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Independent assortment (dihybrid cross)

Let's explore the law of independent assortment. Created by Mahesh Shenoy.

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Video transcript

hi there this is gregor mendel hi Mendel what's up I love to grow pea plants yes he's obsessed with pea plants and today we will see his obsession led us to discover the law of independent assortment what is this independent assortment you ask well let's take an example Mendel this time starts playing with two characters of a plant for example let's say we take a pure tall plant with yellow seeds and cross it with a pure short plant with green seeds see over here we're dealing with two characters the height there's a difference in the height and there's a difference in the color of the seeds as well so two things to take care of and so we'll take it it's really slow because it's very easy to get confused I used to get confused a lot over here and what does this pure mean we'll just to remind you is pure means that if you are to self fertilize this plant then all its kids would be tall and yellow none of them would be short none of them would be green and similarly if you were to self fertilize the other one then all its kids would be short and green none of them would be tall none of them would be yellow that's what we mean by pure okay so let's get rid of this that's another big question is what if we cross them and what kind of offsprings do we get let's see well in the first generation we get all tall plants with yellow seeds none of them are short or green does this look surprising to you that we didn't get any short plans or any green peas or do you think this is familiar think about this for a moment can you recall the law of dominance and see if you can answer why this is happening and if you need a refresher we've talked about them in previous videos on dominance and segregation laws so great you go back and check that ok so let's see let's first write on the traits for the parents and then we'll see what happens to the children so for this parent this one will read in yellow it must have tall tall yellow yellow and the one this one is having must be short short in green and just to be clear what are these things well these things are called jeans don't worry too much about what they are think of jeans as something that decides what your characters are you know how your characters look so this is the hygiene you can think of these are the hide jeans and this is the gene for color and why there are two copies well for every gene you have two copies one you get from your dad and one you get from your mom and why am I writing capital and small letter well the reason for that is again because you can have a dominant type and you have a recessive type so the dominant ones are usually written capital and recessive ones are usually written small so this one has both dominant type of genes both from its both the parents it got that and this one got both from its its parents both recessive same the same is the case with the color so yellow is dominant that we can see over here and that's why it's dominating and the green color must be recessive all right so what's going to happen when you cross them well now to think about that I like to think in terms of sperm and the egg so let's say this is the sperm from the dad let me call this as the dad and let me call this as the mom and here is the egg from the mom so how do these how do these genes pass along well the law of segregation tells us not all of them pass along one copy of each gene passes along so one copy of height will come and since both copies are tall any one if it comes it doesn't matter it'll be tall and one copy of color will come and again both are yellow so both are any one of those will come and similarly in the egg one copy of height which is recessive and one copy of the color will come okay and so now when the sperm goes and fertilizes that egg what will be the genes of that fertilized egg can you write that can you pause the video and see if you can write that all right let's see it now gets the tall copy from the dad from the sperm and gets the short copy from the mom this is the height gene now and similarly it gets the yellow copy from the dad and it gets the green copy from the mom and this is now the color gene and so as a result can you see that these kids are what we call hybrids they're hybrids because the the copies of the genes are not the same these are pure because they have identical copies of the genes but these are hybrids because they have different copies of the genes one dominant the dominant one gets expressed so it's tall because tall is dominant this is becoming this is getting hidden and again yellow is dominant that's why it's being seen the green is getting hidden so it's a hybrid and it's for that reason this experiment is called as a dihybrid cross WiDi hybrid because it's a hybrid for two characters there are two characters they're dealing with and it's a hybrid of them so that's basically why it's called a dihybrid cross now here is the big question that Mendel is trying to ask over here okay so what is asked what is trying to ask us when these genes pass from the parent into it's sperms or into its gamuts the sperm cells and the egg cells are these genes passing as a package or they are independent so for example what I mean is are the tall gene and the the height gene and the color gene is this a single package is this one single package or are these are two individual or independent packages same question over here are these two a single package that gets passed on from here to here eventually or these are individual that's what Mendel is trying to figure out because at this point we don't know what even genes are what exactly are they concert of DNA chromosomes yet to come so how do we answer that question does this experiment answer our question not really at least not this part of it and the reason is think about it even if these things were packaged let's say over here this was packaged then when it fertilized and gets passed on this becomes one package and this becomes one package kind of makes sense right the way I'm writing it over here but I would but the plants would still look the same right they would still look the same on the other hand let's say they were not packaged let's say they were individual these traits were individual then over here inside there these things will be individual but again they would look the same right so how do I know inside these plants whether they are packaged together or they are individual how do we know that this is where Mendel decided and maybe you can guess it now we need to go one step further and see what happens to their children so then we get rid of this and so that's what it does next he takes one off the f1 generation plan which has this it takes a