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Video transcript

my friend Marcus has beautiful blue eyes but you know what's strange both his mom and dad have brown eyes how is that possible well in this video Mendel and his peas are going to help us answer this question so in a previous video we had seen one of Mendel's experiment he took a pure tall pea plant and cross fertilized it with the pure short peopleand and saw that the offsprings were all tall he called him the f1 generation this was shocking because we believe that you know crossing them would give us some medium-sized plants but we didn't we got all tall no short no medium all tall and so Mendel was wondering whether these tall plants were the same as these pure cow heads as well and what do we mean by pure tall plants how do we find out what the pure or not what is the meaning of pure pure means if you were to self fertilize it so if you self fertilize a pure tall plant then all its offsprings would be tall similarly if you self fertilize a pure short plant all its offsprings would be short that's what we mean by pure so manda wanted to know whether this these twelve plants were also pure so what it did is he said let me take these f1 generation tall clans and self fertilize and see what would I get and what he got was again a surprise now he got both tall and short plants in a specific ratio roughly three is two one and so the two important questions to answer over here is in first case why did we get all tall why wasn't the short trait seen at all and another question is in the second case suddenly where did the short trade come from why it's in the ratio 3 h2 1 always it's as if the short rate has skipped one generation and then appears back in the next generation what's going on how do you explain all of this all right so to answer both these questions mental comes up with two laws the first one is called the law of dominance so what does it say so Mendel says by looking at this he is that when you cross the pure tall in the short plant both the tall and the short trait got passed along so these plans have gotten both the trades let me just write it down they got the tall rate which I'm going to represent with the capital T and they also got the short rate from this plant which I'm gonna write as small T and I'll tell you why I'm writing is a small T and not s okay so these plants contain both the traits the heart rate and the short rate but then you might be wondering well if they contain both the traits then why are they all appearing tall why not the short trip why don't they appear short well this is where Mendel guessed that maybe that the presence of tall trait is dominating maybe the short trait is hidden in the presence of the target again let me just write that down so what is trying to say is that maybe the tall trait this trait is dominant that's why this is called the law of dominance okay he believes that this trait is dominant and the short trade he calls it recessive recessive so the meaning of this is that recessive means the trade gets hidden hidden in presence of dominant trait so in presence of dominant trait that trade gets hidden and that's why it's not being seen and that's why what we see is only the dominant trait and so we say the dominant trait is getting expressed the word express means that's what we end up seeing okay so in short to summarize he's saying that both the traits are passed along it's not a mix nothing like that both are being passed along it's just that one of them is dominant and that's what we see the other trade is there but it's hidden in the presence of dominant the tall trade the short trade gets hidden now of course at this point you might have some questions as to why the torn trade is dominant why not the short trade why isn't that dominate well look Mendel is just trying to guess what might be happening so at this point we don't know why one of the traits is dominant what makes it dominant okay of course later on we'll study more about genes and how these traits gets passed and about DNA that's well we'll get some clues about why certain traits become dominant and other becomes recessive or that becomes hidden but as of now let's not worry about why one of them is dominant okay let's just accept this all right and if you're wondering why I wrote over small T and not s for short this is the notation that we write today for dominant traits we use a capital letter and for recessive traits the hidden traits we use the small letter of that same alphabet okay so we use the same alphabet to represent both the traits that's how we basically write it now okay so even this plant is tall has a tall and a short trait in it fine so the next question is how do we explain this why is it that when we self pollinate this plant we get both tall and short plants and why is you get in that same ratio 3 is to 1 what's the speciality of the 3 to 1 where does that come from well to answer this question he comes up with another law so let's move up let's make some space and write down that law so the second law that he comes up with to answer this is called the law of segregation segregation and the word segregation over here you can think of it as separation separation ok so what's being separated so Mendel guesses that when the gametes are formed in this plant meaning the sperm cells and the egg cells are formed in the plant they do not carry both the traits he says that maybe when the traits pass into the gamuts they get separated meaning some of the sperms will get at all rate some of the sperms will get a short trade similarly when the eggs are formed in the ovary of the flower they don't get both of them they get separated meaning some eggs will get tall traits some eggs will get short traits so let me just write that down this way so imagine this is one of the sperms which is found inside the pole grains of course his poems will not look like this you know different plants have different kinds of sperms let me just draw it that way so in that sperm is formed some sperms will get it halted but some sperms might get a short trade and that's what he means by separation the traits get separated when these sperm cells are formed and similarly when the excels are found inside the ovary so imagine this is an egg and the Excel so when the Excel is formed again some excels will get the tall trait and some excels will get the short trait okay law of segregation all right now how does this explain what happens and now think about it when we self pollinate what's happening basically the sperms are fertilizing the eggs now depending upon which sperm fertilizes which eggs we might end up with a tall plant or a short plant for example if this sperm with the tall trait goes and fertilizes this egg with also a tall red and the fertilized egg will have both tall traits and so when that plant grows it's gonna be a tall plant okay but there are other