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Worked example: Punnett squares

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Video transcript

in the last video I drew this grid in order to understand better the different combinations of alleles I could get from my mom or my dad and this grid that I drew is called a punnett square punnett punnett square and I looked up what punnett means and it turns out and this this might be the biggest takeaway from this video that when you go to the the farmers market or you go to the produce and you you see those little baskets you see those little baskets that often you'll see maybe strawberries or blueberries sitting and they have this little grid here right there sometimes grapes are in them and you you have a bunch of strawberries in them like that that green basket is a Punnett that's a Punnett apparently in some countries they call it a pond it I think England's one of them and you UK viewers can correct me if I'm wrong and so I guess that's where the inspiration comes from for but calling these punnett squares that these are kind of these little green baskets that you can throw different combinations of genotypes in and these punnett squares aren't just useful if you're talking about crossing two hybrids this is called a monohybrid cross because you're crossing two hybrids for only one trait it could be useful for a whole set of different types of crosses between two to two reproducing organism and it doesn't even have to be a situation where where one thing is dominating another let's let's do a bunch of these just to make you familiar with the ideas so let's say you have a mom so instead of doing two hybrids let's do say the mom I'll keep using the blue eye brown-eyed analogy just because we're already reasonably used useful to it let's say the cheese homozygous dominant and let's say that the dad the dad is is it is a heterozygote so he's got a brown and he's got a blue and we want to know the different combinations of genotypes that one of their children might have so what we do is we draw up punnett square again so let me draw a grid here draw grid right there and up here will write the different gene that mom can contribute and here will write the different genes that dad can contributor the different alleles I didn't want to write gene I wanted to write dad all right so the mom in either case is either going to contribute this big B brown allele from one of the homologous chromosomes or on the other homologous well they has the same allele so it's going to contribute that one to her child the dad could contribute this one that big brown-eyed big the capital B allele for brown eyes or the lowercase B for blue eyes either one so the different combinations that might happen an offspring could get both of these brown alleles from one copy from both parents this could also happen where you get this brown allele from the dad and then the other brown allele from the mom or you could get a brown allele from the mom and a blue-eyed Anil from allele from the dad or you could get the other brown-eyed allele from the mom right these are when the mom has this she has two chromosomes homologous chromosomes each of them have the same brown allele on them they both have that same brown allele so I could get the other one from my mom and still get this blue-eyed allele from my dad so if you said what's the probability of having a blue-eyed child assuming that blue eyes are recessive and remember this is a phenotype these particular combinations are genotypes in order to have blue eyes you have to be homozygous recessive you have to have two lower case B so what's the probability of having this well there are no combinations that result in that so there's a 0% probability of having two blue-eyed children what's the probability of having a homo let me a homozygous dominant child let me write that homozygous zygous dominant and now we're looking at the genotype we care about the specific alleles that that child inherits well which of these are homozygous dominant whether you have this one right here and you have that one right there and so two of the four equally likely combinations are homozygous dominant so you have a 50% shot and we can do these punnett squares they don't even have to be for situations that where one trait is necessarily dominant on the other for example you could have the situation it's called incomplete dominance so incomplete dominance in complete dominance let's say you have two traits for color in an hour you can have you could have red flowers or you could have white flowers or you could have white flowers and let's say I were to cross a parent flower where that has the genotype capital R I'll just make it in a capital W so that could be the mom or the dad or the analogy breaks down a little bit with parents although there is a male and a female that sometimes on the same plant and let's say the other plant is also a red and a white so the other plant has a red allele and also has a white allele so what are the different possibilities well we just draw our punnett square again let me draw our little grid so the child can inherit both of these red alleles could inherit this white allele and then this red allele so this red one and then this white one right that's that right there and that red one is that right there or could inherit this red one from let's say this