A REAL XX Superpower

Those who would have us all be the same would not like this one: it turns out that human females – that is, XX or XXY individuals, chromosomally speaking, to be PC – may be “tetrachromats”, or have four different colour receptors in their eyes. Thus, while most humans have only red, green and blue receptors, a quite significant proportion of genetic (XX) females can have two different red receptors (eg: orange-red and yellow-red).
This theoretically allows them to see a hundred times more colours than normal, according to the New Scientist article that just caught my (trichromatic) eye – “Dimensions of colour” in the 29 June issue, also available here – http://ophthalmology.washington.edu/sites/default/files/articles/jay_neitz_article.pdf.

This would very adequately explain to me, in my simple understanding of these things, why it is that many females – presumably XX – of my acquaintance seem able to discern so many more colours and tints than I can. But this is by the bye.

Now far more XY human males than XX females have red-green colour-blindness – because, it turns out, that the genes for the two receptors sit next to each other on the X chromosome, and as (XY) men only have one, a defect in one or other gene means no sensing that colour – and up to 2% of (XY) men are in fact missing / deficient in one or other.

And as (XX) human females have two chromosomes, it would take mutations to both sets of genes to have any effect – and that is statistically far less likely, which is why human (XX) females are far less likely to be colour blind.

But also the ONLY humans to be able to use four colour receptors…or to be able to differentiate all of those hues in a standard wall paint catalogue.

This less extreme than the case of squirrel monkeys, incidentally, where females are trichomats, and ALL males (you don’t need to be PC with squirrel monkeys) are dichromats, as they only have blue and green receptors.

Wouldn’t that seem like a superpower, if you were a male squirrel monkey?

Turns out that you can genetically engineer males back to par using a viral gene therapy vector – probably a recombinant adeno-associated virus (AAV), although they didn’t say – and that genengineered male monkeys now pass the colour-blindness test, a trait that is stable over years.

The article speculates – I would never dare; it would mean assuming that there was a genetic difference in ability between men and women, and we all know where THAT leads! – that humans might like to experiment with the same sort of therapy.   A direct quote:

“Suppose you could just have the shot and get the fourth photopigment, so you could see a hundred times more colours – who wouldn’t go for that?”

Who indeed: to be able to differentiate teal from eggshell blue; cyan from lapis lazuli; peach from…who knows what these shades are called?

I don’t – and I don’t much care. I will just count this as a difference worth celebrating, and get on with life.


About Ed Rybicki

Ed is a 60-ish virologist and biotechnologist, formerly a Zambian and presently a South African. He is into family, virology, biotechnology, science in general, science fiction in particular, photography, red wine, wearing loud shirts, 70s rock, blues and smooth jazz...and telling stories. Sometimes, interesting ones. And writing for his own amusement.
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