It’s distinctly uncomfortable for us men, but some women are, biologically, much more equal than we are. Of course, women can do all sorts of things that we can’t, like having babies, but this is far worse.
They have better colour perception. Well, perhaps 20-50% of them do. And they can use it against us, well against some of us. And around 5% of them are really super in this.
Now, I’m sure that you recall that we have three ‘colour’ receptors in the eye, sensitive to red, green and blue light. The sensitivity of the ‘green’ cones extends, weakly, into the red and the blue; this is also true for the other receptors, there is an overlap of sensitivities. We also have ‘rods’ sensitive to light, brightness and darkness. Cows don’t have colour perception, so they can’t see that they are eating clover or ragwort; and the red rag that the matador waves is just movement to the bull. The colour doesn’t matter (rather like the snake and the snake-charmer; it’s not the music of the flute, it’s the movement that gets them.) Even more strangely, some people have degenerative diseases when their rods, light/dark sensitive, and their cones, for ‘colour’ vision are destroyed, yet then can still distinguish between light and dark: there seems to be a third receptor (so far unidentified) for light.
The genes for the red and green cones are on the X chromosome. (The genes for blue cones are on chromosome 7.) The X and Y chromosomes are what make us male or female; two Xs and we are female, an X and a Y and we are male. It’s rather discomforting, for a man, to realise that the default* human embryo is a female, and that it takes the miserable, shrivelled up, runty Y to turn it into a male.
So, a male is genetically XY and a female is XX. If the X is defective, the man can be ‘colour-blind’, having the inability to easily distinguish between red and green; around 8% of men are colour blind. (Women can also have red-green colour blindness—from a colour-blind father and a mother who is a ‘carrier’, but this is much less common, at perhaps 0.5%.) If you have a high-end computer monitor you can ’see’ what a colour bind person will see; vital if you are a webmaster.
Now, it was always accepted that if we had two copies of chromosome, the genes on one of them would be ‘turned off’; so for the normal female with XX, one of the X’s was ‘non functional’. But this isn’t correct. It’s now apparent that both genes can be at work. So, the XX female can have parts of each chromosome ‘on’. In perhaps 20-50% of women, this means that they get two slightly different (usually, as far as I can determine, green) receptor cones in the eye. In a much smaller number of women, perhaps 5%, they can have two green and two red receptors; they have ‘5-colour’ vision.
As the blue receptors aren’t on the X, but doubled on chromosome 7, do we all have the possibility of two blue receptors? If this exists, I haven’t found any reference to it so far.
What, practically, does this mean? The ‘green cones’ on the two X chromosomes may be most sensitive to slightly different wavelengths of light, so slightly different shades of ‘green’. Such women with ‘4-colour’ vision have much better colour discrimination, they can distinguish between shades of red and green that most of us can’t. You can test this with a high quality colour spectrum print: men usually see 5-7 colour changes; these women typically see 7-9.
So? What’s the evolutionary advantage? Well, and here the menz should take a deep breath; if you are part of a pair-bond with a female, but you have been spreading your seed, and on your return you try to get away with it by telling porkies, the slight flushing, the minor facial reddening that comes with lying won’t be apparent to you, but she will see it. Menz, you just can’t win.
* This, strictly isn’t quite correct. There are ‘intersex’ variations, making the concepts much more complicated; see here for more.