Science Puzzle
Why Are More Men Colour-Blind Than Women?
About 1 in 12 men can't easily tell certain reds and greens apart, so they'd struggle to see the number hidden in a dot test card. Among women it's only about 1 in 200.
Why is red-green colour blindness so much more common in men?
The Answer
The genes for the red-sensing and green-sensing cells in your eyes are on the X chromosome. Men have only one X, so a single faulty copy makes them colour-blind. Women have two Xs, and a working copy on one makes up for a faulty one on the other.
Women usually have two X chromosomes and men have one X and one Y. The Y is small and carries few genes, none of them for colour vision. So a man has just one copy of each colour gene, and whatever is on his X is what he sees with. A woman is only colour-blind if both of her Xs carry the fault.
The numbers fit. If about 1 in 12 X chromosomes carries the fault, a woman needs to be unlucky twice: 1/12 × 1/12 is about 1 in 144. The real figure is a bit lower because the faults come in more than one kind, but it's far rarer than in men either way.
It also explains the family pattern. A colour-blind man passes his Y, not his X, to his sons, so they don't get it from him; his daughters get his X and become carriers, and their sons may be colour-blind. The chemist John Dalton described his own colour blindness in 1794. He asked for his eyes to be studied after his death, and in 1995 DNA from his preserved eye showed he was missing the gene for the green-sensing pigment.
The principle: Sex-linked inheritance. A fault in a gene on the X chromosome shows up far more often in males, who have only one X, than in females, who need two faulty copies.