Phenotype plasticity | Heredity | AP Biology | Khan Academy

The folks you see in this picture are two NASA astronauts who also happen to be identical twins. On the left here, this is Mark Kelly; you can see his name on his patch right over there. And then this is Scott Kelly.

The reason why we want to look at these two astronauts in particular is to think about genotype and phenotype that we have been introduced to in other videos. So, just as a bit of a review, when people talk about genotype, they're talking about the actual genetic information encoded in an organism's genes.

So, if you go inside a cell, we have chromosomes, and then on those chromosomes, they're long. Each chromosome is a long stretch of DNA, and then sections of that long stretch of DNA will code for various things, often usually various proteins.

But there's a difference between what is actually what information is in one's genes versus what actually gets expressed—the observable characteristics—that is phenotype. Two people with the same genotype might have a very similar phenotype, but they won't necessarily have an identical phenotype because, above and beyond the genes affecting phenotype, you also have the environment affecting someone's phenotype.

So, as you can see here, Mark and Scott Kelly are identical twins. They have the same genotype, but if you just look at their facial characteristics, you can see that you could tell the difference between the two. And it's not just the fact that Mark has a mustache and that Scott doesn't. We could draw in a mustache; you would still see that they look different.

That's because their bodies developed in different ways based on the environment that they happened to be in. This idea that the same genotype could result in variations of phenotype; this is known as phenotype plasticity.

And maybe in one of the most extreme experiments ever conducted on phenotype plasticity, NASA was intrigued to see, well, what would happen to the gene expression if Scott Kelly spent an extensive period of time in space while his twin brother didn't. So, they looked at how their bodies expressed certain characteristics before Scott spent an extended period of time in space and then after he spent that time in space.

They studied his body, and they saw that there were differences in the mitochondria and differences in which genes were expressed. In fact, there was a seven percent difference in the gene expression after the extended time in space for Scott.

So, that's an example of that space environment changing Scott's phenotype. The NASA scientists theorized that it was a low oxygen environment. There was a lot of carbon dioxide in that environment, and there were stressors from space travel that would have affected how the genotype, which is identical to his brother Mark's genotype, might have been expressed, resulting in the phenotype plasticity above and beyond what other changes in phenotype between Scott and Mark accumulated over the course of their lives despite having the same genetic information.