The gene for color blindness is sex-linked recessive. Indicate the genotype of a man with normal color vision.

Correct answer: XD Y.

In humans, the male sex is heterogametic, which means a combination within the male human genotype of two different sex chromosomes – X and Y. Numbers 1 and 2 represent female, homogametic genotypes.

It is generally accepted when solving problems in genetics to designate a recessive gene with a lowercase letter, and a dominant gene with a capital letter. Therefore, the gene that determines the development of color vision pathology in humans is indicated by a lowercase letter d, and number 3 represents the genotype of a man with color blindness.