The genetics of eye color worksheet answer key

Part I – High School Blues Evan and Alexia had been happily married for seven years and had a delightful fi ve-year-old son named Ryan. One day, while going through his old high school biology textbook, Evan stumbled on some troubling information. In the section on the genetics of eye color, he read that two blue-eyed parents cannot produce a brown-eyed child. Th is was disturbing to him because both he and Alexia had blue eyes, but Ryan had brown eyes.

He and Alexia were very much in love and Evan didn’t believe his wife had been unfaithful. Puzzled, he questioned his wife, who confi rmed she had been faithful to him. Evan had known Alexia long enough to recognize when she was lying and detected nothing but honesty in her response.

What is going on?

Questions 1. What protein does the “eye color gene” encode? Propose what the function of this protein might be, and how

this protein might diff er in people with brown and blue eyes. 2. Which is dominant and which is recessive: the blue eye allele or the brown eye allele? 3. If Evan has blue eyes, what genotype (which two alleles) is he likely to have? 4. If Alexia has blue eyes, what genotype (which two alleles) is she likely to have? 5. Draw a Punnett square showing the genotype of all the possible children this couple could have. Based solely on

this information, what is the likelihood that Ryan is their child? 6. Assuming that Alexia has been unfaithful, what is (are) the possible genotype(s) of the man who is Ryan’s

biological father? 7. Assuming that Alexia has been faithful, suggest as many hypotheses as you can to explain Ryan’s phenotype.

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Page 2“More than Meets the Eye” by Annie Prud’homme-Généreux

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., diff erent types) of the melanin present in the iris (Sturm & Larsson, 2009).

Th e iris has a front layer and a back layer. Th e space in between them, called the stroma, is fi lled with various proteins, including white collagen fi bers. For almost all eye colors, there is a lot of melanin on the back layer of the iris (Sturm & Larsson, 2009). Where people diff er 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 fi brils. Th is scatters the short blue wavelengths to the surface. In other words, when light hits the collagen fi brils, the light is refracted, or bent, and this makes the light appear blue or green. Th is eff ect is also experienced when looking at the sky. Th e 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. Th is eff ect 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 diff erent 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 aff ect 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? Off er several possibilities.

Figure 1: Top fi gure illustrates a brown eye, and bottom fi gure shows a blue eye. Th e small brown squares in the front and back of the iris represent melanin molecules. Th e relative number of brown squares represents the relative density of melanin in various regions of the iris in eyes of diff erent color.

Pupil

Iris Front Layer

of Iris Back Layer

of Iris

Collagen protein in Stroma of Iris

Melanin in Back of Iris

Light hits Melanin in Front layer of Iris

Pupil

Iris Front Layer

of Iris Back Layer

of Iris

Collagen protein in Stroma of Iris

Melanin in Back of Iris

Light Reflects Off Back layer of Iris and Bends When it Encounters Collagen in Stroma

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Page 3“More than Meets the Eye” by Annie Prud’homme-Généreux