The Colorblind Bias: Why Men Are More Likely To Be Colorblind Than Women (L30)

Disclaimer: This may seem confusing and harder to follow for someone without the background in biology that I already have, but I will do my best to have definitions in the writing. I will also be including quite a few photos/diagrams along the way. If anyone would like clarification of a certain word or anything else, feel free to comment & leave questions.

The image displayed below shows a chromosome. Each small section on the chromosome (either purple or white in this case to show separation) is a gene. People have 2 sets of have 23 chromosomes; one of these is either an X or a Y chromosome and this is what decides the gender of two humans’ offspring. Males have an XY genotype (genetic makeup), and females have an XX genotype.
Sex linked traits are traits that are only passed through either the mother or the father to the offspring. These traits are controlled by the X and Y chromosomes.


(Chromosome Map)

The gene for colorblindness is found on the X chromosome. For typing purposes, we will say that the allele for normal vision is XN; (in reality, it would be written as X with a superscript capital or lowercase n to the right of it). Likewise, the allele for colorblindness is Xn. The capital N symbolizes that normal vision is dominant, and colorblindness is recessive. By creating punnet squares using all the possible parent allele combinations, one can discover the likeliness of a person being born colorblind.

punnet squares

If this is too small for you to read, follow this link; I published the page to the web to make it easily visible.

The first punnet square shows that the father (top) and the mother (left) both have normal vision. Oddly enough it also shows that two normal sighted people have a 25% chance of birthing a colorblind son. A male born to these two parents has a 50% chance of being colorblind; a female has zero chance of being born colorblind in this situation. In this case the mother is a carrier of colorblindness.
The second punnet square shows that the father (top) is colorblind, and that the mother (left) has normal vision. The results of the punnet square show that their female offspring will be carriers but will not be affected by colorblindness. This also shows that their male offspring have no chance of being colorblind. For this couple, there is a 50% chance that their offspring will be a carrier.
The third punnet square uses a father (top) who is colorblind, and a mother (left) who is an unaffected carrier. There is a 50% chance that this couple’s offspring will be colorblind. The possibility of having a colorblind girl is 50%, as is the probability of having a colorblind boy.

There are other punnet squares that one would have to observe as well, but I have provided you with the basic template and “pattern”. Feel free to continue on your own and try all possibilities. I sincerely apologize for the small size of the punnet squares, I went through hell trying to format them and insert and enlarge. This is the largest they can be made without becoming so blurry that they’re illegible. 


Chromosome Map: Genes and Disease
. 1998. NCBI Bookshelf. Chromosome Map Genes and Disease. Web. 20 Nov. 2014. <;.

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