Part V – Three Hypotheses Following their reflections, Alexia and Evan contacted
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Question
Part V – Three Hypotheses
Following their reflections, Alexia and Evan contacted Dr. Barry Starr at Stanford University to confirm their three hypotheses.
Hypothesis 1: A Mutation
“Genetics is complicated by the fact that genes don’t always stay the same.[…] Our DNA copying machinery is nearly perfect but it still will make an occasional mistake. If that mistake happens in sperm or egg cells, it will get passed on. And if the change is in the right place in the blue eye gene, blue-eyed parents can now have a brown-eyed child. [...] Genes for things like blue and brown eyes are very, very similar. In fact, they are really just different versions of the same gene. [...] So, to turn a blue eye gene into a brown eye gene, you may only need to change a single letter” (Starr, n.d.)
Hypothesis 2: Epigenetic Effects
Dr. Starr also offered an alternate hypothesis.
“Sometimes a gene can be read in one person but is unreadable in another. What happens if a gene is unreadable in a parent but a child’s cells can read it? That’s right, a blue-eyed parent can have a brown-eyed child. Believe it or not, sometimes what your mom eats while she is pregnant can affect your hair color. Well, if you’re a mouse, anyway... Scientists did an experiment where they fed a mouse one food and her pups were black. A different food resulted in [yellow] pups. And all of the A, G, C, or T’s were the same between the pups. What happened? The food ended up attaching little chemical groups called methyls to the DNA. These methyls made the gene unreadable. So even though genetics would predict the same color pups, the environment changed the outcome” (Starr, n.d.)
Hypothesis 3: Genetic Complementation
Alexia and Evan also contacted another biologist, Ky Sha, while they were at Stanford University.
Ky had a different idea to explain their situation. Because eye color is determined by many genes, it is possible that each gene product collaborates to synthesize melanin as though they were stations in an assembly line (in other words, the gene products work in series; this is called a biochemical pathway). If this is true, different enzymes work one after the other on intermediates in the assembly line. If an enzyme working on the melanin pigment ahead of them “breaks down,” then all the enzymes downstream on the assembly line cannot do their job. The assembly line stalls. Imagine that Parent 1 has a mutation in Enzyme 1 (on both chromosomes) that prevents the formation of melanin. Parent 2 has a different mutation (also on both chromosomes) that affects a station more downstream in the assembly line, but Parent 2 is also not able to put together melanin. Both parents will have blue eyes because they lack the final product: melanin. However, when the parents combine their genes to produce a child, the child inherits one production line that breaks down at Enzyme 1, and one production line that breaks down at Enzyme 2. Since the child has some functional Enzyme 1 and Enzyme 2 floating in his or her cells, a complete and working production line can be assembled. As a result, some melanin can be formed, giving the child brown eyes. This is called “genetic complementation” (Sha, 2004).
Questions
It was recently discovered that almost all people who have blue eyes most likely share a common ancestor that lived 6,000 to 10,000 years ago. This was determined independently by three research groups who came to similar conclusions about the single locus responsible for blue eye color (Eiberg et al., 2008; “Blue-eyed humans,” 2008; Sturm et al., 2008; Kayser et al., 2008). Given this information, which of the three hypotheses to explain Ryan’s eye color are you likely to dismiss? Why?
Given the information that you have been provided about the nature of the mutation that gave rise to the blue iris allele, determine the likelihood of a backward mutation occurring in a genome to create a brown eye allele starting from a blue eye one. What sort of mutation would need to happen? Is this difficult to achieve?
According to a 2002 Loyola University study in Chicago (Grant & Lauderdale, 2002), almost 60% of people born in North America at the turn of the last century had blue eyes. By mid-century, that number had dropped to a third. Today, it is 1 in 6 Americans (Belkin, 2006; Starr, 2010). Propose a hypothesis to explain this phenomenon.
Assuming that the mutation that causes people to have blue eye arose 6,000 years ago (Eiberg et al., 2008; “Blue-eyed humans,” 2008), and given that at the turn of the last century more than half of Americans had blue eyes, it can be assumed that this mutation is extremely successful. It must confer an advantage. Hypothesize why evolution might have favored the selection of blue eye color. What advantage might it confer? Think of the different mechanisms of evolution that could result in this observation. For explanations that rely on natural selection, specify whether it may be a case of sexual selection.
What are some potential implications of the popular state of knowledge about the genetic determinants of eye color? (In other words, eye color genetics is currently taught as being a simple Mendelian trait with the brown allele being dominant to the blue allele. How might that be problematic?)
Explanation / Answer
Question 1: As the study provided data which showed that almost all the blue-eyed people shared a common ancestor. This indicates that people who have inherited the same locus always produced the same phenotype irrespective of environmental conditions. I would like to dismiss the second hypothesis, where epigenetic modification induced by environment affects eye color (because people who inherited the blue-eye color genotype always resulted in the same phenotype)
Question 2: Melanin plays a key role in determining the color of iris. A single change in the nucleotide of melanin synthesis pathway gene results in a change from brown to the blue color of iris. A backward mutation occurring in the gene by replacing the nucleotide with original nucleotide restores gene product which produces brown eye color.
It is difficult to achieve this mutation because mutations happen randomly in a genome. Mutation of a specific nucleotide at specific gene location will take millions of years.