Question: How does the condition of tetrachromacy occur in women?

David Foley answered on 19 Nov 2014:

It seems that the critical genes encoding the parts of our vision that can cause this phenomenon are on the X-chromosome. Women have two X-chromsomes (men have one X and one Y) and so it seems that in rare circumstances mutants on both X-chromosomes are expressed and alter the vision to produce tetrachromacy.

There are many other examples of similar X-linked and Y-linked genetic mutations which predominantly affect one sex over the other. A really well understood example is that of haemophilia, which effects men rather than women because women have, again two copies of the gene whereas men only have one.