Part II – Eye Coloration Puzzled, Alexia and Evan used the internet to research
ID: 208310 • 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
8] The colour of eye is a trait which is passed from parents to offsprings. Therefore, it is a genetic trait and is influenced by more than one gene, in fact as many as 16 different genes ! But the actual number of genes that control eye colour is still unknown and it has been confirmed that even single nucleotide polymorphism [SNP] can cause change in eye colour.
The two main genes associated with eye colour variation are OCA2 and HERC2. Different SNPs within OCA2 are strongly associated with blue and brown colour of iris. The SNP might be in the the regulatory sequence which might affect the gene product which, in turn, will affect pigmentation. A specific mutation in the HERC2 gene might regulate the OCA2 expression. SLC24A4 and TYR are two other genes which cause eye colour variation.
9] In school, one had studied that two genes determine the eye colour. The dominant gene shows the phenotype and the recessive might express itself in future generations. A person could be heterozygous or homozygous for the eye colour. The genes encode for proteins, which in turn control the pigmentation of the iris. However, now we know that these genes are interdependent and do not function alone. This is the reason why there is so much variation in eye colour.
11] The eye colour depends on the amount of pigmentation in the iris and as we have come to know the pigment formation is encoded by the genes HERC2 and OCA2. There is an interdependence between these two genes and they control the functioning of each other. Any SNP in either of the genes can result in their mutation which can disturb the formation of the protein which controls melanin formation.
10] A specific mutation in the HERC2 gene can regulate OCA2 expression which is partly responsible for the blue eye colour. An SNP in HERC2 gene can affect the OCA2 gene which is responsile for pigmentation in the iris.
Genes give instructions for making proteins. Each gene is made up of coding and non-coding sections. While the coding section has the instructions for making protein, the non-coding part controls when, where and how much of the protein is to be made. The OCA2 gene has the instruction for making a protein named 'P' which helps to make melanin, the pigment crucial for giving eye colour.
Since people with blue eyes have pigment in other places in their body but not in the eye was a matter of curosity for the researchers. It has been suggested that DNA diference in HERC2 causes a transcription factor [TF] to bind and shut off the nearby OCA2 gene. But why would this happen only in the eye ? The answer is that different kinds of cells have different kinds of TFs.
The HERC2 and OCA2 genes work together. If a person has blue eye version of the non-coding HERC2 region then the ORC2 does not make the protein 'P' and no pigmentation occurs , giving blue colour to the eye. On the contrary, if the person has brown eye version of the non-coding part of the HERC2 region , the protein 'P' is made giving the brown eye colour,
12] As discussed earlier, a part of pigment making proces involves the genes OCA2 and HERC2. A working HERC2 is required to activate OCA2 which is the gene actually responsible for the making of the pigment. In fact the two genes need each other to make pigments.
If the HERC2 gene is broken or non functional, it will not be able to 'turn on' the OCA2 gene , so no pigmnt will be made ending up in blue eye colour. Also, if the OCA2 gene is non functional or mutated, then no matter how hard the HERC2 gene tries, there will be no pigment formation, resulting in blue eye colour. So, in short, we can say that both the genes depend on each other for pigment formation. If both are active, there will be pigment formation ending up in brown eye colour.
Because the two genes depend on each other, if the blue eyed parents are carriers of the dominant brown eye colour trait, they can have a child with brown eyes . Genetics is unpredictable !