Part II – Eye Coloration Puzzled, Alexia and Evan used the internet to research
ID: 208285 • Letter: P
Question
Part II – Eye Coloration
Puzzled, Alexia and Evan used the internet to research what gives the eye its color.
“Eye color” refers to the color of the iris of the eye. Melanin is a dark pigment produced by cells in the iris that gives the eye its color. What determines the color of the eye is a combination of the amount, location, and qualities (e.g., different types) of the melanin present in the iris (Sturm & Larsson, 2009).
The iris has a front layer and a back layer. The space in between them, called the stroma, is filled with various proteins, including white collagen fibers. For almost all eye colors, there is a lot of melanin on the back layer of the iris (Sturm & Larsson, 2009). Where people differ is in the melanin in the front layer of the iris.
A lot of melanin in the front of the iris makes the eye look brown because, as light hits the front of the iris, the pigments absorb the light.
Blue irises have less melanin in the front layer, so light can go through it. As light travels through the stroma, it encounters the collagen fibrils. This scatters the short blue wavelengths to the surface. In other words, when light hits the collagen fibrils, the light is refracted, or bent, and this makes the light appear blue or green. This effect is also experienced when looking at the sky. The sky is actually black. However, as light travels through the Earth’s atmosphere, it encounters particles that bend the light and cause the sky to appear blue. This effect is called Rayleigh Scattering (Southworth, 2007; Sturm & Larsson, 2009).
A lot of pigment in the front of the iris gives brown, less melanin gives green or hazel, and little pigment gives blue (Figure 1).
While blue irises have little melanin of any kind in the front of the iris, other eye colors vary in the relative amount of the different types of melanin (called eumelanin and pheomelanin), giving a spectrum of eye shades (Sturm & Larson, 2009).
Questions
8. Based on what you now know, how many genes may be involved in determining eye color? Suggest what each gene does to affect this trait.
9. What do you suspect that the blue/brown eye color gene studied in high school does in the cell? What type of protein might this gene encode? Offer several possibilities.
10. Based on your previous answer, how might the blue and brown alleles differ (how might they differ in function, in sequence, in the resulting protein, in structure, etc.)?
11. Eyes can be brown, blue, or green/hazel. How could these three differences be encoded genetically? Suggest several ways to achieve these three phenotypes.
12. Does this information suggest ways in which two blue-eyed individuals could have a brown-eyed child? Explain.
Explanation / Answer
Question 8 Answered
Though it would seem to be regulated by one, Eye color in humans is actually regulated by almost 15 genes. The main two genes are OCA2 and HERC2 on chromosome number 15. OCA 2 produces a protein named as Protein P which functions in the maturation of melanin-producing melanosomes. HERC2 contains a region called intron 86 which contains SNPs to control the switching of OCA2 and thus producing the lighter shades. Other genes like ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, in association with OCA2 and HERC2 produce a continuum of colors in Humans.