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Sex determination

Let's learn why the father determines the sex of the baby. Created by Sal Khan and Mahesh Shenoy.

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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 but certain cases of it it could be the the may be 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 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 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 agender 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 gasps 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 MOT 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 and a why and a why Chrome 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 extra 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 moms 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 dads 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 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 and Reed 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 in the case of Henry the eighth 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 I'm going to hijack this video now because I want to clarify one statement ok it's not entirely right to think that Henry might have made more of these firms and as a result he ended up with a girl as far as we know you know the number of sperms having X&Y are pretty much equal so then what decides whether this one fertilizes the egg or this one fertilizes the egg it's pure chance it's a 50/50 percent chance if this one goes ahead and fertilizes you get a girl which might have happened in his case and if this one wasn't fertilizes it's gonna be a boy but it's a purely 5050 % chance and before we wind up just to clarify this statement imagine a scenario in which a couple have five daughters in a row alright let's say they get five daughters in a row now my question is do you think for their six sixth offspring do you think that there's a higher chance they get a baby boy no well in such case I might have a tendency to think that you know in the first fight tries they got a girl so maybe in the sixth try there is a higher chance of getting a boy and maybe a lower chance of getting a girl because they already got girls for the first five times right but this is wrong way to think about it the history doesn't matter every time you know a fertilization happens it's gonna be 50/50 percent that means even for the sixth time there's a 50/50 percent chance of getting a boy or a girl and this would be true even if they had hundred daughters I know it sounds crazy but even if they had hundred daughters then for the hundred and eleven time again it's a 50/50 percent chance whether they have a girl or a boy