An example of a sex-linked trait is red-green colorblindness. The OPN1LW gene, which codes for a protein that allows humans to tell the difference between red and green, is located on the X chromosome. Without functional OPN1LW protein, a person will be red-green colorblind. In females, color blindness is a recessive trait because the possession of a single functional copy of OPN1LW is sufficient for normal
vision. Males, however, only have one copy of the X chromosome and thus only one OPN1LW gene. So, if there is a functional mutation on the OPN1LW gene of a male’s only X chromosome, he will be colorblind. There is no corresponding gene on the Y chromosome to compensate for this loss of function. This difference in sex chromosomes explains why red-green colorblindness is rare in females, since females need to inherit two mutated copies if they are to be red-green colorblind. However, females must be carriers of colorblindness in order to pass the trait on to any of their sons.