[Music] hi it's Mr Anderson and today I'm going to give you a beginner's guide to punet squares there are a few mistakes that students always make when they're doing pet squares so I I hope to kind of clear those up first of all we should talk about the namesake this is Reginald punet uh he didn't really work with punet squares however he did work with genetics quite a bit he did work with mimicry in the uh butterflies and so uh his name is probably associated with uh genetics just as much as as mendal and it's through the use of the punet square now the punet square is often overused as just a quick way to solve genetics problems without really understanding what's going on with the genetics and so I want to kind of get to that route and if you could remember one thing from this whole video podcast it's this right here the two sides of a punet square represent the Alternatives after meiosis in other words you have a bunch of genes and you give half of those genes to a sperm or an egg and and that happens through meiosis and so the organization of those gametes this case it's just a monohybrid cross are going to be on either side of this just like a flip of the coin this would be for one parent and then this would be the other parent on the other side and so what are the boxes on a punet square stand for they simply stand for all the Alternatives that could occur if we had mating between each of these different gametes and so let's get to some examples and hopefully that'll help so we're going to start with a monohybrid cross a monohybrid cross is simply a cross that is looking at one trait and so let's do one it's really really simple and so let's say we're crossing purple flowers that are homozygous purple with those that are homozygous white flowers in other words this is the dominant trait this is the recessive trait and so if you look at the parents what you want to do first of all is figure out what are the possible gamt that could be produced in meiosis in other words this one could either give a big p or it could give a big p what does that mean it can only give one thing it can only give a big p or that dominant Al and so if you're doing a a problem like this you don't even need a big Punit square one parent can only contribute big p so let's look at the other parent the other parent can either give a little p and I try to make them really small or a little p because P's look the same and so the other parent can only give a little p and so we could put that on the other side and so what are the opportunities that we could have as far as fertilization goes well this one's automatically going to give it big p the other one's going to give a little p and so this is the only possible outcome we could get between a cross of a homozygous dominant and a homozygous recessive and so you don't really need a big punet Square now you could do that you could fill it in big p over here little p over here but if you do that you're going to get the same thing in all of the boxes and so it's still a one to one uh ratio in other words 100% of the time you're going to have that okay so let's get to one that's a little more complicated let's say we have a heterozygous cross and so this is for um purple flowers as well well if you look at this one now the problem's changed a little bit this one could give a big p but it also could give a little p and so we have to show both of those possible gametes of meiosis and so this would be the big p and then this would be the little p over here so half of the time it's going to give the big p half of the time it's going to give the little p if we look at the other parent same thing it's going to give the big p half of the time and it's going to give the little p half of the time as well so we're going to put that here now we simply fill in the boxes and so this would be a big p with a big p because I'm taking this here and that there this is going to go over to here to give us a big p and a little p by convention we usually write the dominant Al first so that would be one alternative here here would be a big p little p as well so so we get the big p here and the Little P here and then finally we're going to get little p over here and a little p over here since they're each contributing a little p and so what do we get from this cross well we get a one to two since these are exactly the same to one genotypic ratio because the genotype is the letter so there's one that's like this there's two that look like this and there's one that looks like that so that's going to be our 1: two: one genotypic ratio what about phenotypic well this one's going to be a purple flower and so are these other two and so if we're looking at the phenotypic ypic ratio the phenotypic ratio now is going to be a 3: one ratio we're going to have three purple to every one white that we have okay let's try another one let's say we're looking at incomplete dominance so incomplete dominance a Snapdragon would be an example of that a Snapdragon has two genes if it has a red Gene and a white Gene then it's going to be pink and so this one actually has two alals that it can contribute and the same on the other side and so we just write those out so this would be the parent this would be a 50% chance of giving the red 50% chance of the white and then the same thing on this side over here so now if we fill in our punet square like that what do we get for all the different choices well now we have a one to 2: one genotypic ratio but we also have a 1: 2: one phenotypic ratio so if you're doing a question where it's incomplete dominance you use a ponent square the same way um coom would be the same way uh the difference is in incomplete dominance the heterozygous or the hybrids are going to be somewhere between the two if it's co-dominant so actually they're going to express both of those genes both of those proteins now let's try one that's uh a sexlink chromosome or an xlink chromosome and so in this one we've got a parent so this is a mom because she's XX chromosome and she's a