What makes a creature male or a female? If you mentioned the X and the Y chromosomes, you are correct. I mean, you're correct if you ignore most forms of life on this planet. If you actually take the time to examine the lifestyles of different life forms, many of the basic assumptions about sex differences don't hold.
I am going to try and explain this to you, using the Tree of Sex. This family tree traces the ancestry of sex in all of its weird and wonderful manifestations. Those Pie charts are coded according to the method of sex, and I will be explaining what each of those colour codes mean below.
Red = X & Y
There are only two orders of organisms that have a straight up X and Y chromosome. Mammals like us and .. Beetles.
Every member of the species has an X chromosome, carried by the egg. If it pairs with another X, we tend to get a female, if it pairs with a Y, we tend to get a male. Simple right ?
Errrm....I hate to tell you this, but no. In humans and other creatures, we can get all sorts of other natural configurations of the XY chromosome system. Some beetles have four copies of the XY chromosomes. Even weirder, some species have entirely done away with the Y chromosome. In these cases, the X chromosome realises it's male when it has nothing to pair with.
Don't even get me started on the platypus. That greedy little bastard has ten pairs of sex chromosomes. And they aren't all XY. They have sex chromosomes we would usually find in other species, such as birds. Such as the sex chromosomes known as W and Z. One class of animal went all in on these chromosomes, and I'm going to talk about them next.
Blue = W & Z
Some of you may have already been suitably horrified by the sex lives of ducks (and for those of you who have not, you're welcome). But like our ancestors, they decided to consolidate all of their sex genes onto just one chromosome. But they didn't go down the X and Y route, they use different set of chromosomes, called W and Z.
In this case, the egg carries either a W or a Z, whereas the sperm always carries a Z, which will create a male if it pairs with another Z, or a female if it pairs with a W. It's basically like our system, but backwards. All because their ancestors thought they were too good for the XY system.
Grey= When Males are born with only half a genome
I get that in the XY system, men are left with basically half a chromosome lees than females. Yeah, feel bad for us guys, but spare a thought for the males of the Hymenoptera. They are left with just one set of chromosomes, half a genome. When a female lays eggs, the male needs to fertilise them to make them into females. If the male misses out on an egg, that unfertilised egg turns into a male.
Black= Creatures that destroy half their genome to become male
Even the bees can look down on the Scale insects , who are born from fertilised eggs, and start off with two copies of the genome. But then they forcibly knock-out the paternal genes, effectively leading to the same consequences as the previous system, except they do it to themselves.
Purple= Best of Both Worlds !
Let me tell you about clownfish. Clownfish are sequential hermaphrodites. They can either be male or female depending on the situation. In an environment with no females, the biggest male turns into a female. Finding Nemo would have been a very different movie.