hi everybody and welcome back to miss angler's biology class i am miss angler and in today's video i'm going to be going over a past genetic exam question this particular one is a little bit tricky um it's one of the harder ones that you might see for dihybrid crosses and if you like these kinds of videos and you want to see more of them you should think about joining my membership i do loads of these kinds of videos there too exclusive explanation videos as well that only members get to watch i do live lessons and you get access to my summary notes that you can use to study from now as always if you've liked this video don't forget to like it subscribe and turn your notifications on so that you get notified when i post the freshest content every tuesday and thursday now it's at this point in the video that if you would like to attempt this question before i explain it to you pause it now otherwise i'm going to get into how to get full marks for this question so this particular question is as i said a dihybrid question and it is located in section 1 of your exam so at face value it might actually seem like quite a simple question because it's the beginning of your exam but sometimes people get caught up on this and they lose out on eight golden marks which is almost three percent of the total paper and that three percent could be the difference between you getting a 50 and a 60 or um a 17 and an 80. so let's get into the breakdown it says about 70 of people get a better taste when a substance called pct is placed on their tongue they are referred to as tasters all the other people are unable to taste pct and we call them taste blind the taster allele is and this is important dominant and taste blind is recessive so essentially if you can taste the bitter test you have a dominant allele if you can't you have too recessive it also goes on to say that also in humans normal skin pigmentation is dominant whereas the albino condition where no pigmentation is not in other words albinosm is recessive and they've given us the letters in a key used to represent the alleles now and they say that it must be used which means that you can only use these letters you can't come up with your own and if you do use your own you run the risk of getting the question wrong um this also gives us some lovely clues as to which of these particular traits are dominant and recessive even if we weren't so clear in the beginning of the paragraph now then it goes on to speak about some important information it goes on to say that a man who is heterozygous for both uh tasting ptc and skin pigmentation so let's pause there on what his two sets of alleles are if he's heterozygous it means that he has a big letter and a small letter for both of these conditions and so he's going to have this combination marries a woman who is taste blind for ptc and is albino now if she's taste blind we've established that to be taste blind you need to have two recessive alleles because it's a dominant condition and in order to have albinosum because it is a recessive condition you also need to have two lowercase letters now doing this little quick calculation at the beginning it makes uh the exam a lot smoother and easier and you run less of a risk of getting the rest of the question incorrect so the first question says states are why the example above represents a dihybrid it's only for one mark and the short answer is well it's a dihybrid because we are crossing two different genes please be careful to use the word gene and not allele allele refers to a variation of a gene moving on to 4-1-2 write down the genotype of the woman so we've already done that which is great it means we don't run the risk of getting that wrong and just make sure that you see the word genotype and you don't by accident write the phenotype then it goes on to say write all the possible gametes of the man so luckily we've already calculated the man above over here and so what you now need to do is create the gametes now as always i tell my matrix and i would say the same to all of you i like to make four little circles and inside my circles representing cells i'm going to place these particular alleles and to keep track of them i underline the allele as i've used it so what i mean is if i'm going to use a big capital t and a big capital n and i'm going to place it in one of my cells over here i'm going to underline it means i've used it once now i get to use this capital t one more time with the lower case so i've got capital t lowercase n now i get to use the smaller t and the smaller n now i've used that smaller n twice so i can't use it again and last but not least i can use the small t and the capital n and i've used them all twice now that means that i've provided all of the necessary gametes and this is a double tech question it's an all or nothing it means you must get it all right otherwise you don't get the marks the next question says the man and woman have a child whose genotype is this what is the child's phenotype so we go back to our key over here and we have a look as to what the phenotype is so this child has two small t's so they are um sorry they are taste blind i beg your pardon we're over here they taste blind and they have a capital n with a small n which means they have normal pigmentation in their skin um because this is a recessive allele so um to have albinos and so you only need to have one capital in order to have just regular pigmentation so that is our genotype converted to a phenotype and last but not least a man and a woman are only able to produce children with the genotype of big t little t big and little n the woman's genotype is all homozygous recessive state and this is a big one the only possible genotype of the man now i don't want you to fall into the trap of seeing this lady's genotype over here and going oh that's the same as the as the woman at the beginning of this question so obviously the dad must be the same that they're talking about it's not it's a completely separate couple it's not the same group of people and to work this out you could actually do a quick little calculation like a very rough punnett square to try and figure this out um or you could know your genetic ratios and your genetic crosses and if you remember back to the basics of genetic crosses if you multiply or you cross a heterozygous dominant with a heterozygous recessive sorry homozygous i occupied a homozygous dominant times a homozygous recessive all the babies will be heterozygous big letter small letter now if you apply that same logic to this next question the mom is already homozygous recessive for all of her alleles the father therefore in order to produce all of those children who are um in this case uh heterozygous if we follow the same logic the only conclusion that we can come to and we can do this very quickly so we don't have to work it out the only conclusion we can come to is that he must be two capital t's and two capital ns because it would work in exactly the same way every child would receive one capital t from dad one lower t from mom one capital m from dad one lower in from mom now here is the memo so by all means go in and have a look at what you got right um another alternative also as an answer is for the first one if you don't call it a gene you can also call it a characteristic a trait um and use this memo to help you work your way through it um it wasn't too difficult was it i think the reason why this question is seen as difficult is because it's very wordy and you can get lost in the wording but as always if you've enjoyed this video please give it a thumbs up and don't forget to go and check out my membership page and i'll see you all again soon bye