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Discussions of conditions for Hardy-Weinberg

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Video transcript

in the introductory video to the hardy-weinberg equation I gave some conditions for the hardy-weinberg equation to hold and what I want to do with this video is go into a little bit more depth and go a little bit more of a discussion on the conditions for the hardy-weinberg equation now just to review a hardy-weinberg equation is all about if we have a population with a 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 well if you add the frequency of P of the blue plus the frequency of the brown they're going to add up to 100% or 1 and if you square both sides of this you would get this this expression right over here and we talked about that this is the probability or you could say the frequency of being a homozygous and homozygous for the blue this is the pop the probability of being 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 because you could get a blue from your mom or 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 here 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 you could somehow not have stable allele frequency so let me write this down stable allele frequency stable allele frequency so a lot of times there's a temptation to memorize a bunch of the stuff you might want to 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 heritable 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 you can have selection so we're going to assume no selection actually there's more than two ways you could have genetic engineering and all sorts of things so we're going to assume but the 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 are make make that organism more fit for that environment well those traits are going to be 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 effects of natural disaster that really gets you to that small population so that's the big picture but given that big picture I want to 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 that introductory video are no selection and that's consistent with no evolution we I also talked about no net mutation also consistent 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 get a mutation and they're not there now maybe a different version or they're not definitely not blue anymore so the allele frequency would change the reason why we care about large population is a set is mainly for genetic drift if you have a very small population just due to random chance it's it's more likely that the allele frequencies can change appreciably now other conditions that you will often see is are things like random random mating that weather weather or organism has a 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 it 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 isn't growing by by other organism entering it or is it 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 want to 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 you know 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 well or 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 there are tetraploid populations especially this can happen in plants where you could get two sets of chromosomes from your mom two sets of chromosomes from your dad we are assuming we are assuming sexual reproduction that we're not dealing with cloning and 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 have what sex you are so allele frequency allele frequency same in 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 have apply the hardy-weinberg where this is we have two alleles we're assuming sexual reproduction diploid you're getting a mom from your mom from your dad and just here all the conditions that help us ensure that we have a stable allele frequency now the one thing you're saying okay I can you know diploid sexual reproduction okay but isn't isn't you know isn't there always a chance for a little bit of genetic drift isn't there you know just the history of the world is that we have this evolution and the answer is yes and so the the actual reality is is that there's very few places where you could point to very few populations if anywhere you could 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
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