now before we get into punnett squares and how to solve problems using them let's talk about a few things that you need to know you need to be familiar with alleles different flavors of genes and there's two types of alleles typically for one trait you have the dominant allele which will be represented by a capital letter such as capital b for brown eyes or you have a recessive allele which will be the lowercase letter in this case it could represent blue eyes now the traits can vary it could be eye color it could be hair it could be if you're tall or short it can vary now you need to be familiar with these terms as well the first one homozygous dominant so what does that mean well think of the word homo homo means the same so a person with a homozygous dominant trait would have two capital letters or two capital dominant alleles in this case b and b someone who's heterozygous has a dominant and a recessive allele hetero means different homozygous recessive they have the same recessive allele in this case two lower case ladders so this information here is referred to as the genotype for that particular trait because it tells you the genes that corresponds to a certain trait now the phenotype is associated with the physical characteristics that relate to those traits now the individual with the homozygous dominant trait he's going to have brown eyes because big b is associated with brown eyes the person with a homozygous recessive trait is going to have blue eyes so this represents the phenotype of the individual the physical characteristics that you can visibly see now what about the heterozygous individual where he has both the dominant and recessive allele what physical characteristic will he display or she what will be that individual's phenotype so what would you say which one is going to win the dominant allele or the recessive allele whenever you have an individual with a heterozygous trait the dominant allele is going to win the battle so this person is going to have brown eyes let's start with this one a homozygous wolf with blue eyes mates with a heterozygous wolf with brown eyes what is the probability that they will produce a wolf with blue eyes so let's begin by drawing a monohybrid cross so this is a punnett square that only focuses on one characteristic one tray in this case eye color so what we're going to do is we're going to draw one square and then we're going to draw a vertical line and a horizontal line so now we have a total of four squares now what is the genotype of the homozygous wolf so homozygous means that he has or it has both the same genes either lowercase b or capital b but they have to be the same now we're dealing with blue eyes and brown eyes typically blue eyes is usually the recessive trait and brown eyes is usually the dominant trait so here we have a homozygous wolf with blue eyes so therefore that wolf has the genotype lowercase b lowercase b which i'm going to put here now the second wolf is a heterozygous wolf with brown eyes so based on that information what is the genotype of that wolf so it's heterozygous which means that it's going to have two different alleles one capital one lowercase and it's still going to have brown eyes since capital b is the dominant allele so now all we need to do is basically fill in the punnett square so in this square we're going to write these two letters and we're going to put the capital letter first so it's going to be capital b and then lowercase b and the same is true for this one and for this particular square it intersects these two letters so it's going to be lowercase b lowercase b and for the last one it will be the same so once we fill out the punnett square now we can answer the question so what is the probability that they will produce a wolf with blue eyes so this represents the genotype of a wolf with blue ice and these genotypes represent a wolf with brown eyes so to calculate the probability you look at how many have been chosen in this case there's two with blue eyes out of a total of four so two out of four that's one half which is fifty percent if you divide this you'll get point five in your calculator and then multiply that by a hundred that will give you fifty percent so that is the probability that they will produce a wolf with blue eyes now what about part b calculate the phenotype ratio and the genotype ratio so let's make some space let's start with the phenotype ratio so we're dealing with the colors of the eyes the physical characteristics and there's only two colors to deal with either the baby wolves have blue eyes or they have brown eyes so out of let's say four baby wolves two of them will have blue eyes and two will have brown eyes which i use the color red instead of brown for some reason and so you could divide both numbers by two and thus you get the simplified ratio so it's a one-to-one ratio so that's the the phenotype ratio blue to brown or if you want to do brown and blue it's still going to be 101. now let's talk about the genotype ratio and there's only two genotypes so it's either big b and little b which we see here or little b and little b so it's still two to two which can be reduced to a ratio of one to one so that's the genotype ratio for the first generation now let's work on another example two heterozygous cats with brown eyes made together what is the probability that they will produce a cat with brown eyes so once again we're going to say capital b is associated with brown eyes and little b is going to be associated with blue eyes so based on this information calculate the probability that the baby cat is going to have brown eyes so let's begin by drawing a punnett square now the fact that both parents are heterozygous means that their genotype