Friday, December 09, 2005

Do These People Work at the Gap?

Individuals who are red-green color blind cannot distinguish between the colors red and green (makes sense). One type of red-green color blindness (deuteranomaly) is due to a recessive mutation on the X-chromosome. Men are more likely to be red-green color blind because they only have one copy of the X-chromosome, whereas women can carry the mutant allele on one copy of the X-chromosome and still have normal color vision if they have a wild-type copy on their other X-chromosome.

A new study reveals a hidden benefit of red-green color blindness -- the ability to distinguish 15 shades of khaki.
"They identified 15 shades of khaki that fitted the bill, and tested their prediction by showing two sets of subjects - one with deuteranomaly and the other with normal vision - a series of cards carrying pairs of different khaki shades. It proved to be almost impossible for people with normal vision to tell the colours apart."
At first thought, one might imagine this mutation would be beneficial for men picking out slacks at their local department store, but it turns out that the recessive allele may have aided the color-blind males when hunting.
"Simmons hypothesizes that because deuteranomaly is quite common in human populations, the gene responsible may have once provided an evolutionary benefit. For example, it may have helped them spot potential food items in complicated environments such as grass or foliage, he suggests."
I find this hypothesis to be a bit of a stretch. A simple analysis of the frequency of the allele may reveal that it is maintained in mutation-selection equilibrium. For example, some people have suggested that the prevalence of cystic fibrosis is due to a beneficial quality of being heterozygous for a mutation that causes the disease, but it can be adequately explained by the mutation rate from the wild-type allele to the disease causing allele.

A final note on color-blindness and powerpoint presentations: always make figures in your presentations as black and white as possible. You never know when an audience member (especially an important one) cannot distinguish between red and green. You may end up discussing two trend-lines that some people won't be able to tell apart.


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