hi everybody and welcome back today we're going to be looking at monohybrid crosses we're going to go through how to lay one out how to work out a Punit square and then we'll work through one or two examples that you'll commonly see before we do that I just want to cover all the terminology first so that you know how it all fits into the whole um Panet square and how to work it out so a monohybrid crosses mono meaning one is that you're going to cross one trait at a time in other words you're going to um cross color or height and you're going to keep it as individual genes so when we speak about Mono we mean one cross and we have genes and we have alil now a gene codes for a trait it's a section of DNA but you can have variations of that trait and that's what we call an alal it's a variation in other words you can have the I Gene color and then a Leal of that would be brown blue and green and so they're variations of the eye color the next important thing about monohybrid crosses is that we use letters to represent our Leal and often a capital letter represents dominant and a small letter represents recessive now the lettering I've chosen here is simply as an example but if you want to express a dominant trait you would use a capital T if you want recessive trait you would need to express it as a small or lowercase T I'm going to explain exactly when and how you use these as well in the punet square and also how mandal's laws dictate how we use these letters now technically you can have combinations of these letters and they will produce the different Geno and phenotypes so when we refer to a genotype we are referring to the combination of those alals two Capital T's a big letter and a small letter t or two lowercase or small T's but those letters represent a physical um or outward appearance that we may be able to see and so we call that the phenotype and that is the physical appearance now I haven't given these letters any meaning in other words I haven't related them to any physical trait but I will use an example now as how you can actually view these correctly and what they actually mean whether it's the genotype or the phenotype the final part of our monohybrid cross and the terminology associated with it is something called homozygous and heterozygous homozygous is the way in which we write the genotype and it tells us the collection of letters if something is homozygous it can be two Capital T's or it can be two small lowercase T's now they will also tell you in the question that it may be homozygous dominant which means that's two capital letters or if it's homozygous recessive it'll be two lowercase letters heterozygous on the other hand means different so that means that hetero will have one capital letter and one small letter it's important to know that we only use these words in terminology when we are referring to the genotype we don't use these to describe the phenotype now the next important component of our monohybrid crosses is to look at mandal's laws and these are really important because they make sure we understand how to read the planet square but also Al how to construct it correctly so we do need to know these off by heart and the first one is the law of dominance and in a cross where organisms are purebred which that means that the alals they carry for example must either be two Capital T's or there will be two lower teas that means they're pure bread they have no other alals to offer in that contrasting in other words short and tall black and white only the dominant trait will appear recessive alals will always be masked by the dominant Al and so that means that you only need one capital letter T in order for it to be visible in the phenotype in other words if you ever have one capital T and one lowercase T the dominant trait which is the capital T will always be visible in the phenotype you'll be able to see it the only way to see the recessive trait is if you have two lowercase T's that then brings me now to the law of segregation now the law of segregation states that characteristics are controlled by pairs of alals which we just saw earlier every time I've drawn my alals I've written them in pairs and what happens is that during meiosis they will separate into different gametes therefore only one Al from each pair will be present in a gamt in other words if I was constructing a gamt from the following alals I had a capital t and a lowercase T when they go into their respective gametes they will go separately from one another so one gamet will have a capital T and the other gamet will end up with a lowercase T so how do we actually lay out a genetic cross and how do we do a pet Square so the first thing that we always need to do is we need to start off with the P1 generation the P1 generation refers to our um parent generation and what you're always going to do is you need to write their physical characteristics first now you would take that information from the paragraph that they would have provided with you and perhaps you will say a black mouse cross a white mouse you then need to write out their genotypes now remember the genotypes are referring to the lettering so let's stay with black and white and I'm just going to use this as an example but if I had a pure black mouse it'd be two Capital B's and a pure white mouse I would use two small letter B's and somewhere in the paragraph they would have either told you that white is recessive and black is dominant or they would tell you what the children are and you can work backwards but I'll show you how to do that in the next example once you've written out those characteristics you then need to formulate the Gam so in the column to the left hand side you would write gamet and right below the uh parental genotype you will write the word meiosis then what you will need to do is you will need to separate your gametes so you're going to place one capital b a little bit of a distance away from the next you can also use commas in between your letters I like to use circles around mine only because it makes it really easy for the marker to know that these individual alals are now separate from one another they're in actual cells they're no longer attached to one another and just make sure that nobody misunderstands what I've written after that you will going to then have to include the word fertilization and that is where you are now going to go into your Panet square and so before you start doing your planet Square in the side column you are going to write fub1 F1 refers to the filial generation in other words the outcome of the children now what we need to do is we need to insert our gamits into our punet Square now we learned also that there's the law of dominance but there's also the law of segregation meaning that when we put in our alal from our parents we need to make sure that we keep the pair of alals that came from one parent in the same column however they're separated by these lines and that's the law of segregation the other column going down is going to have the other parents Al now what you need to do is you need to substitute the values in and it's very similar to maths in that you would use like a the foil rule you would then multiply this letter B with this letter and you would then write the answer in here which would be capital B lowercase b and then you would do it again for the far letter b and you would do the same thing and you would basically multiply the letter in now that you comp to the entire um Planet square and you must do it for the second set of letters as well you now will need to write the pheno and genotype