Chapter 7, Problem 50P

Most mammals, including New World primates such as marmosets (a kind of monkey), are dichromats: They have only two kinds of rhodopsin-related color receptors. Old World primates such as humans and gorillas are trichromats with three kinds of color receptors. Primates diverged from other mammals roughly 65 million years ago (Myr), while Old World and New World primates diverged from each other roughly 35 Myr.

a.	Using this information, define on Fig. 7.32d the time spans of any events that can be dated.
b.	Some New World monkeys have an autosomal color receptor gene and a single X-linked color receptor gene. The X-linked gene has three alleles, each of which specifies a photoreceptor that responds to light of a different wavelength (all three wavelengths are different from that recognized by the autosomal color receptor). How is color vision inherited in these monkeys?
c.	About 95% of all light-receiving neurons in humans and other mammals are rod cells containing rhodopsin, a pigment that responds to low-level light of many wavelengths. The remaining 5% of light-receiving neurons are cone cells with pigments that respond to light of specific wavelengths of high intensity. What do these facts suggest about the lifestyle of the earliest mammals?