 Humans have a very neat way of determining sex, but you're already aware of that. The entire system is driven by two sex chromosomes, X and Y, both of which are loaded with sex genes. All the females have two copies of the X chromosome and all the males have an X and a Y chromosome. Now the females, aka the moms, can only give out an X chromosome to her kids. So all of her gametes will have just this X chromosome, making all of them identical to each other. That's why moms are called homo-gameric, meaning that she always has the same gametes. Meanwhile on the other hand, dads have an X and a Y chromosome. So half of his gametes will have the X chromosome and the remaining half will have the Y chromosome. Because of the fact that he has two different gametes with him, we call him heterogamedic. Now because of this heterogamedic factor, dads can either give out an X or a Y chromosome to his kids. If he gives out an X, he will have a baby girl. If he gives out a Y, he will have a baby boy. Now let's flip this entire system. If we flip it completely, we will have an opposite version of this XY system. So instead of having females with two copies of the X chromosome, we're now going to have males with two copies of the X chromosome and females with an X and a Y chromosome. Now you must be thinking that this is a little absurd, something like this might not even exist. But let me tell you, it definitely exists and it exists in every single bird that you can see around you. Birds have an opposite version of the XY system as their sex germination system. Let's take a closer look at this. Now before we move on and figure out how exactly the sex is getting determined in birds, let me ask you a question here. Don't you find this a little weird? I mean a few seconds ago I just told you that XX are always females and now I'm telling you that these are males? And vice versa, all of a sudden I was so convinced, we were so convinced that XY are males. But now they are females? What is going on? Now we would have still tried to make some sense out of this. If the human sex chromosomes were exactly like the bird sex chromosomes. But that is not even the case because the human sex chromosomes are nothing like the bird sex chromosomes. I mean they don't even share a single gene. Then why are we calling these sex chromosomes X and Y when they are nothing like X and Y? Now scientists were equally as confused as we are right now. So they decided to give new letters or assign new letters to these chromosomes that we can find only in birds. The sex chromosomes of course. So instead of calling them X and Y, they decided to call them Z and W. So instead of having XX males we now have ZZ males and instead of XY females we now have ZW females. Now that we have the whole letter and the name part sorted out, let me present this to you in a much clearer format. There we go. So now we have ZZ males and ZW females. Since birds have it the exact opposite of us, we now have homogematic males or males with the same gametes and heterogametic females or females with different gametes. Now the male birds will always give out a Z to all of his kids. Meanwhile the females will give out either a Z or a W. Now based on this entire information I'm pretty sure you can come to one major conclusion. Can you guess what that is? That's right. This time moms are the one who gets to decide, whoops. So moms are the one who gets to decide the baby sex instead of the fact that dads used to do that for us. If the mom gives out a Z then we're gonna have a baby boy bird and if she gives out a W then we're gonna have a baby girl bird. Now let me give you something to think about here. You already know that the Y chromosome has the main sex gene which kind of goes ahead and activates the male parts in a baby and that's why the baby ends up becoming a boy. So with respect to that or with regards to that, what do you think happens in a bird? Given that they have the exact opposite system to ours. What do you think? Can you guess? Can you pause your screen here for a second and guess exactly what might happen? My first guess or my only guess when I started learning about this was that you know what? It's the exact opposite. So definitely W will have some kind of main sex gene that will go ahead and activate the female parts of the bird. I mean that makes complete sense right? And that's why it ends up becoming a female because why gets to decide everything here? I'm pretty sure W gets to do that too in birds. But turns out that I was wrong and if that is the guess that you had then you're wrong as well. It turns out that W doesn't have the main sex gene. In fact, W doesn't have a lot of sex genes to begin with. It only has maybe a couple. It barely has any sex genes that we can connect the entire sex determination system to. So if W doesn't have the sex genes, who does? I mean there's only one candidate left at this point so we're just going to go with a safe bet that it's them. Z is the larger of the two chromosomes and contains most of the sex linked genes including the main sex gene which kind of determines the entire process. Now you're probably thinking at this point that how is that even possible? Because both the male and the female birds have Z chromosomes in them. Then how exactly is the sex getting determined? Take a closer look. Tell me the one thing that jumps out at you from this particular image. Correct again. It's the number of the Z chromosomes. The number of the Z chromosomes differs in both the birds. The males have two of them and the female has only one. Accompanied by a W chromosome. So you see the sex determination pattern in birds does not really depend on one chromosome with the main sex gene like it does in us. In fact this depends on the amount of sex gene that a bird gets. So this pink colored thing that you see over here that's the main sex gene. So if the bird has two copies of the Z chromosome it will definitely have a higher amount of the main sex gene compared to the females which have only one copy. So if a bird has a higher amount of sex gene or it has two copies of the Z chromosome then it's definitely going to be a male bird. On the other hand if it has a lesser amount of the main sex gene or just one copy of the Z chromosome then it's definitely going to be a female. So now you must be thinking that if it's the number of the Z chromosomes why do we need this W at all? Is W absolutely jobless doing nothing just existing? The answer to that question is we don't really know. We do have some theories which suggest that W kind of supports the Z chromosome in making the bird more female by contributing to a few traits or very specific traits but other than that we don't really have much of an answer. In fact this whole thing about the number of Z chromosomes determining the sex of the bird is also actually a theory but it's the best one that we have so far and it makes the most sense. But this is what really baffles me. Let's assume that W is jobless, that it doesn't really have anything to contribute to this entire sex determination system. In that case we should have found a bird that has only one Z chromosome because that's all it needs for it to become a female bird. So why can't we just have a bird with one Z chromosome and nothing else? So a bird with genotype Z0. But so far we have not found a bird like this anywhere. So these birds, these Z0 birds, they don't exist at least not that we know of but wait, wait, wait, wait. If you think that this is where the weird ends, think again because even though we might not find Z0 birds we do have Z0 butterflies. Wait, my butterfly has flown away, give me a second. There you go. We do have Z0 butterflies even though we don't have Z0 birds and yes, butterflies also follow the same system as the birds where they have a Z and a W chromosome and that's why the world of biology never ceases to amaze me and the world of animal sex is just downright weird but fascinating because if you think that all of this was complicated wait till you find out about the temperature-controlled turtle babies and the platypus which actually has 10 Z chromosomes. Yeah, good luck with that science.