Colorblindness and heredity

Cecil,

My maternal grandfather is colorblind, my mother is colorblind, I’m
colorblind, but my brother is not. My father is also not colorblind.
I don’t know of anyone colorblind on my dad’s side. One of my friends
told me that in some science class they discussed heredity and found
out that if your mom is colorblind you are definitely color-blind.
But my brother is not color-blind. Is one of us adopted, or does that
teacher need some more lessons themselves?

Regards,
Joe (from MN)

My mother is colorblind but I am not colorblind (I’m a male).

I think that if you are relying on straight Mendelian genetics, then this is not a possible outcome. However, the world does not operate according to straight Mendelian genetics. First of all, genetics is not the only thing that can cause colorblindness. Also, some genes can be damaged or more expressive than others which can lead to this seemingly impossible outcome.

Depends on what kind of color-blindness we’re talking about. The classic red-green colorblindness is a recessive X-linked trait. The reason for this is that the genes that code for the red and green receptors are on the X chromosome. According to classical Mendelian genetics, the only way that a female can express a recessive X-linked trait is if she has those genes damaged on both of her X-chromosomes. However, the reality is a bit more complex. Not every X chromosome in females is active; one copy is inactivated, at random, in each cell. If a female has the genes damaged on only one of her chromosomes, but by random chance, that chromosome happens to be inactivated in most of her cone cells, then she’ll be color-blind, though she has a spare working copy. This is an example of a trait with ‘incomplete penetrance’ - just because you have the gene, doesn’t mean that you experience its effects maximally. Typically, a heterozygote’s retina is a mixture of functional and non-functional cones. If the distribution was skewed somehow, a heterozygote would be more or less affected.

At that point, one of her sons can receive the fully-functional red/green receptor genes from her (your brother), and one can receive the affected red/green receptor genes (you). This is without getting into issues of recombination rates between those genes, etc.

Thank you both for responding. Perhaps the question isn’t as simple as I thought it would be. Thanks a lot for your explanations though - it helps.