hi everybody and welcome back today we're going to be looking at a dihybrid cross up until this point you've been working with monohybrid crosses mono being one trait at a time this time we are going to do a dihybrid effectively that means we're going to do two traits at the same time now that means for example if we look at my two cats in this picture we are going to do two traits in one genetic cross and so in this example we've got two genes or two traits at the same time and the example being fur color and eye color now there is a law that governs dihybrid crosses and that is the law from mendel which is the independent assortment law this law states that our genetic traits are inherited independently from each other what that basically means is that your eye color isn't linked to your skin color or in these cats not all dark brown cats will have green eyes and not all cream-colored cats will have blue it means that you inherit these traits individually separately your eye color does not determine your blood group you inherit those things separately or independently from one another now when we do dihybrid crosses it's really important to know what all of your lettering means and dihybrid crosses in its simplest form can follow complete dominance and we are only going to do complete dominant crosses in this video and so for that we need a set of four alleles the alleles that we are going to use are going to represent our dominant and recessive traits so the four alleles are going to be a capital f for dark brown fur and a small f for the cream-colored foe then for green eyes we're going to have a capital e and a lowercase e for blue eyes it's important to have your key with your letters ready if you haven't already been provided with one if you haven't you need to make up your own lettering key and i always suggest that you use letters that has a marked difference between the capital letter and the lowercase letter because sometimes some letters look very similar whether you are writing them in capital or in lower case now the hardest part of dihybrid crosses and getting them right is creating the gametes and we often get overwhelmed because there are so many alleles that you have to keep track of but i can show you a really easy way how to do this normally in a monohybrid cross you would get four gametes but in a dihybrid you actually need eight gametes now that means that if you haven't produced eight gametes at the end of the stage it means that you haven't done something correctly and you need to go back and see and check your work now there are two laws that govern what's happening here in gamete formation the first one i've already mentioned is independent assortment in other words that means that you are going to inherit your fur color and your eye color as a cat separately from one another in other words there's no influence on one another the second thing is a law we've already learned to one of the previous videos which is the law of segregation which means that you are only allowed to inherit one of each type of allele or gene in other words none of your gametes should have two letter f's in them or two letter e's in them and i'm going to show you how you're going to make your gametes so what i do in a test or an exam is i underline the letters that i've already used so i know i haven't used them more than twice and that's the maximum amount you're allowed to use why twice because essentially each of these alleles is going to be used in gamete formation twice and so what that looks like is this what i'll do is i will underline the capital f and the capital e and then i form my first gamete you'll notice i can only have one f and one e i'll then do this again i'll do it with the same letter f and another letter e and i form my next gamete and i will repeat the process now you'll notice on the one letter f i've underlined it twice which means i've used it twice i'm not allowed to use it again however i still have three other letters i haven't finished with yet so i take my second letter f i underline that f and i underline one of the e's i then create my next gamete and i do this again for the last gamete with the final f and the final e and i produce my fourth and final gamete from this individual and that's the first four i'm going to do the second four for the other cat now that i have my eight gametes and i've made sure that i've only used each allele twice no more than that i'm going to need to put these gametes into a punnett square like we normally would keeping in mind that in the punnett square you always need to keep the individual you're taking it from in the same column so that means that if this is the side of our punnett square and these are all the rows that are going alongside it it means that i'm going to put all the dark brown cats alleles in this row and i'm going to put all the light brown cats in this row i need to keep them separate from each other so that i know that they come from separate individuals now when working with these two cats they have what we call true breeding or pure breeding characteristics because our dark brown cat has a homozygous dominance for both color and eye and fur and our lighter colored cat our cream-colored cat has homozygous recessive for both of its eye color and its fur color and we call this true breeding or pure breeding and we cross them in a dihybrid and there's always only one answer that can come out in this and i'm going to show you what that looks like in the next slide now in this example i'm not using cats but it works in exactly the same way and i wanted to expose you to another example besides cats and a common one is actually seeds and so whenever you see an example or a question that says you're going to cross purebred or true bred parents it basically means that those parents have either all dominant characteristics or all recessive characteristics so in our cat example we had a dominant cat with its dominant fur color and eye color and the other cat had all the recessive characteristics in seeds it's the same thing in this one i have round and yellow seeds and these are the dominant characteristics i know this because i have a capital r and a capital y and we know that only dominant alleles have this and then wrinkled and green is our recessive traits and it is a lowercase r and a lowercase y i'm just going to quickly show you what the parents would have looked like in terms of their alleles just so you see where we got these gametes from so i've just wrote in for you so that you can see them here this is what the original parent would have been in terms of its genotype for both the dominant and the recessive and you would make your gametes like normal now what they've done in this punnett square it looks like a monohybrid but it's actually a dihybrid they've just squished it into four blocks and the reason for that is you're not going to get any other outcome but this heterozygous product a big r a little r a big y and a little y and they're all going to come out round and yellow so for simplicity's sake i want you to know that you could do this and you could draw all 16 outcomes but all 16 will be exactly the same so there was no need to and this is a shorter quicker way because they're purebred there is only one outcome and that is they're all going to be round and yellow there is no ratio for this one there is no combinations it is simply all round or yellow [Music] so if you were to write this as a genotype or phenotype essentially they're all round and yellow they're 100 or 16 out of the 16 possible options and this is one of the easiest ones to work out because all the answers are the same but what happens if we were to take one of these other individuals if we were to perhaps take one of our heterozygous individuals here and we crossed it with another heterozygous well this then creates a beautiful ratio that we need to know off by heart and that is in the next slide so what do we do if we are going to cross two heterozygous individuals now these two heterozygous individuals