is the mom plant and then the white allele from the dad plant so that's that one right there or you could inherit both white alleles now what I said when I went into this and I wrote it at the top right here is we're studying a situation dealing with incomplete dominance so what does that mean well that means you might actually have mixing or blending blending of the traits when you actually look at them so if this was complete dominance if red was dominant to white did he say okay all of these guys are going to be red and all and these guy only this guy right here is going to be white so you have a 1 in 4 probability of being white but let's say that a header is a heterozygous genotype so let's let me write that down let's say if I let's say that are when you have one R allele and one white allele that this doesn't result in red this results in pink this results in pink so this is what blending is it's kind of a mixture of the two so if I said if these two people were to these two plants were to reproduce and the traits for red and white petals I guess we could say are incomplete dominant or or incompletely dominant or they blend and if I were to say what's the you know what what's the probability of having a pink plant and now when I'm talking about pink this of course is a phenotype so the probability of pink well let's look at the different combinations how many of these are pink this one is pink and this is pink so to our pink of a total of four equally likely combinations so it's a 50% chance it's a 50% chance that were pink and that we could keep doing this over multiple generations say Oh what happens in the second in the third and the fourth generation actually we could even have a situation where we have multiple different alleles and I'll use the get almost a kind of a more realistic example I'll use blood types as an example so there are three potential alleles for blood type three potential alleles you can have a blood type a you could have a lot of blood type B or you could have a blood type O you could have a blood type O and what happens is if you have a combination here between codominance and recessive genes and I'm going to show you what I talked about when we when we do the punnett squares maybe I'll stick to one color I think you're getting the idea so let's say I have a parent who is a B so that means that they have one on one of their homologous chromosomes they have the a allele and the other one they have the B allele that's what a B means so the phenotype is the genotype there codominant they both express themselves they don't necessarily blend they both express that's an a/b blood type so this is let me write this right here this is a B blood type blood type and then the other parent let's say the other parent is let's say let's say that they are they are fully an a blood type let's say they're an a blood type let's say their phenotype is an a blood type and I'm not confusing you but their genotype is that they have one allele that's an A and their other alleles that's an O so this is what's interesting about blood types it's a mixture o is recessive o is recessive while these guys are codominant so if you have either of these guys with a know these guys dominate if you have them together then your blood type is a B so what are all the different combinations for these for this this couple here for this couple right here well you could get this a and that a so you get a from your mom and you get an A from your dad right there and if clearly in this case you'll have your phenotype you will have an a blood type in this situation you could get the a from your dad and you could get the B from your mom in which case you have an A B blood type you could get the a from your mom and the O from your dad in which case you have an a blood type because this dominates that or you could get the B from your let me don't want introduce arbitrary colors you could get the B from your mom let me write it this way B from your mom that's this one or the O from your dad and there's no what's again introduced to a different color and this is a B blood type so if I said what's the probability what's the probability of having an A a blood blood type well and once again we're talking about a phenotype here so which of these are an a blood type well this one definitely is because it's a a if you have two a alleles you definitely have an a blood type but you also have an a blood type phenotype if you have an A and then an O oh is recessive so this is also going to be an a blood type so a oh these are both a blood a blood so there's a 50% chance because two of the four combinations show us an a blood type and you could do all of the different combinations and you say well how do you have an O blood type well at least both of you both of your parents will have to carry at least one oh so for example to have a to have an A you know that would have been possible if maybe instead of an a B this right here was a oh then this combination would have been a to O's right there so hopefully that gives you an idea of how a punnett square can be useful it can even be useful when when we're talking about more than one trait so let's go to our situation that I talked about before where I said you know you have little B is equal to blue eyes and we're assuming that that's recessive and you have Big B is equal to brown eyes brown eyes and we're assuming