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Genetic Determinism

DNAI got into a discussion with a few co-workers last week on a topic that I'd thought most educated people agreed upon to a large extent - the limits of genetic determinism. In the old argument of nature vs. nurture, I thought most people realized that who we are is a combination of both influences. However, in that conversation, I was the only one who thought environment played a big role, while the other two thought it was mostly down to genetics. Anyway, a few days after our conversation, I sent them an e-mail explaining how environment can contribute to our traits, and decided that it might be worth posting a modified version of it here on the blog. So, to anyone who puts too much stock in genetic determinism, here's some information on how environment also plays a strong role in our development.

First, there's an example that's so obvious that we almost forget about it - muscle size. Genetics gives us a potential muscle size & strength, but our actual muscle size can be greatly affected by diet and lifestyle, particularly by being active or working out. This is a clear example of genetics and environment interacting to produce a trait.

Here's an article on height (since that was one of the traits my coworkers and I discussed specifically). Based on studies between twins and other relatives, it looks like genetics is 60 to 80% responsible for height, and environmental factors, particularly nutrition, are responsible for the remaining 20 to 40%.

Here are a couple more links on height.

One important caveat on twins that doesn't get mentioned in many of these articles - identical twins don't look so similar solely because of their shared genetics (although that is the biggest reason). It is also due to the shared environment in the womb. That's why fraternal twins look more similar that siblings that didn't develop together. So, it's not enough to look at identical twins in these studies - you have to use fraternal twins as a control for early developmental factors.

Here's a really good site on the 'nature vs. nurture' debate that focuses on intelligence. I'm giving the link to the conclusion, but if you follow the links on the site, you can find the evidence they list. To quote part of that site:

Through the research we have done, it seems that heredity, as well as environment plays an important role in humans’ mentality; but these are not exactly equal in influence. A person’s entire environment seems to be more effectual in determining his mental ability than heredity is. The most fundamental way to explain our opinion is quite comprehensible. It is that heredity determines one’s potential, but environment determines how far one will reach that potential during his lifetime. In other words, every individual has a destined mental potential, but how much of that potential the individual will be able to gain solely depends on the environment that the individual grows in.


Here's another article that touches briefly on genetic determinism, mentioning an experiment where cloned plants (i.e. genetically identical) were grown in different environments, and the plants grew differently depending on the environment they were in.

Another point against genetic determinism is the fact that our cells aren't perfect machines, where given inputs give precise outputs. Cells are a cluttered stew of molecules inside a membrane. Depending on how molecules are dispersed throughout the cell, two genetically identical cells may have different reactions to the same conditions. Carl Zimmer's book, Microcosm, has a good explanation of this, if you ever get a chance to read it. A good example, one which made headlines, is the first cloned cat. Although it has identical nuclear DNA to its mother, it has a different color pattern, because the activation and inactivation of the responsible genes is more or less random.

Here are two more links, dealing with related themes that we discussed. The first link is to an article on the Flynn Effect (the fact that IQ scores have been increasing). We also discussed abstract thinking, and whether or not it's a learned skill. The second link below includes a discussion of a study done in Uzbekistan which seems to confirm that abstract thinking is learned (though the article also mentions potential problems with the study).

Okay, so what's my point in all this? Genetics plays a significant role in who we are, but so do environmental factors, and even random chance has a part. So, given the long complicated history that has led to the current conditions in the world, unless two people have had very similar upbringings, it would be nearly impossible to tell how much of the difference between them was due to genetics.

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