- Allele frequency
- Hardy-Weinberg equation
- Applying the Hardy-Weinberg equation
- Discussions of conditions for Hardy-Weinberg
- Allele frequency & the gene pool
- Mechanisms of evolution
- Genetic drift, bottleneck effect, and founder effect
- Genetic drift
- Natural selection in populations
- Selection and genetic drift
Discussions of conditions for Hardy-Weinberg
The Hardy-Weinberg equation assumes stable allele frequencies in a population. Key conditions include no mutation, random mating, no gene flow, infinite population size, and no selection. Diploid organisms with sexual reproduction are also assumed. Although real-world populations may not strictly adhere to these conditions, the equation remains a useful approximation. Created by Sal Khan.
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- I still don't quite understand the use of the Hardy-Weinberg Equilibrium off paper because it only deals with nonevolving populations. Aren't all populations evolving somewhat which would skew the data? I know he mentions there are a few populations which group under the equilibrium but what are some examples if any?(4 votes)
- You are correct that it is unlikely to find a population in Hardy-Weinberg Equilibrium (HWE).
Generally what people look for is how a population deviates from this equilibrium — those deviations can suggest what sort of processes might be going on in a population.
For example, if you were examining a population and found that organisms homozygous for a dominant color allele are much more rare than the HWE predicts. This might suggest that allele has a recessive phenotype that negatively effects the organisms survival (i.e. there is selection against individuals that are homozygous recessive for that allele).
One situation where this useage of the HWE might be really helpful would be if the homozygotes were susceptible to dying just after fertilization and thus the death would not be easy to observe.(3 votes)
- When the frequencies of each allele at a given locus are added together they do not equal 1. Why is this?(2 votes)
- Because that population is not under Hardy Weinberg equation? In nature, you'd hardly find a population in HW equilibrium.
I do not know what frequencies of allele you are referring to.
Can you give timestamp from the video?(2 votes)
- Does death of organisms for certain alleles in a population not change the allele frequency and hence deviate from hardy weinberg law?(2 votes)
- Yes you are correct. We are looking at a snapshot of a population at one moment in time. When organisms die, their alleles are removed from the gene pool and the gene pool changes slightly. Importantly, organisms with a 'bad' combination of alleles are more likely to die than organisms with a combination of alleles that works well in their environment. The gradual changing of the gene pool is part of how evolution works.(2 votes)
- the HWE doesn't apply to single celled organisms, right? (since they basically make identical copies of themselves?)
thanks in advance.(2 votes)
- If single-celled organisms do make identical copies of themselves, then yes, HWE doesn't work on them. However, if they reproduce sexually, and if they are diploid, then they do approach HWE.
I got the information from:
Under the explanation titled
"[Are there other Assumptions?]", which is a subtitle for the section titled "Hardy-Weinberg assumptions and evolution."(1 vote)
- What is an example of genetic drift?(1 vote)
- I was wondering:
How do you determine if a situation will produce a large p value without calculating it but looking at the conditions? For example, my class did a simulation with about 30 people all starting with a heterozygous genotype, without immigration and emigration. Is there any way to predict what kind of values of p and q I would get?(1 vote)
- For your first question:
There is only one genotype that directly relates to a phenotype — can you say which one it is?
Now, assuming the population is in Hardy-Weinberg equilibrium — how would you determine p?
For your second question:
What are the values for p and q at the start?
What does the Hardy-Weinberg equation predict will happen to those values over time?
What would you expect to happen if two of those 30 individuals mated and had 20 children?(1 vote)
- What is the purpose of the Hardy Weinberg Equilibrium if no species fit the model because all species are currently evolving, as well as natural selection based off of environment? All species deviate from HWE right?(1 vote)
- Evolution is not that fast so it does make sense. Evolution does not happen on ontology time scale.(1 vote)
- Many generations go by and a group turns into a random mating population of 10,000. Two of the original group were carriers of the autosomal recessive disease allele for Tuberculosis. Tuberculosis is now at 6% frequency in the gene pool. What are the genotype frequencies now? Can someone show their work!! I can't seem to get it to equal to 1.(1 vote)
- This is almost identical to the question he works through in the previous video:
I suggest (re)watching that video and then trying again.
(Note: Tuberculosis is caused by a bacteria and thus is not a genetic disease ...)(1 vote)
- I still don't understand why allele frequencies in a population generally stay the same. Can someone please explain it for me? Much appreciated.(1 vote)
- For anyone who has the same problem with understanding it as me, go to the following link:
Read the section titled "Hardy-Weinberg equilibrium." From there, I think I understood why the allele frequencies should stay the same :)(1 vote)
- What sort of variables do you use if there is more than two alleles present in a given population?(1 vote)
- Do you mean what letters (e.g.
