Why more males are color blind.

Being color blind is a recessive trait that needs allele combinations of two recessive alleles such as “aa.” That combination is the only way recessive traits will be expressed in the individual’s phenotype. When a recessive allele is paired with a dominant one (Aa) then the recessive allele will not be expressed—but rather suppressed— by the dominant allele because it is, well, dominant.  Sadly, being color blind is a sex-linked gene. As it is commonly known, females have the combination of “XX” (two X chromosomes) while males have the “XY” combination (one X and y chromosome). The gene for being color blind is located on the X chromosome. For females, since they have two of these, the X chromosomes must both carry the recessive allele in order for that female to be color blind (aa). Most of the time females are less likely to be color blind because one of those X chromosomes, or even both of them, carry the dominant allele, saving them from being color blind (this can result in an AA or Aa allele combination). For males, however, they only have one X chromosome and it is not the same as a Y chromosome. So, if the X chromosome has the recessive allele, they have no choice but to express it because there is no other allele that would dominate or “suppress” it.


-Nicole Acnam