Page 110 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2
P. 110
Problems 269
responds to light of a different wavelength (all cones. Such people, if they exist, could potentially de-
three wavelengths are different from that recog- tect 100 million colors! For parts (a) and (b), assume
nized by the autosomal color receptor). How is that each X chromosome has one red and one green
color vision inherited in these monkeys? photoreceptor protein gene. For all parts, assume
c. About 95% of all light-receiving neurons in hu- that mutant alleles can produce photoreceptors with
mans and other mammals are rod cells containing altered spectral sensitivities.
rhodopsin, a pigment that responds to low-level a. Explain why scientists expect that many more
light of many wavelengths. The remaining 5% of females than males would be tetrachromats.
light-receiving neurons are cone cells with pig- b. In X-linked, red/green color blindness, mutation of
ments that respond to light of specific wavelengths either the red or green photoreceptor gene results
of high intensity. What do these facts suggest about in a rhodopsin-like protein with altered spectral
the lifestyle of the earliest mammals? sensitivity. The mutant photoreceptor is sensitive
51. Humans are normally trichromats; we have three dif- to wavelengths in between the normal red and
ferent types of retinal cones, each containing either a green photoreceptors. Why do scientists think that
red, green, or blue rhodopsin-like photoreceptor pro- a woman with a son who is red/green color-blind is
tein. The reason is that most humans have genes for red more likely to be a tetrachromat than a woman
and green photoreceptors on the X chromosome, and a whose sons all have normal vision?
blue photoreceptor gene on an autosome. Our brain in- c. Suggest a scenario based on Fig. 7.33d that could
tegrates the information from each type of cone, mak- explain how extremely rare males might be
ing it possible for us to see about one million colors. tetrachromats.
Some scientists think that rare people may be
tetrachromats, that is, they have four different kinds of
