Science Sundays: Dishing on Genetic Drift

By: Meradeth Houston on 9:37 AM

Welcome back to Science Sundays :) Today I thought I'd hit on another genetic concept that I've been hitting home with my students all week (well, at least attempting to). Genetic Drift: sounds cool, right? It is a powerful force, and would have a big effect if, say you had a new population being founded, or lost a huge chunk of a population.

That new population that was founded on Mars or the moon? They might have some issues with genetic drift.

So, what's genetic drift? Well, at its most basic, it's sampling error. Where you have a sample of something that isn't totally representative of the underlying diversity. Take flipping a coin: if you do it twice, you may get two heads, so you're not sampling the tails side of the coin. Say you flip it ten times and you get seven heads and three tails: you're not seeing the true 50/50 variability that exists in the coin's nature. (Okay, I know that's a random example, but it works :)

This skew from the actual total variability and range in a population in each sample that's taken can work from generation to generation. Just by chance (sampling error) not all the variability that exists in a population may get passed down from one generation to the next. Say someone with a unique gene variant (allele) doesn't have children, or doesn't pass along that specific allele: well, then it's lost. Over time, just through random chance, genetic drift works to get rid of variability in a population. A population will eventually hit what is known as fixation, where they've lost all the variability for a gene. It's crazy how this happens, but drift is one of those forces that is totally random and therefore super powerful.

Okay, so the true interesting stuff that happens is when genetic drift principles are applied to founding a new population. This is where Founders Effect comes into play. Simply, it's where a new population in formed and brings with it a random sample of the variability in the parent population. What usually happens is that variability is lost this way, as the migrants take less variation with them as they pick up and leave.

It's also possible to get a strange concentration of certain genes that may have been really rare in the parent population (take schizophrenia in the Hutterite populations--it's at like 1/43 people are affected, which is insanely high compared to regular populations, all because the 101 couples that founded the population had a randomly higher concentration of alleles for the ailment). It works on a larger scale, too: like the peopling of the New World with Native Americans: they have almost all blood type O, having randomly sampled only that blood group when they split off.

The other effect of genetic drift is a genetic bottleneck. When a population event occurs and leads to a large part of the population being lost, random variation can be lost. The difference here is that this happens within one population, not by founding another one. So, say a giant meteor hits the earth, killing off a huge portion of the population: the variability of the population that's left may be very different, if say, only one population in the jungles of the Amazon manages to survive (more on the true nature of human variability later).

This has turned into a giant post, but hopefully it makes sense. Anyone have a good example of genetic drift they want to share?