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Conditions

Dad is color blind, mom isn’t. They have 2 daughters and both are not color blind. When the girls marry someone who isn’t color blind, will their kids be color blind?

-A curious adult from Maryland

October 18, 2017

Each of their sons has around a 50% chance for being color blind. Most likely none of their daughters would be (although there is a chance the sons of these daughters, the grandsons, could be).

This is because both daughters are carriers. They have the bit of DNA that can lead to color blindness but they are not color blind themselves.

The reason they aren’t color blind but their sons are at risk has to do with where that bit of DNA is and how it works.

Three Things You Need to Know

OK let’s dive right in and see what is happening in your case.

The DNA that leads to color blindness is on the X chromosome. (Click here to learn more about chromosomes.)

Here are the three key things you need to know:

  • Biological males have one X chromosome and one Y chromosome (XY)
  • Biological females have two X chromosomes (XX)
  • To be color blind, all your X’s have to have the color blind DNA

(At the end of the answer I have put a table where I put this all together so you can see the odds of having a color blind child with the six possible parent combinations.)

Your Case

So the father was colorblind which means all of his X’s had the color blind DNA. We will show this like this:

XcY

He has an X and a Y and his X has a little “c” to show that the X has the color blind DNA.

When he has a child, he will either pass his X or his Y down. If he passes the X, he will have a daughter and if he passes his Y, he will have a son.

This means that all his daughters inherited his Xc. Since neither daughter is color blind, their other X must not have the X with the color blind DNA. Their other X is just an X:

XcX

If they had two X’s with the color blind DNA, they’d be color blind too.

These daughters are carriers, they have one of each type of X. Let’s now see what can happen when they have kids with a man who is not color blind.

Here they each are:

To make things easier to follow, I have color coded the X’s. So mom has a blue one that can lead to color blindness and a black one that can’t. And dad has an orange one that can’t lead to color blindness.

Remember that when they have kids, each parent will only pass down one chromosome from their pair.

Let’s say that mom happened to pass down her blue X and dad passed down his Y. Here is the combination their son would have:

He would have an X with DNA that can lead to color blindness and a Y. In other words all of his X’s would lead to color blindness. This is why he is most likely color blind.

Here are all four of the possible combinations:

XCX         Carrier, daughter

XX           Daughter

XCY         Color blind son

XY           Son

Each daughter has a 50% chance of being a carrier and each son has a 50% chance of being color blind.

The daughters who are carriers can have color blind sons with the same logic. If they have children with a man who is not color blind, then each daughter has a 50% chance of being a carrier and each son has a 50% chance of being color blind.

The daughters without that blue X will not have color blind sons.

Up until now I have only dealt with the situation where mom is a carrier and dad isn’t color blind. In the last section, I’ll give the odds for a color blind child with all six possible combinations of parents. 

Every Genetic Combination

Here is a table with all the possible outcomes for the most common form of color blindness. It is a big table: 

(Click here for a larger version of the table.)

Dad is in the first column and mom is in the second. Dad can’t be a carrier so he is either color blind (first three rows) or not color blind (last three rows).

The second column includes the three possibilities for mom—color blind, carrier and neither.

The way to use this is to find the row that describes the parents. Let’s say dad is color blind and you know mom is a carrier (more on that later).

In that case, you would look at the second row. There you can see that each son has a 50% chance for being color blind. Each daughter has a 50% chance for being color blind and a 50% chance for being a carrier.

Of course the tricky part in using this table is when mom isn’t color blind. In that case it can be hard to know if she is a carrier or not.

If she is not color blind but her dad is, then she for sure is a carrier (see the second and third rows). But if dad and/or mom aren’t color blind, it is hard to know which row applies to her.

At least for now, if you don't know if mom is a carrier or not, you just have to look at both possibilities and realize that at least one applies to her situation. Things might change if a "direct to consumer" genetic test becomes available that can determine if she is a carrier or not.

But until then, this is the best we can do.

By Dr. D. Barry Starr, Stanford University


This answer deals with red-green color blindness (number II in this image). (Wikimedia Commons)


The part of our DNA that can lead to red-green color blindness is on the X chromosome, shown here in red. Biological males have just one X (along with a green Y). (Wikimedia Commons