hi everybody and welcome back today we're going to be looking at sex linked disorders and we're going to go through exactly how you inherit these diseases how to calculate them in a punnett square and also we are going to look out how do you explain these in longer questions when you have to explain it perhaps in a paragraph so a fundamental aspect of this topic is understanding the structure of the chromosomes now in the photograph in front of you you have what we call our gonozomes or the sex chromosomes now when we speak about gronosomes we are talking about the x chromosome and the y chromosome essentially these are the 23rd pair in humans that dictate our biological sex autosomes are the 22 other chromosomes that humans have and they are not linked to our sex determination now in this photograph the very large chromosome is represented by our x so that is the x chromosome the smaller chromosome in blue is the y chromosome now depending on whether you are male or female females will have two x chromosomes and males will have an x and a y it's important to know these combinations of by heart because it's going to help you learn about these sex link disorders a lot more easily now you will notice that especially and fundamentally females have two x chromosomes which makes them a lot less susceptible to disorders because if one of their x chromosomes is damaged the other one can help it out it can fix the problem in males on the other hand you will notice that they only have one x chromosome and because an x and a y are actually not homologs in other words they're not an actual homologous pair they're not perfectly identical to match um whatever is on the y actually can't help if there's something broken on the x now in this video i'm only going to talk about x linked disorders which means that these are sex links disorders but they're only found on the x chromosome you do also get ones on the y so a very common example of a sex link disease is red green color blindness now essentially red green color blindness means that you have the inability to see certain shades of color and alongside here i have included a very simple red green color blind test if you cannot see the number in the circle there's a possibility that perhaps you do have color blindness you might see the incorrect number or a partial number if you're hoping to know what the correct number is it's a 2 that's hidden inside of there often people with red green color blindness confuse the reds the oranges and the yellows and greens with each other now this particular kind of sex-linked disease is carried on the x chromosome and it is a recessive disorder that means that we are going to use a small letter to represent it but this time we're actually going to write it as a superscript at the top of the x if you don't have the disorder then your x chromosome is going to carry a capital r on it and that means you don't have it in other words in order for us to see the variations of how this turns out let's have a look at this example along on the left so what we have here is an unaffected father and we have a carrier mother now let's talk about the unaffected father if the unaffected father doesn't have the disorder that then means that his x chromosome will have a capital r on it and he will have a y and there's nothing on the y because this is not a y linked disease it is an x chromosome disease the mother on the other hand is what we call a carrier what that means is that one of her chromosomes carries it and the other doesn't so if we were to do her genotype she would have one capital r on her x chromosome and one small case r or lowercase r that means she's carrying that disorder now what they've done lower is that they've done a genetic cross like a punnett square but with pictures this time and what we have is four possible outcomes of the offspring first of all we have an unaffected son and an unaffected daughter now the only way that that's possible for the unaffected son is that his x chromosome which he gets from his mother because you can only get wise from fathers the x chromosome he got from his mother can only be the capital r he can't inherit the small r if he did he would have the disorder and so that then means that the only possible thing that is on his x chromosome is going to be a capital r and a y our daughter who's unaffected on the other hand she has two x chromosomes and now we need to know well what are her r's does she have two big r's little one what does she have well if we look at the fact that she is not a carrier it means that she is going to inherit one allele from each parent and she's going to inherit one capital letter from her father and she's going to inherit the other capital letter from her mother making her therefore have a genotype of a capital r on the one x and a capital r on the other then we look at the carrier daughter the carrier daughter just like her mother will have the same phenotype she will have an x with a capital r on it where did she get that capital r from she received that one from her father because that's all he can give to his daughters he can't give a y chromosome the other r on the other x however was inherited by and from the mother because that's the only other place she could have got this small r which she carries and so that's where her small r comes from finally we have the affected sun the affected sun is going to be x y and what do we find on his x chromosome well he inherited his y chromosome from his father so there's nothing that we can receive from the dad that will tell us if they're colorblind what's important is where did he get that affected allele from and he received it from his mother so that