Chapter 7, Problem 45P

Humans are normally trichromats; we have three different types of retinal cones, each containing either a red, green, or blue rhodopsin-like photoreceptor protein. This is because most humans have genes for red and green photoreceptors on the X chromosome, and a blue photoreceptor gene on an autosome. Our brain integrates the information from each type of cone, making it possible for us to see about one million colors.

Some scientists think that rare people may be tetrachromats, that is, they have four different kinds of cones. Such people, if they exist, could potentially detect 100 million colors! For parts (a) and (b), assume that each X chromosome has one red and one green photoreceptor protein gene. For all parts, assume that mutant alleles can produce photoreceptors with altered spectral sensitivities.

a.	Explain why scientists expect that many more females than males would be tetrachromats.
b.	In X-linked, red/green “color blindness,” mutation of either the red or green photoreceptor gene results in a rhodopsin-like protein with altered spectral sensitivity. The mutant photoreceptor is sensitive to wavelengths in between the normal red and green photoreceptors. Why do scientists think that a woman with a son who is red/green color-blind is more likely to be a tetrachromat than a woman whose sons all have normal vision?
c.	Suggest a scenario based on Fig. 7.33d (p. 243) that could explain how extremely rare males might be tetrachromats.