of the resultant Offspring and so what we always like to include is the genotype now the genotype is going to have to either be hom homozygous or heterozygous and you need to include the lettering with that now in this example there are only heterozygous examples so you would write um big letter B small letter be but then you also need to include the amount now you can decide you can have four out of four you could have 100% it just depends on the question and whether or not they want you to write a fraction or whether they want you to write a percentage the final piece of your planet square is the phenotype or the physical outcome now I said to you in the beginning that I did a cross on a black mouse and a white mouse now if black is dominant and and it is represented by this capital B that we wrote up here earlier and if each one of my resultant Offspring in my punet Square have at least one that then means that all the individuals in this cross will be 100% black and you will need to write that down under your phenotype now there aren't any ratios in this example for example there aren't three black to one white mouse but I'm going to show you how that also looks in the next example so let's look at this very basic example and it will just reiterate what I was explaining in the previous punet square example so let's say you get this question and this question says in a genetic diagram cross a true breeding yellow pod with a true breeding Green Pod now when they refer to true breeding what they mean is this they mean that the alals must be the same in other words there must be two capital letters or two lowercase letters but let's read the rest of information so we know what we need to do it then says that green is the dominant color now When selecting a letter to use often we use the capital letter from the dominant trait now in this example green is the dominant color and therefore we're going to use the letter G now if green is the dominant color then my true breeding Green Pod should be two Capital G's because it's true reading it's pure my yellow pod on the other hand is what would be termed the recessive trait and we now know that the recessive trait can only be represented by two smaller lowercase G's now if we cross them what would they look like in their Planet Square now of course if we follow the whole outline of the planet Square diagram there are a few things that are missing in my picture but the most important part I want to show you is the cross in the Panet Square itself and you can see here quite nicely as well with the picture as a visual and you will notice that when we take our true bread Green Pod with our true bread yellow pod there are our two recessive alals for the yellow and two dominant for the green and when you multiply it into the planet Square you will notice they all come out as being heterozygous and all these green pods are um a mixture of the two parents now this is important because what it does is it shows you that every time you mix one pure bread or true bread individual with another you will always get 100% the dominant trait in terms of the phenotype but what happens if you take two of these individuals and you cross them with each other in other words I might take this individual and this individual and I do another cross with them so now here's my second example what happens if we were to cross two heterozygous flowers with each other now I'm going to use a different example now with flower color I'm not going to use pods this time just so you can visually see the difference and so it says we're going to take two heterozygous Flowers Crossing with each other and it also tells us that purple is the dominant color now because purple is the dominant color we are going to have to make sure that purple is represented by capital letter now in this cross they've chosen to use the letter B I know that I said that you can also use the letter P um but this example they've used B and essentially what it means is that these two flowers are a big letter and a small letter so if I make it match the diagram I'm going to put up now that means that these flows will be a capital b and a small B and we're going to cross them with each other now this is what it looks like if we were to take the two purple flowers because they will be purple why because they both have a capital B in their alals and we do a cross with them we get a slightly different combination in the first cross you will notice that we have three individuals that have a capital B in them and then one that has none and what you're seeing here is the ratio that heterozygous crosses create in other words whenever you cross a heterozygous organism with another heterozygous organism you always get a ratio of three is to one in other words you will get three organisms that will have at least one capital letter like the three purple in this picture and the one refers to the white recessive individual over here now the final component of this cross is that we must make sure we write the phenotype and genotype out correctly now the phenotype I represented just now by this ratio of 3:1 and they may ask you for the r ratio in the question but if you have to just simply write out the phenotype 75% will be purple and 25% will be white but it's important that that just doesn't translate automatically into what the genotype is there is 75% purple however not all the purple flowers have the same genotypes 25% of them are homozygous dominant which represents if we look back at the diagram this individual over here then we have 50% that is heterozygous and that then represents these individuals which have a mixture of our two lowercase and uppercase letters and lastly we have a homozygous recessive individual 25% and that is this white flower so you'll notice that the Geno and phenotype percentages are not always the same and it's important not to just copy the phenotype percentages straight down so let's quick look at some terminology recap we learned about a monohybrid a monohybrid is when we cross one trait at a time we either do color only height only and within those monohybrids we have alals and genes Gene refers to the code and that means that we are looking for a code for a characteristic and the leals of variations of genes in other words you can have the eye color Gene and the variation of those eye colors makes the alals we then looked at two very important laws the law of dominance and the law of segregation and we also looked at the fact that organisms that have a dominant Al present will physically express it in other words that's when we see organisms that have capital letters in their uh genotypes and so whenever there is at least one capital letter the dominant Al um will show in the phenotype in recessive organisms on the other hand they need two lowercase letters in order to be seen and that refers also back to the law of dominance in that in order to be able to see a recessive um structure you need to have two recessive alals we then looked at homozygous and heterozygous which refers to the way in which you write the AL homo meaning two of the same hetero meaning two different letters we looked at how to um use a planet Square to work out a genetic cross and finally we spoke about Pure or true Brea reading individuals I'll remind you that a pure bread or a true breing individual is an individual that either has two capital letters or to lowercase letters and often they have contrasting characteristics like tall and short white and black thanks everybody and I hope this video helped you and I'll see you again soon bye