Inherited blue-yellow defects are rare in either sex. They see only different degrees of lightness. For them, the world appears to be shades of gray, black and white. To have rod monochromacy, someone must inherit a gene for the disorder from both parents. This condition occurs in approximately 1 in 30,000 of the population.
Dichromacy is a less severe form of color defect than monochromacy.
Dichromats can tell some hues apart. Protanopia and deuteranopia are red-green defects. Persons with red-green defects have difficulty distinguishing between reds, greens and yellows but can discriminate between blues and yellows. Protanopes often can name red and green correctly because green looks lighter to them than red. Males have red-green defects if they inherit a defective gene from their mother.