I’m the only colorblind person in my family. Do I have a mutation?

March 23, 2018

A high school student from the UK asks:

"None of my family is colorblind in the last 3 generations, except me (I am male). I believe that it’s a genetic mutation that just happened to make me colorblind. Is this the most likely explanation?"

The quick answer is that you most likely inherited your colorblindness from your mom. And she got the DNA for colorblindness from her mom (your grandmother).

You probably don’t have a new, random mutation to blame. Instead, you have regular genetic inheritance to thank!

This is because women with regular color vision can secretly be carrying the DNA for colorblindness, and pass it on to their children. That means it can stay hidden in a family for generations and pop up unexpectedly.

So if both you and your mom have this bit of colorblind DNA, why are you colorblind while she isn’t?

The reason has to do with where the bit of DNA that causes colorblindness is located. There are 4 key things you need to know:

  • The DNA that leads to colorblindness is located on the X chromosome
  • Most biological males have one X and one Y (XY)
  • Most biological females have two X’s (XX)
  • To be colorblind, all your X’s have to have the colorblind DNA

What this all means is that it is easier for men to be colorblind than women. And that women who aren’t colorblind can have kids who are.

Here are the possible combinations for a biological female:

Color vision vs colorblind.

Here, the X chromosome that can cause colorblindness is shown in red. As you can see, it is only when both X’s are red that she is colorblind.

The same is not true for biological males.

As biological males only have one copy of the X chromosome, they only need one colorblind X to be colorblind. Because of this, men are much more likely to be colorblind than women.

Color vision vs colorblind.

Since you are male, you got a Y from your father, and an X from your mother. As the colorblind DNA is on the X chromosome, it must have come from your mother.

Color vision vs colorblind family tree.

Since your mom isn’t colorblind and you are, she most likely has one X that can lead to colorblindness and one that does not. She isn’t colorblind but can pass it to her kids. She is a carrier.

Colorblind carrier.

So where did her colorblind DNA come from? She inherited two X chromosomes from her parents, one from her mom and one from her dad.

Let’s consider your grandfather first. As a man, he has one X and one Y. Since you know he has normal color vision, he must not have the colorblind DNA. He has a black X:

Colorblind no carrier.

What about your grandmother? As a woman, she has two X chromosomes (XX). Since she has normal color vision, she must have at least one non-colorblind version. However, since your mom is likely a carrier, her mom probably is too. This means she has one colorblind X that she passed on to your mom.

Colorblind carrier.

So your colorblindness is not a random mutation, but actually from your mother and grandmother (and maybe your great-grandmother and so on!).

Color vision vs colorblind family tree.

Author: Dr. Abbey Thompson

Abbey is the Director of Educational Outreach for the Stanford Genetics Department. As part of this, she acts as “The Tech Geneticist”, reviewing all questions submitted to Ask-a-Geneticist and editing all new articles. She also leads the Stanford at The Tech program, which brings Stanford scientists to The Tech Interactive to run science activities with visitors in the Biotinkering Lab.

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