Tritanopia is a rare genetic condition where the individual is unable to detect blu and yellow colors but red and green vision remains normal. The symptoms are impaired blue vision, impaired yellow vision, normal red and green vision and abnormal blue cone electroretinogram. Home eye tests and home vision test are used to diagnose tritanopia. It is sometime called blue-yellow color blindness, however, the word blue-yellow color blindness is misleading. People affected by tritan color blindness confuse blue with green and yellow with violet. So the term blue-green color blindness would be more accurate because the colors blue and yellow are usually not mixed up by tritanopes. Tritan defects affect the short-wavelength cone (S-cone). There are two different types which can be observed:
Tritanopia: People affected by tritanopia are dishromats. This means the S-cones are completely missing and only long- and medium- wavelength cones are present.
Tritanomaly: This is an alleviated form of blue-yellow color blindness, where the S-cones are present but do have some kind of mutation.
Blue-yellow color blindness can be observed only very rarely. Different studies diverge a lot in the numbers but as a rule of thumb you could say one out of 10'000 persons is affected at most. In contrary to red-green color blindness, tritan defects are autosomal and encoded on chromosome 7. This means that tritanopia and tritanomaly are not sex-liniked traits and therefore women and men are equally affected. It can be observed that tritanopes usually have fewer problems in performing everyday tasks that do those with red-green dichromacy. Maybe this is because our society associates green with good/go and red with bad/stop, a pair off colors which accompany us everyday, yet a clear reason has not been found yet by researchers.
Here is a video that helps to explain tritanopia: