Tuesday, March 1, 2011

Dominance: Why?

As I was reflecting over the last week, I realized that, although I've heard quite a bit over my life about the dominance of alleles, I'd never heard a real explanation of why certain traits express themselves over others. So...I found out.

On this page, Stanford University scientist Ruth Tennen answers this question. Apparently, there are several different reasons this happens. The most basic one occurs when the gene's function is to make a protein. If the recessive allele does not make this protein, then because the other allele will anyway, its presence is apparent in the phenotype.

For example, consider red hair. Because the protein that the MCR1 gene makes removes red pigment, as long as a person has one working copy of the MCR1 gene, they will not have red hair. A person has to have two copies of the broken MCR1 gene in order to have red hair.

Then, believe it or not, the opposite can be true--the dominant allele can be the broken gene. This was somewhat confusing at first to me, so I'll do the best job I can to explain it, often by stealing the metaphors used on the aforementioned site.

This situation of the recessive allele being the functional gene often occurs when the broken protein made by the dominant allele gets in the way of the protein made by the recessive allele. Consider a relay team. The first runner on the relay does his job just fine--he runs his 100m. But the next runner always drops the baton on the handoff, and because of this, this relay never wins. The first runner here is like the functional protein made by the recessive allele--it does its job just fine. But then, because the broken protein can't help out along the way, it is the work of the broken protein--the dominant allele--that is expressed.

Now, of course, there are more situations than this. There's codominance and incomplete dominance and...well, you get the idea. However, the basic idea here is that it's all about the proteins and the way they get along.

See you soon! 


  1. That's just what I was thinking!

  2. That's just what I was thinking!


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