means the sun will have a small letter r on his x so to round this section off just to ensure that we know exactly how this works if you are two x's you're female you must have two small r's on both of your x's to have the disorder if you are a boy you only need one small letter r on your x and that's because you only have one x allele to work with on the other hand in our dominant or capital r you can have a girl who has two capital rs you can have a boy with a capital r which means he doesn't have the disorder and the final option oh i've made a mistake let's just quickly rub that out the final option is if you have a female who is a carrier and the female i'm just going to put it below here because i'm running out of space is going to be a capital r and a small or lower case r now we need to look at how do you actually calculate the sex linked inheritance and for this one i'm going to use hemophilia which is another very common kind of sex-linked disorder that we do in school and these are three different examples with three slightly different outcomes it's been a little bit challenging also to write these h's as superscript i know that they're written here lower remember when we write them we want to write the x and then we want to put it at the top like that uh just for reference when you're writing this in school so let's have a look at the first example so our first example is a male with hemophilia now you are going to set out your genetic cross like you normally do you're going to have your geno and phenotype of the parents meiosis gametes fertilization and then you're going to draw your punnett square like you normally would you you basically draw your normal genetic diagram except the lettering is just slightly altered and with this one the question would have said a male with hemophilia so here is our male with hemophilia there is his one allele and there is his other sex chromosome and then we have a female who is a non-carrier sometimes they call them non-carriers sometimes they can call a female without hemophilia sometimes they will also say it's just a normal female essentially what that means is that this female has two capital h's in other words she is not a carrier of the disease and she does not have it either now you would do your genetic cross like you normally would and these would be our outcomes now the way in which you write your answer is really important and so that's what i'm going to show you now in terms of calculating it now what we need to do is we need to group together our genome and phenotypes and so looking at our punnett square we can first of all group together our genotypes we're going to look at the females first and these two females carry the same genotypes we group them together so that's 50 percent and then these two male genotypes are also the same and so we group those together also 50 the phenotype is a reflection of that where there are 50 females without hemophilia it's important to write that they are without hemophilia and that they are females this is a sex-linked cross so you must give the sex of the individual and then 50 males without hemophilia now let's have a look at the second example the second example will have something in the question along the lines of a male with hemophilia and so here are his alleles and they are always placed together let's not forget that and then the next important component is that this cross now has a female carrier it's important to remember that the word carrier is actually not a descriptive word that you're allowed to use when you talk about the phenotype at the very end and i'll show you exactly what i mean by that they can use it in the question to describe the female but you can't use it to describe them later on when you write out the phenotype but i'll show you what i mean and here is our female carrier we know she's a carrier because she has one capital h on her x and one lower case h on the other now we are going to multiply this into the table and let's write out our pheno and genotype all right now this genome phenotype is a little bit longer because we've produced essentially four very different individuals and so let's have a look at the genotype so we're going to group all the same genotypes together now in this particular punnett square all the genotypes are different this female represents 25 this female here represents 25 this male is 25 and this male is also 25 the phenotypes follow a very similar distribution 25 percent of females without hemophilia we then have 25 females with hemophilia it's important to note that these 25 percent of the females with hemophilia won't actually be born but because a punnett square is working out probability you still need to provide the probability of a child inheriting the disorder don't get caught out on questions that are asking about hemophiliac females just because you know that they're not actually born but they will essentially be formed and often a mother goes through a miscarriage we then have 25 males without hemophilia and then 25 males with hemophilia now let's get to the third example in this final one we look at a normal male which as we can see here he has his dominant h on the x and the y chromosome and then we have very importantly now a female carrier you will now notice that whenever i'm going to write about female carriers we don't actually call them that when we write our phenotypes but you'll see now as i do the calculations and i write out our genome and phenotypes you multiply into the punnett square and you write down your pheno and genotype results now this final genome and phenotype reveals a very important way in which we need to group our results the genotype is