The Genetics of Color Blindness

The X chromosome connection

The genes that build the red and green cones sit on the X chromosome. Biological males have one X and one Y (XY); biological females have two Xs (XX). Red-green colour blindness is recessive, meaning it only shows up when there's no working copy of the gene.

• A man has just one X. If that X carries the affected gene, he has no back-up copy, so he's colour blind. This is why a single affected gene is enough in men.
• A woman has two Xs. She'd usually need both to carry the affected gene to be colour blind. With one affected and one typical X, she has normal colour vision but is a carrier.

That single difference, one X versus two, is why red-green colour blindness affects about 1 in 12 men but only 1 in 200 women.

What it means to be a carrier

A carrier typically sees colour normally but can pass the affected gene on. Each son has a 50% chance of inheriting her affected X and being colour blind; each daughter has a 50% chance of being a carrier herself. Interestingly, some carriers may even have subtly enhanced colour discrimination, the basis of the rare "tetrachromat" idea.

Why it "skips" a generation

A common pattern: a colour-blind grandfather passes his affected X to all his daughters, who become carriers with normal colour vision. Those daughters can then pass it to their sons, the grandsons. So the trait appears in the grandfather and grandson but not the mother in between, giving the impression it skipped a generation.

Blue-yellow and total colour blindness

Blue-yellow (tritan) deficiency is inherited differently, the blue-cone gene is not on the X chromosome, so it affects men and women about equally and is much rarer. Total colour blindness (achromatopsia) is rarer still and follows its own recessive pattern, usually needing an affected gene from both parents. See all the types.

Want to know your own status? Take the color blind test, and if it matters for family planning or a career, a professional exam and, where relevant, genetic counselling can help. Related: can women be colour blind?