that this is dominant dominant and let's say we have another trait another trait we call it well I introduced that tooth trait and before so let's say little T is equal to small teeth small teeth I don't know what type of bizarre organism I'm talking about although I think I would fall into the big tooth camp and let's say Big T is equal to big teeth big teeth so an individual can have for example I might be I might have I might be heterozygous brown eyes so my genotype might be heterozygous for brown eyes and then homozygous dominant for teeth so this might be my genotype this and the phenotype for this one would be a big tooth brown-eyed person right big let me make that clear this is big tooth phenotype big tooth and this is the phenotype what you see is brown eyes a big tooth brown-eyed person now if we assume 2d the genes that code for teeth or eye color are on different chromosomes and this is a key assumption we can say that they assort independently let me write that down independent assortment and dependent assortment assortment I shouldn't have assortment so this is a case where if I were to look into my look at my chromosomes let's say this is one homologous pair maybe we call this homologous pair 1 and let's say I have another homologous pair and obviously we have 23 of these but let's say this is homologous pair 2 right here if I color is the eye color gene is let me pick a is here and here remember the both homologous chromosomes code for the same genes they might have different versions those are alleles and if teeth are over here so if teeth are over here they will assort independently so when when after meiosis occurs to produce the gametes the offspring might get this chromosome or copy of that chromosome for eye color and might get this a copy of this chromosome for for teeth eyes or tooth size or it could go the other way maybe it's another offspring gets this one and this chromosome for eye color and then this chromosome for teeth color and gets the other version of the allele so because they're on different chromosomes you can there there's no linkage between whether if you inherit this one whether you inherit big teeth whether you're going to inherit small brown eyes or blue eyes now if they were on the same chromosome let's say the situation where they are on the same chromosome so let's say let's say let me pick another trait hair color hair color let's say the gene for hair color is on chromosome 1 so let's say hair color the gene is there and there these might be different versions of hair color different alleles but the genes are on that same chromosome in this situation if someone gets let's say let's say this is if this is blue eyes here and this is blond hair then these are going to always travel together you're not going to have these assort independently and these are called linked traits let me highlight that so these are these right there those are linked traits but for a second and we'll talk more about link trait and especially sex-linked traits in the in probably the next video or a few videos from now but let's assume that we're talking about traits that assort independently and we cross 2 hybrids so this is called a dihybrid cross very fancy word but it's just a gives you an idea of the power of the punnett square so let's say both parents are so they're both hybrids hybrids which means that they both have they're both have the dominant brown eye allele and they have the recessive blue I&E allele and they both have the dominant bigtooth gene and they both have the recessive little toothed gene so this is the genotype for both parents both parents are die hybrids they're hybrids for both genes both parents what are all the different combinations for their children and you know I could have done this without die hybrids I could have made one of them homozygous for one of the traits and a hybrid for the other and I could have done every different combination but I'll do the die hybrid because it leads to a lot of variety and you'll often see this in in classes so if I'm talking about the mom so the mom what are the different combinations of genes that the mom can contribute well the mom could can Tribute the brown so for each of these traits she can only contribute one of the alleles so she could contribute this brown right here and then the big yellow tea so this is one combination or she could contribute the big brown and then the little yellow tea or she can contribute the blue-eyed allele and the big tea so these are all the different combinations that she could contribute and then the final combination is this allele on that allele so the blue eyes the blue eyes and the small teeth so that's for Mom and of course dad could contribute the same different combinations because dad has the same genotype let me write that down so brown eye let me just write it like this so I don't have to keep switching colors actually I want to make them a little closer together because I'm going to run out of space otherwise nope let me do it like that okay brown eye so the dad could contribute the big the big teeth or the little teeth along with the brown-eyed gene or if he could occur if you the blue-eyed gene or the blue-eyed allele and in combination with the big teeth or the yellow teeth a synapse elt the little teeth that would be a different gene for yellow teeth or maybe that's an environmental factor so