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Video transcript

by this point in the biology playlist you're probably wondering a very natural question how is gender determined in an organism and it's not an obvious answer because throughout the animal kingdom it's actually determined in different ways in some creatures especially some types of reptiles reptiles its environmental environmental not all reptiles with certain cases of it it could be the D maybe the the temperature in which the the embryo develops will dictate whether it turns into a male or a female or other environmental factors and in other types of animals especially mammals of which we are a one example its genetic basis it's a genetic basis and so your next question is a Sal so let me write this down in mammals in mammals its genetic its genetic so like hey maybe there's a maybe there are different alleles a male or a female allele then you're like hey but you know there's so many different characteristics that make that that differentiate a man from a woman maybe it would have to be a whole set of genes that have to work together and to some degree your second answer would be more correct so let me just draw a a it's not even more than just a set of genes it's actually whole chromosomes determine it so let me draw a nucleus that's gonna be my nucleus and this is gonna be a nucleus for a man so 22 of the pairs of chromosomes are just regular ngon sex-determining chromosomes so I could just do you know that's one of the homologous 2 4 6 8 10 12 14 I could just keep going and you have and you eventually you have 22 pairs so these 22 pairs right there they're called autosomal and those are just our standard pairs of chromosomes that code for different things each of these right here is a homologous pair homologous which we learned before you get one from each of your parents they don't necessarily code for the same thing for the same versions of the jeans but they code for the same genes if eye color is on this gene it's also on that gene and the other gene the homologous pair that you might have different versions of eye color on either one that determines what you display but these are just kind of the the standard genes that have nothing to do with our gender and then you have these two other special special chromosomes I'll do this one it'll be a long brown one and then I'll do a short blue one and the first thing you'll notice is that there they don't look homologous how could they code for the same thing when the blue one short and the brown ones long and that's true they aren't Amala guess and these will call our sex-determining chromosomes sex-determining chromosomes chromosomes and the long one right here it's been the convention is to call that the X chromosome let me scroll down a little bit and the blue one right there for that as the Y chromosome and to figure out whether something is a male or a female it's a pretty simple system if you got a Y chromosome you are a male so let me write that down so this what this nucleus that I drew just here obviously there you can have the whole broader cell all around here this is the nucleus for a man so if you have an X chromosome and I will talk about in a second why you can only get that from your mom an X chromosome from your mom and a Y chromosome from your dad this you will be a male if you get an X chromosome from your mom and an X chromosome from your dad you're going to be a female and so we could actually even draw a Punnett square there's almost a trivially easy punnett square but it kind of shows what all of the different possibilities are so let's say these are your month this is your mom's genotype for her sex-determining chromosome she's got two x's that's what makes her your mom and not your dad and then your dad has an X and a why and a Wycombe a seadoo a capital and has a y-chromosome we could do a Punnett square what are all the different combinations of offspring well your mom could give this X chromosome and in conjunction with this X chromosome from your dad this would produce a female your mom could give this other X chromosome with that X chromosome that would be a female as well or well your mom is always going to be donating an X chromosome and then your what your dad is going to deter donate either the X or the Y so in this case it'll be the Y chromosome so these would be female and those would be male and it works out nicely that half our female and half our male but a very interesting and somewhat ironic fact might pop out of you when you see this what determines whether someone is or who determines whether someone is mate whether their offspring are male or female is it the mom or the dad well the mom always donates an X chromosome so in no way does a mom what what the the the haploid genetic makeup of the mom's egg of the gamete from the female in no way does that determine the gender of the offspring it's all determined by whether you know let's say these are all of let me just draw a bunch of you know dad's got a lot of sperm and they're all racing towards the egg and some of them have an X chromosome in them and some of them have a Y chromosome in them and obviously they have others and obviously the one if if this guy appear wins the race or maybe I should say this girl if she wins the race then the fertilized egg will develop into a female if this firm wins the race then the fertilized egg will develop into a male so the iron and the reason why I said it's ironic is throughout history and probably the most famous example of this is is Henry the eighth Henry the eighth you have these I'm it's not just the case with Kings it's probably true because most of our civilization is male-dominated that you've had these men who are obsessed with producing a male heir to kind of take over the family name and you know in the case of Henry the take over a country and they become very disappointed and they tend to blame their wives when the wives keep producing females but it's all their fault Henry the eighth I mean the most famous case was with Anne Boleyn hee hee you know she's some you know I you know I'm not an expert here but the the the general notion is that look she he became upset with her that she wasn't producing a male heir and then he found a reason to get her