The right answer is: recessive gene on his X chromosome.
The structure of visual pigments is coded by three genes whose pathology modifies pigments and determines color blindness. The two genes coding for the L-cone and M-cone pigments are located on the long arm of the X chromosome.
Since it's a recessive disease. In the female, carrying two X chromosomes, the genes are in duplicate, while in humans, the carrier of a single X chromosome, the gene is in single copy and its color vision is determined solely by the X chromosome received from its mother.
An X chromosome can be pathological in two ways: either because one of the genes is abnormal or because it is absent.