DEC colours

Ben Franchuk wrote: Some people (all of them women) have four different colour receptors. There are two different kinds of "Red" cone, chosen genetically. Yours may be different from mine. We might well match two different Pantone cards against, say, a piece of cloth, or a switch on the panel of a large computer. Now, the gene that codes for which type of red receptor you have is in the X chromosome. One of the reasons why boys are many, many times more likely to be colour-blind than girls is that girls have two X chromosomes, so the "good copy" will be used. What happens then is a bit more interesting - the retina of the girl's eye will have colour-blind spots. You can find these by shining a very narrow coloured beam into the eye, and moving it about. Some bits of the retina won't let her see what colour the beam is, some will. It's exactly the same way that a black-and-white cat and a ginger cat will have tortoiseshell female kittens. So, if one parent has one kind of red cone, and the other parent has the other kind of red cone, then the resulting retina will be a patchwork of both types of cone. Women who have tetrachromatic vision can tell the difference between tiny gradations of colour, so small that most people cannot see it. Gordon JC Pearce.