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Genetic Variation: Responsible Factors

by Georgina Cornwall, PhD
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    00:01 Now, it’s time to take a step back from the Hardy-Weinberg equation itself.

    00:09 Think again about this Hardy-Weinberg equilibrium because as I mentioned and as we know, it’s not a very realistic situation. Because it’s not very realistic, we can use it as a measure to compare reality against. But what happens when they can’t be met? What do we call these situations? You probably already have an idea that we break pretty much all of the Hardy-Weinberg assumptions, right? We know that mating is not always random.

    00:48 For example, in the United States, we may have stratification in which there’s different cultural groups that associate together and marry within their culture, different ethnic groups.

    01:02 We also have somewhat of a hierarchy of class structure where it may be more common to marry within your group than outside of your group which is actually selective mating.

    01:18 It’s certainly not random mating as if running into any old person would really work out, right? Assortative mating is what we actually do in reality. We have positive assortative mating and negative assortative mating. This is a really interesting field all on its own, to consider that some individuals are more likely to positively assort, like brings like and other individuals are more likely to negatively assort and mate with someone opposite of themselves.

    01:56 Both of those provide their role when we consider population genetics. I think it’s a fascinating field.

    02:02 But we can increase diversity by outbreeding, so to speak and negative assortative mating.

    02:11 But positive assortative mating is usually what happens. We mate within our groups.

    02:18 Of course, we can take that to an extreme, consanguinity where we have on occasion, mating within families. Either way, I’m just covering this because they are forces that move us away from completely random mating. We all, by some means, make selections about our mates consciously.

    02:40 These mechanisms in selecting mates can often act to reduce the genetic variation in a population.

    02:48 Another violation of Hardy-Weinberg equilibrium is the idea of mutation. In Hardy-Weinberg, mutation doesn’t happen. But we all know that mutations happen in a population, albeit heritable mutations are fairly infrequent. So, genetic change by mutation is actually very slow.

    03:14 But on occasion, we see that mutations occur and they are highly beneficial, for example, the CCR5 receptor mutation that we have been discussing. Sometimes we have slightly beneficial mutation, sometimes neutral. Sometimes they’re deleterious, meaning we’re selecting against them because they are perhaps like sickle cell anemia, preventing our ability to carry as much oxygen as we need. Those things may be less selected for which is why we see and sometimes they may be more selected for because we all know that sickle cell anemia prevents against malaria.

    03:55 This is a great example of where in one population we see the allelic frequency being higher for sickle cell allele versus another population. Let’s say we look at certain regions, Africa versus Europe, we’ll have a distinct difference in the allelic frequency. This is something that you need to keep in mind if you are discussing genetic counseling with any of your patients. Mutation rates in general are quite low. So, mutation changes or allelic frequency changes due to mutation are actually quite slow.

    04:37 But historically, there are those mutations there. This scenario of sickle cell allele is definitely something that lends itself to natural selection, just covered that. This hemoglobin molecule in its normal form, we know that the beta chains are mutated. That is one of the things that is actually a protective effect against malaria. In that case, we have AA normal hemoglobin.

    05:12 We see a higher frequency of the big A alleles, say in Europe versus a lower frequency of the big S allele. We consider this a codominant trait. Think back to the molecular series where we were looking at codominance. Can you recall what codominance is? Exactly, so both cases are expressed, right? The case of sickle cell is a deleterious effect because all of the cells are sickled.

    05:47 But an individual that is heterozygote actually has an advantage in the case of sickle cell anemia.

    05:53 So, we call this the heterozygote advantage. Either way, natural selection, another violation of Hardy-Weinberg equilibrium. It does happen. We can see the evidence of these cells here.

    06:09 I kind of like this picture because it shows the full cells as well as the sickled cells.

    06:16 The sickled cells obviously have multiple different effects, so pleiotropic effects, another concept in genetics that we need to keep our minds on. Moving on to look at gene flow is another violation.

    06:33 Looking back at the CCR5 receptor here, we can see that that mutation has a much higher prevalence in Northern Europe and has spread throughout Europe as interbreeding, outbreeding with different populations of individuals has occurred. So, gene flow is simply the movement of genes from population to population with immigration and emigration. That is a concept that is violating the Hardy-Weinberg equilibrium. Genetic drift and the founder effect, we see that when a small group moves from one population to a new location, perhaps emigrating from their home country to the United States. Maybe we have a misrepresentative portion of people so that the founder group has not represented the entire population equally. In this example, we have fairly even red and green on this side. On the right side, we see that there is more representation of the red individuals.

    07:54 So, we’re going to see in that future population a genetic drift towards the red individuals.

    08:03 Finally, we need to consider genetic drift, again a violation of Hardy-Weinberg equilibrium because we certainly see immigration and emigration. Often, when a small subpopulation moves away from the greater population, perhaps you could use the example of people moving from their countries into the United States, we see a misrepresentative gene pool or a change in the genetic diversity.

    08:34 Because for example here, we have green and red individuals in pretty equal proportion and the founder group has hence, the founder effect as they move to their new place. In this case, we have three red ones and one green one. We’re going to have in the future a misrepresentation of the original population or a founder effect or genetic drift towards more of the red alleles in this population. In short, we’ve looked at Hardy-Weinberg equilibrium and how each of these conditions are violations of Hardy-Weinberg equilibrium because Hardy-Weinberg state doesn’t really exist, right? It’s a theoretical state against which we compare the present state so that we can see if shifts in allelic or genotypic frequencies have happened. You have also got a new grasp on the Hardy-Weinberg equation. You should be able to use it to predict genotypic frequencies based on allelic frequencies and allelic frequencies based on genotypic frequencies and the number of individuals in a population. Because for your exams, you need to know how to use that equation. So, practice. I look forward to seeing you in the next lecture series.


    About the Lecture

    The lecture Genetic Variation: Responsible Factors by Georgina Cornwall, PhD is from the course Population Genetics. It contains the following chapters:

    • Factors Responsible for Genetic Variation
    • Mutation
    • Natural Selection
    • Gene Flow
    • Genetic Drift - Founder Effect

    Author of lecture Genetic Variation: Responsible Factors

     Georgina Cornwall, PhD

    Georgina Cornwall, PhD


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