Monya Olchowy, Second Life asked:
A light has to interact with our tissue for us to see it. Is it true that women have a variant of the pigment to see in the red spectrum?
Chris - The answer is, yes, they do. The gene which enables us to see red light, in other words, encodes the red detecting pigment in the retina is carried on the X-chromosome, and because women have two X-chromosomes that they inherit from their parents – one from the mother and one from the father, they therefore have two genes in their body that are capable of detecting red.
But during development, one of the X-chromosomes is randomly inactivated. This is called ‘X-chromosome inactivation’ because you don't want two copies of the chromosome active because you only need one because men only have one copy.
But that process of inactivation is random in the tissues which means that some cells will be using one X-chromosome whilst other cells could be using the other X-chromosome, and therefore, if you extend that to the retina, there’ll be some cells in the retina that are seeing red, using the gene on one X-chromosome, and some cells seeing red using the gene on the other X-chromosome.
And this means theoretically, there could be different genes running in different bits of the retina, and therefore, the perception of red could be slightly different in different positions on the retina in women.
But because men have only one X-chromosome because our genotype is XY, we therefore don't inactivate an X-chromosome, and therefore use the same gene throughout the retina, and that’s why you also find men who can be colour blind because they can inherit a red receptor that doesn’t work.
Whilst women, because they have two X-chromosomes, even if one of them has a defective copy, the other one is almost certainly still going to work and therefore, in general, you don't find women who have colour blindness. So the answer is yes.