Red-green color blindness is in the majority of cases provoked through a defective X-chromosome. Human beings have 23 different pairs of chromosomes whereof one pair is the so called sex-chromosome. This pair consists of two X-chromosomes on women and one X- coupled with one Y-chromosome on men. Color vision in the red-green area is coded on the X-chromosome which is called a sex linked trait.
This concludes if a man is a carrier of a defective X-chromosome he will suffer from color blindness. On women the not defective chromosome is in charge and therefore she is not colorblind but a carrier for color blindness. Because a women needs two defective X-chromosome to be affected this symptome is called X-linked recessive. A very interesting conclusion of this: If you are male and your father suffers from a red-green color vision deficiency you can not inherit it from him. Only women can be carriers for color blindness who pass it on to their sons.
Let’s have a look at some illustrations. On the left you can see how the disorder is passed on from an affected father to his children. The sons are unaffected and do not have the mutation. The daughters are not affected but are both carriers of the disorder because they inherited the defective X-chromosome from their father. The illustration on the right side shows a mother which is a carrier and a father which is unaffected. Their son is at a rate of 50% affected i.e. red-green colorblind and their daughter is at the same rate either are carrier or unaffected.
In the last illustration we coupled an affected man with a women which is a carrier. As you can see their children are at a rate of 50% affected. This is the only case shown here, where a women can be affected i.e. suffering from a red-green color blindness. If the children are unaffected the daughter is anyway a carrier of the disorder. The not shown combinations where man and women are either both affected or both unaffected are left to the reader…
I hope this could give a better insight into the biology behind color blindness. It has to be noted that these remarks are only true for red-green color blindness. Blue-yellow color blindness (tritanopia) is linked to the chromosome pair 7 and therefore sex independent. Further readings on this topic and more details can be found under the following links:
The above illustrations are provided by the U.S. National Library of Medicine, 7th March 2006.