I'll take a hack: 1. Imagine the retina as a field of neurons that are receptive to light. Imagine different types of neurons are sensitive to different types

I'll take a hack: 1. Imagine the retina as a field of neurons that are receptive to light. Imagine different types of neurons are sensitive to different types of light (RGB). Each person has two copies (or alleles) of the R, G, and B genes. The R & G genes are on the X-chromosome. 2. In a color-blind person, the R & G genes merge to form a R-G allele that is sensitive to some red-green combination. Thus, men who carry this R-G gene have cells that express B and R-G genes and thus are red-green colorblind (they can't distiguish between the two because they only have one type of receptor for the two). Remember they only have one copy of these R and G genes. 3. Women might carry the R-G allele, but in those cases their retina as a whole display R, G, B, and R-G. These people are known as heterochromats can can distinguish colors better than normal people. Note that X-inactivation leads to cells even having R-G allele, or R or G gene. 4. It's rare for a woman to have two R-G alleles. When they do, they are red-green colorblind.