World and Old World monkeys had diverged from a common ancestor, which had two visual pigment genes.
• Human males with a mutated form of the red or green pigment gene experience the same color vision of our male primate ancestors.