carrier of let's say color blindness to the Gene and this is a dad that's normal so I'm going to put Dad up here here X Y because half of the time he's going to give the X and half of the time he's going to give the Y if we put mom over here I'm going to put that carrier up there she is not colorblind because she has one deficient Gene colorblind Gene but she has another Gene that works well on our other X chromosome so if I fill in this one here it's going to be X CX so this would be a female because two X chromosomes but they're going to be a carrier of that Gene if we look down here this would just be a normal female F male if we look down here I'm grabbing the X from here and the Y from up there so that'd be XY so that'd be a normal male and then if we look at this one right here this would be a male who's color blind the reason he's color blind is that he doesn't have an X chromosome or another Gene as a backup copy to that so those are monohybrid crosses and usually students do fairly well on those next are dihybrid crosses and this is where the mistake really start and so if we look at this parent this is a typical di hybrid cross um let me tell you what the letters stand for the r stands for round p seeds and the y stands for yellow seeds if it's the the recessive that stands for wrinkled and if it's a little y that stands for green and so we're looking at a die hybrid so that means two traits we're looking at seed shape round or wrinkled and Seed color yellow or green so now we have to do a DI hybrid cross and so the tendency if we look at this parent the tendency is to see the that there are four letters over here there's four boxes over here and then you just simply write them out Big R little r Big Y little Y and then you get a weird answer and you don't know what to do with it okay that's wrong that's a mistake okay whenever you're figuring out the gametes remember that you have to give one of each letter in other words each of the gametes is going to have one of each of the alal and so let me clear this mess out of the way so what do we do let's say this parent right here and I'm going to write it up here so it makes a little more sense Big R little r Big Y little Y what possibilities could they produce they're giving one of each letter remember well they could give the Big R and the Big Y so Big R Big Y that'd be one possibility they could also give the Big R and the little y so they could give the Big R little y they could also give the little r Big Y or the little r little y since they're giving one of each color there's only four one of each letter excuse me there's only four possibilities that they can give so it's getting to be kind of a mess but those are the four right here and so those four are going to go across the top so we got Big R Big Y Big R little y little r Big Y little r little y so those are the four gametes that you could produce in other words with this parent you can only get four combinations of each of the two letters um same thing down this side so it's going to be same same thing on written down this side because this other parent is going to be exactly the same okay so it would take me a long time to write all of those out so let me throw those in here so these would be the parents all the possible gametes you could get and if I fill those in these first nine if you look at them let me let me get a color that's different let me grab a yellow color and so this one right here is going to be round and it's going to be yellow so it's going to be round and yellow and this one you can see it's going to be round and yellow and round and yellow and round and yellow round and yellow and round and yellow and round and yellow and round and yellow and round and yellow in other words nine of them are going to be round and yellow in shape and the only reason why is that the r is dominant and so you could have one like this where they're hybrid for both and they're still going to be round and yellow and so let me try to add the next ones so what about these next ones well the next ones are going to be round and green so let me get a different color so these ones are going to be round and green because the round is dominant but they don't have the dominant for the yellow so they're not going to be let me get rid of that let's add the next ones so these ones are going to be wrinkled and yellow so again I got to get a different pen so these ones are going to be wrinkled in yellow and then if we look at the last one it's going to get all of the recessive alals and so that one's going to be getting green it's going to be green and wrinkled okay and so when we say there's a nine to3 to 3: one ratio that's just a a typical dihybrid cross um we're going to have nine of the phenotypes that are round in yellow three of the phenotypes that are round in green three of the phenotypes that are yellow and wrinkled and then only one of the ones that's not and that's only going to work if you are able to set up your gametes correctly on this on the on the side um now it's super hard for you to answer a question like this on a test you're rarely going to have to draw a DI hybrid cross but it's important that you understand the concept of it because most of the genes inside your body are not caused by one gene they're caused by multiple genes so how tall you are is caused by probably a dozen different genes inside your body so you can imagine how big the punet square is going to be for that um an example of uh that I'll leave you with it be this one so let's say we have a parent here and the parent is Big R little r Big Y little y little r little r little y little y the question I'm asking you is um how big would your punet Square have to be and so you're going to have to figure out what are all the Poss possible gametes that you could get from both of those and then um then then draw it out figure out all the possibilities you can get if you're thinking it's going to be a 4x4 um you're doing way too much work and so those are ponet squares and I hope that's helpful