Normal and defective red–green vision

One gene for red and one to three genes for
green lie close together on the long arm of the X
chromosome in humans (1). Since the
sequences of these genes are very similar, unequal
crossing-over is not infrequent (2). Intergenic
crossing-over leads to loss (green blindness)
or duplication; intragenic crossing-over
leads to a hybrid gene (red blindness). Green
blindness results from loss of a gene for the
green receptor; red blindness, from a defective
or absent red receptor. With red–green blindness,
neither a normal red nor a normal green
receptor is present. About 1% of all men are red–
green blind and about 2% are green blind. About
8% show weakness in differentiating red from
green.