hybrid and then self fertilizes it okay and then he takes the seeds and he sews them and you know what he gets he gets a lot of tall plants with the yellow peas which may not be all that surprising he gets some short plans with green peas again may not be surprising but this is the surprising part he also gets some tall plants with green peas and some short plants with the yellow peas ooh this is interesting because now we are getting new variations waiting tall plant with green peas and short plants with yellow peas what could this mean [Music] Mendel looked at these f2 kids f2 generation kids and he got the answer for the question he was looking for so how does this answer our question well let's see let's take it slowly ok remember for what follows remember the yellow ones remember they had the ones that comes from the dad and the green ones comes from the mom now again the question is whether they're packaged or not right so let's now talk look at the sperms and the eggs of this particular plant ok what could be the different possible sperms that this plant might get let's first assume that they were packaged together the genes of the dad and the genes of the mom were packed together what would have happened let's see if that was the case then the spawn could get the tall the height gene from the dad but that spa must also get the color gene from the dad if they were packaged together they should travel into this firm as a single package right and another possibility is the spawn will get both of the traits from the mom again if there was a package T and Y and same would be the case for the eggs as well these will be the only possibilities if these were a package you should either get everything from your dad or everything from your mom okay now let's see what would have happened in this situation if this sperm fertilizes this egg we would get tall yellow that makes sense so we'll get this plant if this sperm fertilizes this egg again we'll get tall yellow because it will dominate on this and so again we'll get this plant what happens if this sperm goes indomina fertilizes this egg again we will get tall yellow right so these three possibilities will only give meet Holly yellow and in the last possibility if this perm goes and fertilizes this egg then I would get short green because they're both recessive characters then I would only get short clean now think about this is the important part okay if this was true then we saw that the only possible children we should have gotten is tall yellow or short green nothing in between that means our children should have looked exactly like their parents but the experiment shows otherwise in the experiment we got these two others as well these two variations as well how is this possible Mendel asked how can you have a tall with green how is that possible Mendel thought that can only be possible if these traits are not packaged together because if they are not packaged together if these are not packaged then we can have other possibilities for example I can now have a sperm which gets the height trait from the dad but might get the color trait from the mom if they are not packaged this is a possibility and similarly another sperm might get the other way around it might get hi trade from the dad sorry from the mom from the mom and it might get the color trade from the dad and of course the same for the eggs as well and so you see the in order for the new variations to occur this has to happen which means the traits that get passed along for different characters can pass independently the height character does not depend on what color character passes or the color character does not depend on what height character passes they pass on independently and this is what we call the law of independent assortment and so Mendel said in order to be absolutely sure that this is what's happening let's count the number of plants and see what ratio we end up getting and then let's see theoretically if we get the same ratio so when he counts them he finds that this comes is 9 this is about 3 3 1 I'm writing the ratios okay not exactly the number of plans there are definitely hundreds of fans but the ratio turns out to be 9 is to 3 is to 3 is to 1 and that's true for all the experiments that he does so he does this with maybe color and shape he does that maybe with color of the flower and maybe the color of the peas all the different experiment that he does he gets the same ratio and now let's work out what are the different possibilities we might get over here and see if we get the same ratio and so just like before we're gonna make a box to nicely represent all the possibilities and since this time we have 4 different possibilities for eggs and sperms there are total 4 times 4 16 different possibilities so again great idea to pause the video and see if you can fill up this square yourself alright hope you have filled it yourself this is what we will end up with now let's see what these plans look like and the way I will do it it I will first do the color let's see which is yellow and which is green and then I will do the tall and the short so whichever has at least one why one yellow definitely has going to be yellow so this is yellow this is yellow this is yellow remember yellow is dominant so one why is enough one yellow color enough yellow yellow yellow yellow yellow yellow yellow and these don't even have that yellow eyes these are the ones will be green green green green green all right now let's see which is tall and short again if there is a capital T it's gonna be definitely a tall that is one capital T tall so T this is tall tall tall this is tall tall tall tall tall tall tall tall I think if I story short so rest all would be short short short short now let's count how many tall yellow one two three four five six seven eight nine that's what we got over here how many tall green one let me use a different color one two three that's what we got here how many short yellow short yellow one okay different color one two three and how many short green only one perfect Mendel was spot-on on his theory so indeed this conclusively proves that the traits passed on are sorted independently as not as a package throughing his law of independent assortment good job Mendel thank you thank you so that's pretty much it let's see if we can now recall what we learned in this video can you recall what a dihybrid cross is what is the law of independent assortment and finally can you see if you can work out what will be the f2 generation offspring ratio if we cross two short plants one which has round green seeds versus one which has wrinkled yellow seeds so the two traits over here is the shape of the seeds and the color of the seeds and by looking at the letters you can kind of figure out which is dominant and recessive if you have difficulties answering these questions no worries you can always go back and re-watch the video