 In humans, sex is determined by a specific pair of chromosomes which we call sex chromosomes. You might remember that humans have 23 pairs of chromosomes. One of those pairs is what we call sex chromosomes and that determines the sex. How? Well, it turns out that if you look at the females, then both the sex chromosomes are identical. We call them as X and therefore we say females have an XX sex chromosomes. Whereas when it comes to males, it turns out that they have one X chromosome but the second chromosome is slightly shorter. We call this the Y chromosomes and therefore we say males have XY sex chromosomes. So when a sperm fertilizes an egg, if the zygote has XX, it becomes a female. If the zygote has XY, it becomes a male. Now let's see what are the chances of an offspring being a male and being a female. So if you look at the male, since they have two chromosomes, the sperm cells can either take a Y chromosome or an X chromosome. Remember, sex cells are always haploid which means they always carry one copy, not both the copies. Therefore some sperms will carry this copy versus some other sperms will carry this particular copy. What about female eggs? Well, in females, both the chromosomes are X, therefore the X can only have one kind of copy, X chromosomes. And by the way, just to be clear, I don't mean that males will have only two sperms and females will have only one egg, no, no, no. We have lots of sperms and eggs. What I mean over here is that we'll have a lot of sperms but about half of them will be X and about half of them will be Y. That's what we mean over here. Similarly, we'll have lots of eggs but all the eggs will only have X chromosomes. That's why I have drawn only one egg over here. And so now if you look at the fertilization, we will see that if this sperm fertilizes this particular egg, we'll get the offspring will have X, X chromosome versus if this sperm fertilizes the egg, then the offspring will have X, Y chromosome. And that's all the combinations we have. This means that one out of two will be female and one of two will be males. So look, we have a 50% chance for female and 50% chance for males. So in humans, the chances for getting a female or male are always 50%. In other words, it's purely random. And the second thing we see is that which sex cells determine the sex of the baby? Is it the egg cells or the sperm cells? We see that it's actually the sperm cells that determine it. When the sperm cells carrying the Y chromosome fertilize with the eggs, then you get a male versus if the sperm cells carrying the X chromosome fertilize the eggs, then we'll get a female. So it's the sperms eventually that decide whether we get a male or female. But that does not mean that the father decides it because the father cannot control this. Remember, this part, whether the sperm has an X chromosome or a Y chromosome is completely random. So it's wrong to say that the father or the mother decides the sex. The sex determination is random because the sperms get randomly assigned X and Y and even genetically there's a 50-50% chance of getting a male and a female.