very simple all we need to do is group them so we have 25 percent two x's with capital dominant h's we have an individual a female individual who has a capital h of a small h 25 have a capital h a dominant allele and 25 percent of the males have a a recessive h on their x and that's how we record it what i want to bring your attention to is how you write the phenotype 50 of the females are without hemophilia you do not write 25 percent without and then 25 female carrier technically being a carrier is not a phenotype you can't actually write that down because it's not a physical characteristic that can be observed it is only a genotype i know in questions that they will refer to them as carriers and they have to because that's how they're indicating that they're heterozygous that they have a big letter and a small letter or a dominant and a recessive allele present but you cannot write them that way we then still write however the males the same and that we have 25 percent males without hemophilia and 25 males with hemophilia the last thing that is really critical that goes with any sex linked inheritance is questions like the following let's say for example three they ask you to tell them of the children what percentage has hemophilia now of the four children in example number three how many have hemophilia now of all the children that's boys and girls and of all three only 25 now that refers to all the children but what happens if the question was what percentage of the males have hemophilia now if we only look at the boys we only look at the males and there is only one of the two males that have it that then means that of the boy children 50 percent of the boy children have hemophilia it's really important to read the question carefully and to see are they asking about the children as a whole or are they asking about the boys or the girls only next we need to look at how are you going to explain this type of inheritance often they will ask you how do you explain the sex-linked inheritance you've just seen now in the hemophilia or perhaps in the red green color blindness and what you need to do in it's a very basic structure is you're always going to state the mother's genotype and that means that you need to give what her letterings are on her x's and her y's on her x's excuse me the same goes for then stating the father's genotype so you're going to state their genotypes for example let's say that our mother's genotype is a x with a capital h and a x with a lower case you need to state that and then for the father you need to do the same let's say for example he has a dominant allele on his x chromosome then what you need to do is you need to explain how this occurs through the following things you are going to then explain how you inherit one allele from each parent that's important males inherit only a y chromosome from their father and an x chromosome from their mother females on the other hand inherit an x chromosome from their mother and their father now depending how long the question is and what exactly it's asking they may also ask why are boys more affected by sex-linked disorders than girls this is where you will then include a section in your answer about the fact that males only have one x chromosome and because they only have one x chromosome they are more likely to inherit the disease why are they more likely to inherit the disease because they do not have another x chromosome to mask it in other words if we look back up at this father's genotype you'll see he only has one x chromosome with the capital h on it the dominant allele if he had a recessive allele there's no other allele that can protect him from that disorder the y chromosome is too small it doesn't have enough information on it and it can't mask anything on the x however on this mother's genotype you'll notice she has a dominant allele and she has a recessive allele which technically means she's carrying hemophilia but because she has two copies of the alleles one dominant and one recessive her dominant allele masks the recessive allele therefore she does not suffer from the disorder last but not least let's do a quick terminology recap we spoke about autosomes and gonozomes autosomes are the first 22 chromosomes and this is where you find autosomal diseases if you've come across an autosomal disease you are not going to use x's and y's you still use the standard a and small letter a or you can even use any other lettering depending on what they want you to use but essentially you only need to use your letters of the disease you do not need to use any other x's and y's gronosomes on the other hand are the sex chromosomes the x and the y and that's what we dealt with today you get recessive and dominant traits and this will then depend on whether or not the sex link disorder is a dominant or a recessive sex-linked disorder remember that for it to be recessive in females you're going to need two on each of her chromosomes so one on each in males however they only need one recessive allele because remember they only have one x in hemophilia we looked at a bleeding disorder that is found in males and carried by females no female hemophiliacs are born or exist red green color blindness is a type of sex lick disorder which both males and females experience although males are more commonly found to have red green color blindness and then we spoke about the phenotype which is the physical characteristics of the individual and what they look like remember we can't have a carrier as a phenotype and lastly the genotype essentially the alleles that we find on their sex chromosomes thank you everybody i hope this video has helped you and i will see you again soon bye