Color Blindness Inheritance

Inheritance calculator

Pick each parent's colour vision status to see the odds for each child. The complete answer table is printed below as well, so nothing here depends on the calculator loading.

Every parent combination, in one table

Red-green colour vision deficiency, X-linked recessive. Odds apply independently to each pregnancy.

Not every form is inherited the same way

How is color blindness inherited?

The genes that build the red (L) and green (M) cone pigments sit next to each other on the X chromosome. Men have one X and one Y; women have two X chromosomes. That single fact drives the entire pattern. A man's only X came from his mother, so if it carries an altered pigment gene he is colour blind — there is no second copy to fall back on. A woman with one altered copy still has a working copy on her other X, which is normally enough for typical colour vision, so she is a carrier rather than affected.

The consequence is that colour blindness travels through women and shows up in men. A carrier mother has an even chance of passing her altered X to each child. If that child is a son, he is affected; if a daughter, she becomes a carrier in turn. Nothing about a previous child changes the odds for the next one — each pregnancy is an independent coin flip.

An affected father contributes the mirror image. He passes his X to every daughter and his Y to every son, so he cannot pass red-green colour blindness to a son at all, and every daughter he has becomes at least a carrier. That is the mechanism behind the classic pedigree where a trait appears in a grandfather, vanishes in his daughters, and returns in his grandsons.

Is color blindness dominant or recessive?

Red-green colour blindness is recessive, and X-linked. Recessive means one working copy masks one altered copy — which is why carrier women usually have normal colour vision. X-linked means the gene sits on the X chromosome, which is why the recessive pattern produces such a lopsided result between the sexes rather than the even split an autosomal recessive trait would give.

The other forms follow different rules, and conflating them is the most common mistake in this topic. Blue-yellow (tritan) deficiency involves the S-cone gene on chromosome 7 — an autosome, not a sex chromosome — and is inherited in an autosomal dominant pattern, so one altered copy is enough and men and women are affected about equally. Complete achromatopsia is autosomal recessive and requires an altered copy from both parents.

So the answer to "is colour blindness dominant or recessive" depends entirely on which colour blindness. For the common red-green kind that nearly everyone means: recessive, X-linked.

Drawing the Punnett square

A Punnett square for an X-linked trait uses the parents' sex chromosomes rather than a single gene pair. Write the mother's two X chromosomes along the top — one normal, one altered, if she is a carrier — and the father's X and Y down the side. Each of the four cells is an equally likely combination of one chromosome from each parent.

For a carrier mother and a father with normal colour vision, the four cells come out as: an unaffected daughter, a carrier daughter, an unaffected son, and an affected son. That is where the two familiar 50% figures come from — half of daughters are carriers, half of sons are affected — and it is the single most commonly assigned version of this exercise.

The calculator above runs the same square for every combination of parents, including the cases that are easy to get wrong, such as an affected father with a non-carrier mother, where no son is affected but every daughter is a carrier.

What this does and does not tell you

These are probabilities for each pregnancy, calculated from the standard X-linked recessive model. They describe the odds before a child is born, not a measurement of any actual person, and they assume the family history you enter is accurate — which matters, because carrier status is invisible without testing and is frequently unknown.

The model also assumes the common red-green forms. It does not apply to blue-yellow deficiency, to achromatopsia, or to colour vision changes acquired later in life from eye disease or medication, none of which are inherited in this pattern.

If the question is a real family-planning one rather than a curiosity, a genetic counsellor can work from actual family history and, where appropriate, genetic testing. Colour vision testing of the people involved is a cheap and useful first step: knowing a father's type and degree, or confirming that a mother's own colour vision is typical, sharpens the picture considerably.

Color blindness inheritance — frequently asked questions

How is color blindness inherited?

Red-green colour blindness follows an X-linked recessive pattern. The red and green cone pigment genes are on the X chromosome, so a son needs only the single altered copy he receives from his mother, while a daughter needs an altered copy from both parents. This is why about 1 in 12 men and about 1 in 200 women are affected.

Is color blindness dominant or recessive?

The common red-green forms are recessive and X-linked. Blue-yellow (tritan) deficiency is different — it sits on chromosome 7 and is autosomal dominant, affecting men and women about equally. Complete achromatopsia is autosomal recessive.

What is the chance that a son will be color blind?

It depends entirely on the mother. If she is a carrier, each son has a 50% chance; if she is colour blind herself, every son is affected; if she carries no altered copy, no son is affected. The father makes no difference to a son at all, because he passes a Y chromosome rather than an X.

What is the chance that a daughter will be color blind?

A daughter needs an altered copy from both parents, so she can only be affected if her father is colour blind and her mother is at least a carrier. With a carrier mother and an affected father the chance is 50%; with an affected mother and an affected father it is 100%. In every other pairing a daughter may be a carrier but is not affected.

Can color blindness skip a generation?

It appears to, and the mechanism is carrier daughters. An affected man passes his X to every daughter, who becomes a carrier with normal colour vision, and each of her sons then has a 50% chance of being affected. The trait moves from grandfather to grandson through a woman who never shows it.

Is color blindness autosomal dominant?

Not the common kind. Red-green deficiency is X-linked recessive. Blue-yellow (tritan) deficiency is the form that is autosomal dominant, and it is rare — under 0.01% of people — which is why the autosomal dominant answer is wrong for almost every case people are actually asking about.

If I'm color blind, will my children be?

If you are a colour blind father: none of your sons will inherit it from you, and all of your daughters will be carriers. If you are a colour blind mother: all of your sons will be affected, and all of your daughters will be at least carriers. Use the calculator above with both parents' status for the specific answer.