these are all the different combinations that can occur for their offspring so let's draw call this with maybe a super punnett square because we're now dealing in a set of four combinations we have 16 combinations 16 combinations looks like I ran out of ink right there let me write that it's it's strange why 16 combinations let me write that out something's wrong with my tablet maybe there's something weird okay so there's 16 different combinations and let let's write them all out and I'll just stay in one maybe neutral color so I don't have to keep switching I could get this combination so this big blue big brown eyes from my mom brown eyes from my dad allele so it's brown brown and then big teeth from both I could have this combination so I have capital B in a capital B and then I have a capital T and a lowercase T and then I let's just keep moving forward so I could get a capital B and a lowercase B with a capital T and a capital T a big B lowercase B capital T lowercase T and I'm just going to go through these superfast it's going to take forever so capital B from here capital B from there capital T lowercase T from here capital B from each and then lowercase T from each you have a capital B and then a lowercase B from that one and then a capital T from the mom lowercase T from the dad I think hopefully you're not getting too tired here and so then you have a capital B from your dad and then a lowercase B from your mom two lowercase T's actually let me just pause and fill these in because I don't want to waste your time there I've saved you some time and I've filled in every combination similar to what happens on many cooking shows but now that I've filled in all the different combinations we can talk a little bit about the different phenotypes that might be expressed from this dihybrid cross for example how many of these are going to exhibit brown eyes and big teeth so big teeth brown-eyed kids big teeth big teeth let me write this down here so if I want big teeth big teeth and brown eyes brown eyes so my pen doesn't brown eyes so how many are the big teeth and brown eyes so we have to just they're both the dominant so if you have either a capital B or a capital T in any of them you're going to have big teeth and brown eyes so this is big teeth and brown eyes this big teeth right here brown eyes they're big teeth I'm going to say this is a brown eyes and big teeth because that's the order that I wrote it right here brown eyes and big teeth brown eyes and big teeth even though I have a recessive trait here the brown eyes dominate and I had a small teeth here but the big teeth dominate this is brown eyes and big teeth this is brown eyes and big teeth let's see this is brown eyes and big teeth brown eyes and big teeth and let me see if I all of them well no this is brown eyes little teeth this is brown eyes and big teeth right there and this is also brown eyes and big teeth they're heterozygotes for each trait but both brown eyes and big teeth are dominant so these are all phenotypes of brown eyes and big teeth so how many of those do we have we have 1 2 3 4 5 6 7 8 9 of those so we have 9 9 brown eyes and big teeth now how many do we have of big teeth so let me write big more let me write different colors so let me write brown eyes brown eyes and little teeth little teeth something my pen tablet doesn't work quite right over there so brown eyes and little teeth so see this is brown eyes and little teeth right there this is brown eyes and little teeth right there this is brown eyes and little teeth right there so there's three combinations of brown eyes and little teeth three and if I were to say blue eyes blue eyes blue and big teeth big teeth what are the combinations there well this is blue eyes and big teeth blue eyes and big teeth blue eyes and big teeth so there's three combinations there and then if I want to be recessive on both traits so if I want let me do this I'm going to I want blue eyes blue and little teeth little teeth there's only one after they're out of the sixteen there's only one situation where I inherit the recessive trait from both parents for both traits so if you look at this and you say hey what's the probability so there's only one of that what's the probability of having a big teeth brown-eyed child and these are all the phenotypes there are 16 different possibilities here right there's 16 squares here a nine of them describe the phenotype of big teeth and brown eyes so there's a 9/16 chance so it's 9 out of 16 chance of having a big teeth brown-eyed child what's the probability of a blue-eyed child with little teeth one in sixteen one in sixteen so hopefully in this video you've appreciated the power of the punnett square that it's a useful way to explore every different combination of all of the genes it doesn't have to be only one trade it can be in this case where you're doing two traits that Express that show dominance but they assort independently because they're on different chromosomes you could use it where did I do it over here you could use it to explore incomplete dominance when there's blending where red and white made pink genes or you could even use it when there's codominance and when you have multiple alleles where it's not just two different versions of the gene there's actually three different versions so hopefully you've enjoyed that
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