possibilities as well right so you know what I want to do at this point I want you to pause the video and see if you can find out how many such possibilities are there and out of those possibilities how many do you think will be tall and how many do you think will be short and see if you can get that magic ratio 3 is to 1 so great idea to pause the video and see if you can figure this out now ok hopefully you've tried let's see so one possibility is this sperm fertilizes this egg that's one or maybe another possibility is this form goes and fertilizes this egg that's 2 another possibility is third possibility is this sperm goes in fertilizes this egg and the last possibility would be this sperm goes in fertilizers this egg so total we have four different possibilities alright and the way we usually like to write down that possibly those four possibilities all the possibilities over here we like to draw a a box so here's how we like to do so let me just redraw those sperm cells this is the tall sperm and this is the short sperm okay it's not tall it's just that it's a sperm that contains tolerate and the sperm that contains a short rate and let me draw the egg which contains stall rate here and the egg which contains short rate over here and usually what we like to do is we like to draw a box like this okay all right now let's look at all the possibilities if this sperm fertilizes this egg then we get the fertilized egg has both tall traits so TT but if this sperm goes in fertilize is this egg then we'll have one tolerate coming from the egg and one short rate coming from this pump okay if this happens then we get a tall trait coming from the sperm and the short rate coming from the egg and finally if these this fertilizes then the fertilized egg will have both short rates and these are the only four possibilities we have right so now let's look at what will happen when these plants grow so when this plant grows this one it's going to be tall because both its traits are tall when this plant grows look the law of dominance tells us that it is still going to be tall because this one will dominate this is a recessive character recessive trait it'll dock it will it will hide so this will dominate so this is still going to be tall what about this one even this is going to be tall remember it doesn't matter from where the traits came with it when it comes from the egg or whether it comes from the sperm that doesn't matter as long as you have one tolerate it's gonna dominate the plant is gonna be tall but what about this last one well notice it doesn't have any tall traits both traits are recessive when both traits are recessive there is no tall tree to dominate then and only then the plant is going to be short and so right in front of you you can now see what are the chances of getting tall it is 3 out of 4 and what are the chances of getting short it is 1 out of and that's the reason we get the three is to one ratio and just to be clear this does not mean that if you grow four seeds and the four plans then you'll get three out of them will be tall and one will be short you know chances doesn't work that way it just means that if you grow lots of plants maybe hundreds of clans and then you count them then you will see about roughly three to one ratio will turn up four tall and short plants now one question you might be having is let's see what happens if you apply the same logic to the first generation let's apply the same logic to these these this pumps and the X cells over here what happens then well let's see this particular plant was a pure tall plant which means it must have had both tall traits does that make sense why it must have had both tall traits let me use the same color why it must have had both portraits because we know that if we self pollinate this one all its offsprings are going to be tall we have already seen that and that's only possible if both of them at all if one was tall and one was short we would have hot we would if I had this now we just saw and similarly we would same logic we can argue this one must have both short traits right that's the only reason it's looking short if it had any tolerate it would have been tall by the law of dominance and so now think about it now when a sperm from here goes and fertilizes an egg from here the sperm will carry the tolerate right and the egg will definitely carry the short rate and so when they fertilize you get one tall one short does that make sense and even if it's the other way around even if the sperm from here goes in fertilize an egg over here same thing one tall one short makes sense right and now you can use the same logic to explain what happens with the flowers also so when you cross a pure purple flower with a pure white flower in the first generation we got all purple flower now can you guess what was the reason for that well we can guess that when they crossed it both the purple and the white trait got passed along but maybe the purple Trade is dominating and that's why we see all purple over here and no whites and the when you take one of those purple and yourself pulling it by the law of separation now we'll get the same ratio three two one make sense right and so finally can you now see how a recessive trait can skip one generation but then come back in the next generation whoo so can you now explain how my friend got blue eyes from his brown-eyed parents think about what traits might be dominating and what traits got passed along okay let's see since we know that the blue trait got skipped at least one generation we can guess that the blue trait must be a recessive trait and so we can guess that my friend has both blue and I'm writing small letter because it's recessive if we had even one brown trait then his eyes would have looked brown because brown I'm guessing must be dominant so what must his parents be having so his parents must be having one brown trait that's what makes them brown eyes but they must also be carrying a blue trait the fluid is hidden it's recessive it's hidden and that's why they're it's not being expressed and so when the zygote was formed basically when fertilization happened we can guess now maybe the sperm from the dad carried the blue trait and it went and fertilized the egg from the mom which also carried the blue trait the chances of that happening is pretty small as we saw it's a three to one ratio we just saw the chances but can definitely happen and that's the reason why my friend ended up with both blue traits giving him those beautiful blue eyes that's it so let's see if we can recall what we learned can you recall what dominant and recessive traits are what the law of segregation is and can you work out what would be the ratio of the offsprings if we cross a short plant with a tall plant which has both the tall and the short trait in it and if you get stuck at any point no worries you can always go back and rewards that part of the video