is big b little b so we can write that here so in this spot that's going to be the intersection of big b and big b here we're going to have big b little b same over here and for the last one that's going to be the intersection of two lowercase b's so now we could focus on answering the question what is the probability that they will produce a cat with brown eyes so which cats will have brown eyes so this one will be brown and the same is true for these two and this particular cat will be blue so out of four cats three of them will have brown eyes so the probability will be the three cats with brown eyes out of a total of four three divided by four is point seven five if you multiply that by a hundred you're going to get seventy five percent so that is the probability that they will produce a cat with brown eyes now part b what is the probability that the baby cat will be homozygous so out of all the traits listed here which one is homozygous so homozygous means that they have the same letters in this case it's a big b and big b that's homozygous or little b with little b so for part b there's going to be two cats out of a total of four with a homozygous trait or a homozygous genotype so that's 50 percent now let's talk about calculating the phenotype and the genotype ratio in part c so let's start with the phenotype ratio so once again we only have two colors two eye colors so that's brown and blue and we have three genotypes that correspond to the color brown and only one that corresponds to the color blue so it's going to be three to one so that's the phenotypic ratio now let's calculate the genotypic ratio so first let's list the different types the different genotypes that we see so this is one big b and big b these two are the same capital b and lowercase b and this one is the last one remaining so we only have one homozygous dominant genotype and we have two heterozygous genotypes and one homozygous recessive genotype so the genotypic ratio is one two one number three consider a situation where incomplete dominance occurs in flowers so capital r is associated with red capital w is associated with white but when you have a heterozygous genotype rw you get something in between in this case the color pink so with that in mind let's answer the question for part a what is the probability that a red flower will be produced from two pink flowers so we're only dealing with one trait so we just need a punnett square a two by two punnett square with four squares inside so each pink flower will have the genotype rw now let's go ahead and fill the table so this is going to be rr rw rw and ww so what is the probability that a red flower will be produced so this is gonna be a red flower here we're going to get a white flower and let me look for a pinkish color so these two will be they will represent the pink flower so we have a total of two we're looking for red flowers so we only have a total of one red flower out of four so one divided by four is 0.25 times 100 so that gives us a probability of 25 percent so there's a 25 chance that a red flower will be produced from the two pink flowers if we wish to calculate the probability that a pink flower will be produced it's going to be two out of four and so that's going to be 50 percent and the probability of getting a white flower it's just one out of four which is 25 now let's move on to part b what is the probability that a pink flower will be produced from a red and pink flower so we no longer have two pink flowers for the parents we have one red one pink so let's write the genotype so the genotype for the red flower will be rr and for the pink flower it's going to be rw so rw for pink rr for red so let's go ahead and fill out the square so this is going to be r and then we're going to have rw so these two flowers will have a red color and these two will have a pink color so the probability that a pink flower will be produced there's two pink flowers out of a total of four and so that's going to give us a 50 percent probability so that's it for part b number four a beer with black fur and blue eyes mates with another bear that has white fur and brown eyes what is the probability that the baby bear will have black fur and brown eyes so in the last three example problems we only dealt with one particular characteristic so we use a monohybrid cross but in this problem we're dealing with two traits the color of the fur and the color of the eyes so we need to use a dihybrid cross so this problem will involve more work so let's begin by drawing a punnett square hopefully this is big enough and this one is going to have four columns and four rows so four times 4 is 16 and so we're going to have a total of 16 squares now the genotype for the first parent is going to be big f little f little b little b and the genotype for the second parent is going to be little f little f big b little b now what should we place here and here here's what you shouldn't do you shouldn't just write one letter in each column this is not the right way to do it because you're dealing with two characteristics so you need to place two letters in each column so that in each box you're going to get four letters so how do we know which two letters go where so hopefully you've taken algebra and you remember how to foil and that's what we're gonna do so we're gonna take one letter from the first box and pair it up with a second letter from the second box that represents the second characteristic so we're going to pair up capital f with lowercase b so that's going to give us big f little b and then we're going to pair up big f and little b again so if you remember the word foil it's first that's for the blue line and for the red line outer so that's going to be big f