came from the previous cross that we've just done where we produced offspring seeds that were all round and they were all yellow and it's important to know what their genotypes are and before we fill out the punnett square but make sure that we also don't make a mistake when we produce the gametes so here are the alleles for these two individuals we have them as heterozygous because they're heterozygous for their round trait and they are heterozygous for their yellow then if we were to plug all of these answers into a cross it would produce the following outcome so most importantly let's make the gametes because this is often where we go wrong it's important to know that in this kind of cross because they're both heterozygous they will produce the same alleles and so you can do it for the one and you can be sure that the other one has exactly the same set so use my method of underlining the ones that you use as you go along so i'm going to use the one capital r with the one capital y this then produces our first gamete i do it again i underline the capital r that i use this time i'm going to use the smaller case y and that will produce my next gamete and i do it for all four of the alleles and they will produce the following gametes now you will notice that i have used each of the alleles over here only twice each because i've underlined them and i've produced these four gametes now remember in a dihybrid you need to produce eight which means you need to do the same for this individual now because these two individuals are actually identical to one another they are going to produce the exact same set of alleles and so it's safe to say that you can actually just copy them if they are not identical however please don't do this because it will affect your overall answer now let's take our gametes put them into the pilot square and look at the results so once we take our two individuals we put their gametes into our punnett square we can now foil or essentially that means multiply into the bracket each time we can figure out all of our crosses and so for example we cross this individual with this one and it produces this outcome we cross this individual with this one and it produces this outcome and we do that for each individual so that's why we call it foil it's sort of like maths where you multiply into the brackets or in this case into the punnett square each time and so that means every time you multiply you should have four separate outcomes and then once you're done with that row you can move on to the next but now what happens is you end up with these 16 outcomes now when you do this in class or a test you're not going to have time to draw the round and yellow seeds and some of them are wrinkled and some of them are not wrinkled you're going to need to only be able to work with these letterings and so what i suggest you do in order to identify all the similar ones is you're going to need to use a highlighter or a colorful pencil or something so that you can highlight all the ones that are the same now it's important to note that what i'm going to teach you now is the phenotypic ratio in other words it's the physical ratio that is made and it's actually a golden ratio that you have to know off by heart and it works like this so i'm going to take my highlighter and i'm going to highlight all the individuals that have produced the exact same phenotype or physical and that is going to be first of all round and yellow now in this picture it's nice and easy because they've already made all the round and yellow in a picture format for me so it's really easy but if you were doing this on your own essentially you are looking for any individual that has at least one capital r and one capital y and that will give you a round and yellow individual now in total there should be nine of these and that's very important it's going to make our ratio then we go into looking for the next set of individuals that are the same and in this instance they are green and round so we have one two three that are green and round how did i know that because i'm looking for individuals that have at least one capital r but have two lowercase y's so that gives me three that follow that pattern then i look for anybody else who can be grouped together and i see here that i have one two three wrinkled and yellow and i know they're wrinkled and yellow because they must have a minimum of two small r's to be wrinkled and at least one capital y to be yellow so i've got three of those individuals now last but not least i have one individual that hasn't been counted and it's this one over here and that little one on their own is the only individual that has a recessive round trait so they're going to be wrinkly and they have a recessive color trait which makes them green so we've got one and this is our golden ratio for dihybrid phenotypes when you are crossing two heterozygous individuals and the ratio is nine is to three is to three is to one and it will always be like this when you are doing the phenotype it's important for me to acknowledge that this is the phenotype ratio this is not the genotype phenotype remember is the physical you know the color the roundness or the wrinkliness the genotype on the other hand is not going to create this ratio the genotype is when you are writing the alleles the r's and the y's and that's going to also require you to count how many have the same letters how many have two big r's and two big y's or maybe how many have a big letter and a small letter for r and a big letter and a small letter for y the easiest way to keep track of them is to circle or cross or star a little key or something that allows you to group them all together so that you can write their percentages at the end so let's finish this video off looking at some terminology and recapping what we've done so we've done a dihybrid cross a dihybrid is when you are crossing two different contrasting traits perhaps you are crossing fur color with eye color you're crossing blood group with a eye color two very different things and a law that governs dihybrid crosses is independent assortment which essentially means that as i've used before your blood group is not linked to your eye color you inherit them independently of one another and so when you make your gametes you need to make sure that you have one of each of those traits which is a part of our segregation law and you need to ensure that you understand how independent assortment doesn't affect how the animal is produced in other words organisms that have green eyes don't necessarily always produce more organisms with green eyes then we looked at genes and alleles and remember that the gene is the trait it is the overall product but you have variations of the gene and that is an allele so for example the gene is blood grouping the allele is the type of blood group you have so you have a alleles b alleles o perhaps in our cat example you have our green eye allele or your blue eye allele and then we looked at something called pure breeding or true breeding this is when you take an individual who is purebred in other words they don't have any other alleles that could perhaps pop up they're not heterozygous they're homozygous that means that you have two capital letters for one trait and two capital letters for the other or if you're homozygous recessive you are going to have two lower letters for your eye color and two lower layers for your fur color your pure bread you don't have any other mixtures inside of you then we looked at a phenotypic ratio and there's one very important phenotypic ratio in dihybrids and that is the nine is to three is to three is to one that ratio will always be produced when you take two heterozygous individuals in other words they are mixed for their traits we used round yellow seeds that were heterozygous we cross them and we produce this nine is to three is to three is to one and it will work for any dihybrid cross when you cross a heterozygous individual i hope that you've enjoyed this video and i will see you again soon bye