You can read about this here:
• search for "multiple alleles"(1 vote)
- [Voiceover] In the introductory video to the Hardy-Weinberg equation, I gave some conditions for the Hardy-Weinberg equation to hold, and what I wanna do in this video is go into a little bit more depth, and have a little bit more of a discussion on the conditions for the Hardy-Weinberg equation. Now just to review what the Hardy-Weinberg equation is all about, if we have a population with the gene, say for eye color, and let's say that gene comes in two versions, one is the allele that produces blue, one is the allele that produces brown, if p is the frequency of the blue allele, q is the frequency of the brown allele. Well, and if they're the only two versions, if you add the frequency of p, of the blue, plus the frequency of the brown, they're gonna add up to 100%, or one. And if you square both sides of this, you would get this expression right over here. And we talk about that this is the probability, or you could say, the frequency, of being a homozygous for the blue. This is the probability of having two alleles for the brown. And then right here in the middle, this is the probability of being a heterozygote, and why is that? Well cause you can get a blue from your mom and a brown from your dad, or a blue from your dad and a brown from your mom. So there's two ways to get that pq combination. Now the key idea is Hardy-Weinberg assumes a stable allele frequency, so let me write that really big. Because all of these other conditions that you might see are really like, well, what are all the different ways that you could somehow not have stable allele frequency? So let me write this down, the stable allele frequency. Stable allele frequency. So a lot of times there's a temptation to memorize a bunch of this stuff, you might wanna do that. But the more important thing is to get the underlying idea. And the underlying idea is, well, will something somehow cause the allele frequency to be unstable? And actually, another way to say stable allele frequency is no evolution. No evolution. Evolution is a change in the inheritable traits in a population, and that will include a change in allele frequency. And if you think about the two ways that you could have a population evolving, well, you can have selection. So we're gonna assume no selection. Actually, there's more than two ways, you could have genetic engineering and all sorts of things. So we're gonna assume the mainstream ways, I guess you could say, we can assume no selection, we can assume no genetic drift. Remember, selection is certain traits that make that organism more fit for that environment, well those traits are more likely to be passed on. Genetic drift is random chance changes in the allele frequency. It could be due to small populations, it could be due to members of the population migrating or some type of bottleneck effect, some natural disaster that really gets you to that small population. So that's the big picture. But given that big picture, I wanna dive deep into some of the assumptions that you might see in your biology class, just so you feel comfortable with them and you see that we're talking about the same thing. So the ones that I mentioned in that introductory video are no selection, and that's consistent with no evolution. I also talk about no net mutation, also consistent with no evolution. Once again, we don't want to change the allele frequency. If there was net mutation, one of those, maybe some of those blue versions of the gene got a mutation and they're now maybe a different version, or they're definitely not blue anymore, so the allele frequency would change. The reason why we care about large population is mainly for genetic drift. If you have a very small population, just due to random chance, it's more likely that the allele frequencies can change appreciably. The other conditions that you will often see are things like random, random mating. That whether an organism has the blue or the brown version of the gene, that that doesn't make them any more or less desirable to a member of the opposite sex. And if you think about, you might say, well isn't that a form of selection? And you'd say well yes, it kind of is, but this is sometimes broken out as another way. Now also, no migration, that you don't have, the population isn't growing by other organisms entering it or isn't shrinking by other organisms leaving or there's not a mixing of population between two populations. And once again, it's all in, it's all because we care about stable allele frequencies. Now if we wanna go even further than that, and sometimes you will hear these types of things mentioned, although I just mentioned the five mainstream things, which all boil down to stable allele frequency, no evolution, no selection, no genetic drift. But sometimes, we are assuming that we are dealing with diploid organisms, that you're getting one set of chromosomes from your mom, one set of chromosomes from your dad, one version of an allele from your mom, one version of an allele from your dad. And you might say well, how can you be other than diploid? Well you could be a, there are tetraploid populations, especially this can happen in plants. Or you could get two sets of chromosomes from your mom, and two sets of chromosomes from your dad. We are assuming sexual reproduction. That we're not dealing with cloning or just budding, where you're just a copy of another organism from generation to generation. We're assuming that whether you are blue or brown, whether you have those versions, that that's not correlated with what sex you are. So allele frequency, allele frequency same in all sexes, in all sexes. And we're assuming sexual reproduction, once again, we're assuming one where there's only two sexes, so you could, you know, if you were to think about, if you were to let your imagination go wild, you could imagine a lot of other constraints to put here or other ways that the, where you could no longer apply the Hardy-Weinberg. Where this is with two alleles, we're assuming sexual reproduction, diploid, you're getting from your mom, from your dad, and just here are all the conditions that help us ensure that we have a stable allele frequency. Now the one thing you're saying, okay, I can, diploid, sexual reproduction, okay. But isn't there always a chance for a little bit of genetic drift? Isn't there just, the history of the world, is that we have this evolution, and the answer is yes. And so the actual reality is that there's very few places where you can point to, very few populations, if any, where you can say, oh, that's a pure, we can purely apply Hardy-Weinberg there. But like a lot of things in the applied sciences, it's a very good approximation for many populations. And so that's why it is useful.