little f and then i for inner little f little b and then the last little f the little b again now let's do the same thing for the second genotype or the genotype for the second parent so it's going to be little f and big b and then we have little f little b and then it's going to be little f big b and little f little b so now let's fill in the punnett square so we have big f little f and then big b little b so this is going to take a while what i recommend doing is pause the video and fill it out yourself and then you can play the video again and see if you have what i have as well so i'm just going to take a minute and fill everything out so i'm going to try to double check my work as i do this so i don't make any mistakes almost done you could fast forward this if you want to so now at this point we can answer part a so what is the probability that the baby bear will have black fur and brown eyes so i'm going to use i can't use black because the background is black i'm just gonna use like blue for black so black fur is capital f all we need is just one capital f so let's identify every genotype that will show up with the black fur physical characteristic so all we need is just one capital f and the bear will have black fur now let's identify all the bears that will have brown eyes so all we need to identify is at least one capital b and that's it so now we need to identify all the bears that have both black fur and brown eyes so this is one two three and four so we have a total of four well this 4 that's been selected out of a total of 16 possibilities because there's a total of 16 squares so 4 out of 16 if you divide both numbers by 4 you can reduce the fraction to 1 over 4 which is 0.25 or 25 so this is the probability that the baby bear will have black fur and brown eyes so now let's move on to part b what is the probability that the baby bear will have white fur and blue eyes go ahead and try that so let's identify all the bears with white fur so that means that they need to have the genotype lowercase f and lower case f so this bear has white fur this one too basically all the ones on the second half of the punnett square they will all have white fur because they're homozygous recessive now let's identify all the bears with blue eyes so they should have the genotype little b little b since uh blue eyes is gonna be the recessive trait or the recessive allele so let's use the color blue so here's one here's another one that's another one and that's it so the bears that have both white fur and blue eyes is one two three four so once again it's four out of 16 which is one fourth and so the probability is 25 now let's move on to part c so what is the probability that the baby will be homozygous dominant for at least one trait so let's think about what that means homozygous dominant so that means that they have to have the same alleles either uh big f big f little f little f big b big b little b little b now so those are the homozygous genotypes now we want the dominant ones not the recessive ones so we can eliminate these two so we either want big f and big f or big b and big b so let's see if we can identify any of those and i don't see it at all so the probability is zero percent because there's none with a homozygous dominant trait now part d what is the probability that it's going to be heterozygous for both traits so heterozygous means that we're going to have one big letter one little letter so for the first trait it's going to be big f little f for the second trait big b little b so here is one here is the second one that doesn't count not those here's another one and not those as well so once again it's 4 out of a total of 16 which seems to be 25 25 percent is the number of the day we're getting this answer a lot so that's the probability that the baby bear will be heterozygous for both traits now let's move on to part e so we need to calculate the genotype and the phenotype ratio so what i need to do is make some space so let's start with the genotype ratio so this is the first one big f little f big b little b and so let's identify each one with those characteristics so it's just four now the next one is little f a little f big b little b and i see four of those and then the next one is going to be big f little f little b little b and so there's also four of those and finally little f little f with little b little b so four four four four if we divide everything by four then the genotypic ratio is one to one to one to one now the phenotypic ratio the characteristics it turns out it's going to be the same now the genotype big f little f big b little b it corresponds to a bear with black fur and blue eyes actually not blue eyes but brown eyes because of a big b now the second genotype little f little f that's for white fur big b little b that's for brown eyes the third genotype big f little f that's for black fur little b little b that's for blue eyes and the last one little f little f that's for white fur little b little b blue eyes so that's there's going to be four bears that have black fur and brown eyes four that have white fur and brown eyes four have black fur and blue eyes and the other four has white fur and blue eyes so it all depends on the genotypes of the parents and so that's gonna affect the genotypic and the phenotypic ratio of the first generation of baby bears so this will also be reduced to one one one one and that's basically it for this video so now you know how to fill out punnett squares and how to use them to solve problems thanks again for watching and don't forget to subscribe to this channel so for those of you who need help in algebra physics chemistry and things like that i do have other videos on those topics thanks again for watching