essentially decapitated even though even though it was all his fault his fault his the he was maybe producing a lot more sperm that looked like that then was looking like this he eventually does produce a male heir so he was and if we assume that it was his child then then obviously he was producing some of these but for the most part it was all Henry the eighth's fault so that's why I say there's a little bit of irony here is that the people doing the blame are the people to blame for the lack of a male heir now one question that might immediately pop up in your head is Sal is everything on these chromosomes related to just our you know maybe sex-determining traits or are there other stuff on them and so let me draw some chromosomes so let's say that's an X chromosome and this is a Y chromosome now the X chromosome it does code for a lot more things although it is kind of famously gene poor it has it codes for on the order of 1,500 genes and the Y chromosome is pretty I mean it's it's the most gene poor of all the chromosomes it only quote codes from the order of 78 genes and I just looked this up but you know who knows it's exactly 78 but it tells you is it does very little other than determining what the gender is and the way it determines that it does have one gene on it called the sry gene you don't have to you don't have to know that sry that plays a role in the development development of testes or the main the main the the the male sexual organs so if you have this around this gene right here can start coding for things that will eventually lead to the development of the testicles and if you don't have that around that won't happen so you'll end up with a female and I'm I'm making gross oversimplification here but the other everything I've dealt with so far is this is okay this is clearly plays a role in determining sex but you do have other traits on these genes for example and the famous cases all deal with specific disorders so for example color blindness the genes or the mutations I could I should say so the mutations the mutations that cause color blindness called a red-green color blindness blindness which I did in green which is maybe a little bit maybe inappropriate color blindness and also hemophilia hemophilia this is an inability of your blood to clot actually there's several types of hemophilia but hemophilia is an inability for your blood to clot properly and both of these are mutations on the X chromosome mutations on the X chromosome and there are recessive mutations there assess 'iv so what does that mean it means both of your X chromosomes have to have let's take the case for hemophilia both of your X chromosomes have to have the hemophilia mutation in order for you to show the phenotype of having hemophilia so for example if there's a woman and let's say this is her genotype she has one regular x chromosome and then she has one x chromosome that has the I'll put a little superscript there for hemophilia she has the hemophilia A mutation she's just going to be a carrier her phenotype right here her phenotype is going to be no no hemophilia no hemophilia she'll have no problem clotting her blood the only way that a woman could be a hemophiliac as if she gets two versions of this because this is a recessive mutation two versions now this this individual will be will have hemophilia now men they only have one X chromosome they only have one X chromosome so for a man to exhibit hemophilia to have this phenotype he just needs it only on the one X chromosome he has so and then the other ones the Y chromosome so a man will have so this man will have hemophilia so a natural question should be arising is hey you know this guy look let's just say that this is a relatively infrequent mutation that arises on X chromosome the question is who's more likely to have hemophilia a male or female all else equal who's more likely to have it well if this is a relatively infrequent allele a female in order to display it has to get two versions of it so if the frequency of it let's say that the frequency of it and I'll you know I looked it up before this video roughly they say between one and five to ten thousand men exhibit hemophilia so let's say that the allele frequency of this is one in 7,000 is is the frequency of X H right the hemophilia version of the X chromosome and that's why one in one in seven thousand men display it because it's completely determined whether you know there's a one in seven thousand chance that this X chromosome they get is the hemophilia version who cares what the Y chromosome they get is because that essentially doesn't code at all for the the blood clotting factors and all of the things that drive hemophilia now for a woman to get hemophilia what has to happen she has to have two x chromosomes with the mutation well the probability of of each of them having the mutation is one in 7,000 so the probability of her having hemophilia is is one in 7,000 times 1 in 7,000 or that's one in what 49 million so as you could imagine the the incidence of hemophilia in women ISM is much lower than the incidence of hemophilia in men and in general for any selects link straight if it's recessive it's a if it's a recessive sex-linked trait which means you either men if they have it they're gonna show it because they don't have another X chromosome to dominate it or for women to show it she's happy to have both versions of it the incidents in men is going to be is going to be I guess we could say it's good so let's say the let's say that P is or let me write em is the incidence in men in incidence I'm spelling badly incidents in men then the incidence in women will be what you could view this as the allele frequency of that mutation on the X chromosome so women have to get two versions of it so the woman's frequency is M Squared and you might say hey that looks like a bigger number I'm squaring it but you have to remember that these numbers the frequency is less than one so in the case of hemophilia that was one in 7,000 so if you square one in 7,000 you get 1 in 49 million anyway hopefully you found that interesting and now you know what how we all become a men and women and and even better you know whom to blame when when when some of these I guess male focused parents are having trouble getting their son