Why red-green colour blindness is primarily observed in males?

Red-cone sensitivity is coded on the X chromosome, while green-cone sensitivity is coded on the Y chromosome. Because men only have one X chromosome, if this chromosome is mutated for green-cone sensitivity, the man will display the red-green color blindness. Women, on the other hand, have two X chromosomes, and so unless both copies are mutated, they will not display red-green color blindness.

Mathematically, this is why red-green color blindness is more common in men:

Men:

- P(Color blindness) = P(X-chromosome mutation)

Women:

- P(Color blindness) = P(X-chromosome mutation)^2

As the probability of a mutation is very small, the probability that a woman has two mutated X chromosomes is very low.