The explanation behind the 20/300 color blindness is the gene for the S cone pigment is situated on autosomal chromosome number 7, though the genes for the M and L cones are located on the X chromosome. Color blindness is triggered by a hereditary absence of one or more type of cones, typically the M (green) or L (red) cones. These individuals have only two working types of cones, and their vision is consequently dichromatic. People who have only one cone in the mid to long wavelength area (M or L) have trouble differentiating reds from greens. Because the photopsins of the M and L cone pigments are implicit on the X chromosome, and since men have only one X chromosome, such red-green color blindness is far more common in men than in women.