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.
Genetically, there is no difference between "male" and "female" hermaphrodites, for obvious reasons. They have both male and female genes within them, and whether they get activated depends on the environment.
Green = It's all about the hotness
Once upon a time, there was a laboratory, whose well meaning denizens decided it was their mission to save a certain species of turtle from extinction. Their plan was almost foolproof. The species of turtle they wanted to save buried its clutch on the beach. When the eggs hatched, they would be attacked mercilessly by predators, and only a few would make it to the safety of the surf. So a bright spark thought, what if the turtles eggs were hatched in a lab, and raised in safety, and then released en masse into the wild. One problem, someone left the incubator temperature too high. The researchers only realised their error twenty years later, when their brood arrived, and they realised that they were all female.
Genetically, there is no difference between male turtles and female turtles. So what makes them one or t'other ? Temperature. Warmer eggs will become female, cooler eggs become male. This kind of system works in crocodiles, snakes and other reptiles, which makes them really OCD about nesting temperature.
Yellow= Why even bother with sex chromosomes
There are a tonne of species who have sex determining genes spread all over their genome. They can have multiple different systems for determining sex hosted on different chromosomes.
So, wait, what does the "Tree of Sex" actually tell us about what makes us male and female?
There is no set way of determining the difference between male creatures and female creatures. The sexes that we tend to think of as fixed can change drastically over the course of evolution. Some orders of life haven't even decided on a set system. The system of sexual determinism for turtles may only be a million years old. Some organisms have re-evolved asexuality, with occasionally disastrous results.
The truth is that sex is simply a way for DNA to diversify its genetic portfolio. It doesn't care whether you are XY, WZ or a hermaphrodite. It doesn't care whether the sperm producing organisms carry the children or not. It just wants a way to acquire new forms of genes, and evolve new ones safely. It will use any strategy that doesn't encumber its host organism.
Only mammals and birds have plumped for one specific system of sex determination. There is so much sexual diversity within the tree of life. The idea that sexual characteristics are solely determined by genetics simply doesn't hold for many life forms. For the turtles and the hermaphrodite fish, sex is a phenotype, not a genotype. The Tree of Sex tells us that when it come to determining the sex of an organism, there are no hard and fast rules.
Tree of sex was concieved by Professor Judith Mank of UCL and the Tree of Sex consortium, from a publication to be featured in PLoS Biology. This article was based off a talk given to the London Skeptics in The Pub.
Thanks to commenter eleniRPG for the correction about Bees and Hymenoptera