Two cents of variation…

My favorite t-shirt right now is black and says “98% chimp.” Someone asked me once if I really believed that was true. I said it wasn’t a question of belief, it’s a fact. And it is, if you look at it the right way. If you sequenced my DNA and compared it to a chimp’s, you would find they were about 98% identical. If we then sequenced your DNA, we would find it was about 99.98% identical to mine, although surely your 0.02% puts you further away from a chimp than myself.

Sure, we have blue or brown eyes, black or blond hair, we’re taller or shorter. But looking at it from the point of view of a cell, which is to say looking at proteins and other molecules, we are more or less exactly the same. Slight genetic variations and living in different environments (as we learned about last month) combine at a basic biological level to make humans the diverse people we are now. We might be different, but the building blocks are largely the same.

One main source of the differences between you and me at the genetic level are changes lumped into a category we refer to as SNPs (pronounced “snips”). It stands for Single Nucleotide Polymorphisms, which aren’t really so complicated to understand as they might sound. Each of the A, C, T and G elements of your DNA are molecules called nucleotides. A polymorphism is a fancy-pants five-dollar word derived from greek meaning “many forms.” So a single nucleotide polymorphism is just one spot in our DNA where some people have a C and others have a G.

I said that you and I are about 99.98% identical. Which seems like a lot, and is possibly quite a nasty thing to say about you. But when you think about the numbers, it gets put into perspective. We have about 3.4 billion nucleotides in our DNA, which means we differ in almost a million places in our genomes. This 0.02% variability is still enough to give a practically infinite number of variations to the human theme.

Most SNPs are pretty minor changes. They don’t actually show up as an eleventh toe, bigger muscles, or an anxiety disorder. Most of them seem to do nothing at all, from what we can tell right now. Maybe a study will show that one form of a SNP increases your odds of skin cancer by 1.24%. A large number of these polymorphisms are probably completely inconsequential.

Some aren’t, though. Some can predict responsiveness to different drug treatments in cancer; maybe even change the composition of a protein and impair its function. Douglas researchers like Ridha Joober and Howard Steiger have found interesting associations between certain SNPs and psychiatric illnesses like major depression and eating disorders.

For roughly the price of a Playstation 3, you can buy a kit from a private company and send away some cheek cells to have you own DNA examined. For example, an outfit known as 23andMe will take your DNA and check it for thousands of known SNPs. This isn’t going to give you the actual 3.4 billion nucleotide sequence that makes up your DNA, but it is going to tell you about some key places where your genome might be different from someone else.

Just don’t expect it to show that you have an anxiety disorder or you’ll develop cancer. Even people who work in this seemingly serious biological industry say that “the 23andMes of the world are more in the entertainment realm…” Like insurance companies, we can calculate the probability that a given SNP will increase a person’s chances of skin cancer by age 55. But we can’t look at this data for a given individual and know they’ll have cancer or undergo a major depressive episode in their 36th year of life.

Even if we could, there’s nothing we could do about it at the genetic level. SNPs aren’t diseases, and there’s no way to “cure” a SNP. Which one is the “right” one? Genetic variation is good for us as a species, and frequently has benefits that aren’t immediately apparent – sickle cell anemia, for example, is a heritable genetic disorder very common in some areas of the world because it hampers the ability of the malaria parasite to survive in the host.

So, as always, it’s true what our mothers used to say: even down at the genetic level, we’re all unique and special in our own way. Less true is what my father used to say – that me and my friends are a bunch of little monkeys…

Tagged as , , , .

Posted in General science, Molecular biology.

Posted on 30 Apr 2009

Comments are closed.