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Current time:0:00Total duration:4:54

Example punnet square for sex-linked recessive trait

Video transcript

hemophilia is an x-linked recessive trait that affects blood clotting if someone has hemophilia their blood has trouble clotting if a carrier woman and a hemophiliac man have a daughter what is the percent chance that she the daughter will have hemophilia so if you're so inspired pause this video and try to work through this on your own alright now let's work through this together and to do this let's think about the genotypes of both the mother and the father so let's start with the mother and since it is an x-linked recessive trait we're gonna be dealing with the sex chromosomes so let's just remind ourselves at the mother because she's female is going to be xx have two X chromosomes the father the father is going to have an X chromosome and a Y chromosome but now let's think about the hemophilia they say it's a carrier woman so a carrier woman means that one of her two x chromosomes would have the hemophilia allele and the other one wouldn't carrier implies that she doesn't show the trait she doesn't have hemophilia but she's carrying one of these x-linked recessive hemophilia alleles so we could say that one of her chromosomes one of her X chromosomes does not have haemophilia I use capital H for the dominant allele which you would say is no chemo philia we tend to use the capital for the dominant and then the lowercase H where this would be the X chromosome that has the hemophilia allele and I didn't lowercase H because it is a recessive trait and so this is hemo hemophilia and now what about the father well it's x-linked and the father only has one x chromosome and they say it's a hemophiliac man and so that one X chromosome is going to have the hemophilia allele and so the reason why he exhibits hemophilia is that there is no other X chromosome with the dominant allele to be dominant over the hemophilia allele but now let's cross these two with a punnett square so the father to the offspring could contribute the X chromosome that has the hemophilia allele or a y-chromosome and the mother can contribute the X chromosome that doesn't have the hemophilia allele or the X chromosome that does have the hemophilia allele and now I'll draw my little punnett square here so there you go and so what are the different scenarios well this top-left scenario would be I get the X chromosome from the mother that does not have the hemophilia allele and I get the X chromosome from the father that does have the hemophilia allele this scenario is the X chromosome from the mother with the hemophilia allele and the X chromosome from the father with the hemophilia allele this scenario is the X chromosome from the mother with the without the hemophilia allele so no hemophilia or with the non hemophilia leo and the y chromosome from the father and this is the X chromosome from the mother that has the hemophilia and the Y chromosome from the father now what is the percent chance that she that a daughter would have hemophilia so which of these scenarios show a daughter well the daughters are going to be the XX scenarios so these are the two scenarios in which they have a daughter these two because there's a y chromosomes these would be them having a son so out of the two scenarios where they have a daughter one of those scenarios you have two of the recessive alleles and so this is a situation where you have a daughter daughter with hemophilia emo philia right over here and so if we know they have a daughter what is the percent chance that she will be haemophiliac well one out of two so there is a 50% chance now what's useful about this punnett squares you could answer all sorts of questions you could say what is the percent chance that they have a hemophiliac son well one out of these four scenarios is a hemophiliac son then you would say there's a 25% chance that they have a hemophiliac son similarly there is a 25% chance that they have a non haemophiliac son if someone said what is the percent chance that they have a hemophiliac offspring well that would be these two scenarios right over here so two out of the four scenarios are haemophiliac offspring so that would be 50% if you said given that they had a son what is the percent chance that they don't have hemophilia well you would say given that they have a son so that would be those two scenarios one out of those two scenarios the son does not have hemophilia so that would again be a 50% so anyway hopefully this was useful
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