Why is Red-Green Colorblindness a 'Guy Thing?'
Colorblind girls and women are rare, while men who can't match their socks are relatively common. The reason is a genetic phenomenon called sex-linked inheritance. Humans have 23 pairs of chromosomes. One of those pairs, called X and Y, determines sex. Most females have 2 Xs. Most males have an X and a Y. The Y chromosome carries the genes that cause an embryo to develop as a male, but not many others. The X chromosome carries many genes that have nothing to do with sex and many that have no counterpart on the Y chromosome. So, for those characteristics, females get two genes, but males get only one.
Colorblindness is caused by a recessive gene. Recessive means two copies of the gene are required for the characteristic to show up. If one member of the gene pair is normal, then color vision is normal. The gene for colorblindness is carried on the X chromosome. Since it is relatively rare compared to the gene for normal color vision, most women who carry it have a normal gene on their second X chromosomes, so their color vision is normal. Unless their father was colorblind, they may not even suspect that they carry the gene--until they learn that they have a colorblind son.
Females can be red/green colorblind, but only if they get the recessive gene from both parents. For that to happen, their mother must be a carrier (or colorblind herself) and their father must be colorblind, too.
