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 (c.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 cumelanin and pheomelanin), giving a spectrum of eye shades (Sturm & Larson, 2009). NATIONAL CENTER FOR CASE STUDY Light hits Melanin in 1 Front layer of Iris "More than Meets the Eye" by Annie Prud'homme-Généreux Pupil Light Reflects Off Back layer of Iris and Bends When it Encounters Collagen in Stroma Iris Pupil Front Laye of Iris Iris Collagen protein Din Siroma of Iris Stroma of Iris Front Laye of Iris Back Layer of Iris Melanin in Back of Iris Collagen protein Din Stroma of Iris Stroma of Iris Melanin in Bac 28ack Layer of Iris Figure 1: Top figure illustrates a brown eye, and bottom figure shows a blue eye. The small brown squares in the front and back of the iris represent melanin molecules. The relative number of brown squares represents the relative density of melanin in various regions of the iris in eyes of different color. 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. Page 2 NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE 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.)?