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Current time:0:00Total duration:10:40

Thomas Hunt Morgan and fruit flies

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Video transcript

where we left off in the last video we were in 1902 1903 and Mendelian genetics had been rediscovered at the turn of the century and Boveri and Sutton independently had proposed the chromosome theory that the chromosomes were the location for where these inheritable factors that Mendel first talked about where they actually were located but we talked about in that video that that was just a theory this was based on some observations of meiosis and seeing how chromosomes behaved and they seemed to behave in in analogous ways to some of these inheritable factors but they really didn't have good cellular proof that chromosomes indeed were the location for these inheritable factors and we don't really start to get that until we start looking at the work of Thomas Hunt Morgan now 1908 he decides to study fruit flies so why does he want to study fruit flies have you ever seen a fruit fly they're very very very small so you could actually put a ton of fruit flies in one jar so that's convenient you oftentimes don't think about the practical logistics of science but you could put a lot in one jar they were actually cheap and that's another practical concern of science is you don't always have a lot of resources to do your work and they had short lives and they reproduced a lot so you could very quickly get many many offspring in many many generations if you wanted to study how the different traits were passed on or not passed on and so he spent some time he started this in 1908 working with the fruit flies and he kept breeding them in search for some type of a mutant trait in general when you look at when you look at traits in a species the wild type let me write this down the wild type is the one that's typically seen while the mutant trait is something that seems unusual and after two years he finally discovers a mutant trait in his fruit flies he finds a white eyed male so this is the white-eyed male right over here he says okay now this is interesting let me take this white-eyed male and begin to cross it with other with well with with the females and you say well how does this well what you do is you take a jar full of females and you put the white-eyed male in there and then the crossing happens and what was interesting was the inheritance pattern that he saw for this wide-eyed trait because you have the parent generation here but then in the f1 generation all of the females were red-eyed and all of the males were red-eyed and so just off of that first generation it wasn't clear that anything interesting was going on but then when we when he crossed these to each other and I know what some of y'all are thinking wait aren't they all brothers and sisters being crossed to each other oh well yeah they were probably half-brothers and sisters if they came from different different mothers but some of them could have been brothers and sisters but yes that's that's what people are talking about when they're crossing the f1 generation but when they crossed these with each other he saw a pretty interesting pattern he saw a three-to-one ratio of red eyes to white eyes so for every four fruit flies he would see let me let me underline these he would see three red-eyed and he would see one wide-eyed so the wide eyed the wide eyed trait makes a reappearance which in and of itself is interesting it shows that this can be passed on genetically and that's interesting because this was a mutant that just showed up after he did many many many many many generations of observations but what was even more interesting about this three-to-one ratio and that three-to-one is something that popped up a lot in Mendelian genetics but was even more interesting was that he only observed he only observed the white eyes in the males in this f2 generation in this second the second generation of the crosses right over there and so you're thinking well why is that a big deal well he would use he was a pretty astute guy and he says well look if I'm only seeing it in the males and it's not like he only got four offspring years in the ratio he might have had hundreds of them but it was in the ratio of of two red-eyed females for every one red-eyed male for every one white-eyed male and so across these hundreds of in this in this generation he only observed the white eyes on the males and he said maybe this is in some way related to the chromosome that determines sex and so what he was able to do is say well let's just assume that it is let's assume that that trait that that mutant allele that mutant variation of the gene for eye color let's assume it's carried on the X chromosome and so the genotype for that first mutant fly that white-eyed male that he found we could call it and this is the notation that people typically use because it because this is a gene that we're assuming sits on a it's sex-linked it sits on a sex chromosome in this case the X chromosome the way that you would specify the genotype of that wide-eyed male is well on his X chromosome he had the white variation he had the white allele the white variation of that gene and then on his Y chromosome he had no variation for that gene so we assume that it's only contained on the only on the X chromosome you've probably heard of heterozygous or homozygous well this is a case where you have hemizygous you only have a version of the allele on one of your two chromosomes one of the two that you've gotten from each of your parents so this would be the genotype right here of the white-eyed male the genotype for the red-eyed female is specified by so it's on the X chromosome and the females have two X chromosomes just like in the situation for humans so on each of the x chromosomes we assume that the females start off with the red allele and the red allele the notation is the W plus W plus and you might say well why don't we just use the letter R well we could have but the general convention in in genetics is to use the letter of the first mutant type discovered for that gene and then to use this little plus type for the wild type so the wild type is the red eyes and then W which is the first mutant discovered for this gene is the first mutant allele that we do so we named it after that white so this is the white the white allele and these right here these represent the red alleles so these are the genotype of the red-eyed female and so when you cross that first-generation well the wide-eyed male he can either if he's with the he'll either produce sperm that have the X chromosome in it which is going to contain the allele or sperm that have the Y chromosome in it which is not going to contain the allele and the red-eyed female will they produce eggs either way either which of these X chromosomes they contribute they're both going to have the wild-type allele and we can see how this crosses you could get an X from both parents if you get an X from both parents are going to be female because you're going to be xx and each of these females since you've got one wild-type and one mutant type and the wild-type turns out to be dominant they still show their phenotype is still red eyes they still have red eyes but now they are heterozygous they're heterozygotes they are carrying the white allele now the male offspring right over here well in order to be male they got the Y chromosome from their dad so they're not able to get that white allele and they get the red the wild-type from their mom and you could see here and this is why all of the males in that first generation were red they only got one copy of the allele from their wild-type mother but then what was interesting is what the crosses that you see in that next generation if you took these red-eyed females that we already established these are all going to be heterozygotes and so you can see they have the red allele and and they have the white allele and you cross that with red-eyed males you cross it with red-eyed males what is going to happen well the females in this generation in order to be female you have to get an X from your mom and your dad and so they get an X from their dad which has the which has the mutant sucker which has the wild-type they're the dominant red allele and so regardless of which one they got from their mom they're still going to be red females some of them might be homozygotes some of them might be heterozygotes but now we see something happen in testing happening in the mails you could have heterozygote male male flies here where they got the X from where they got the the red X from their mom or you could get the hemizygous wide-eyed males where they got the where they got the white allele the white X from their mom and this is the exact this is this is the exact observation that Morgan made so it was a very interesting thing that he was able to see he started breeding these in 1908 he started breeding these flies in 1908 it wasn't until a couple of years that he finally found that first mutant white-eyed male and it was in 1910 and 1911 that he publishes these discoveries in nature and the reason why this is a big deal is he says look this is my observations are completely consistent with this with this eye trait this gene being on the X chromosome so he was able to show a direct linkage between in this case sex chromosomes and these heritable factors that Mendel first talked about and he would go on and his students that he worked with would go on to study this for for many many many many years and he actually ends up getting a Nobel Prize for this work and this is a big deal because he's finally able to draw pretty substantive connections between these heritable factors of Jin of Mendel this theory of Bavaro furry and sutton that maybe chromosomes have something to do with these in haribol factors and he's showing that this is actually the case these sex chromosomes seem to carry the trait or in case in this particular case the x sex chromosome seems to carry the the gene for eye color in these fruit flies
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