Inheritance of colour blindness
Your eyes detect colour with special cells that contain chemical pigments. People with red-green colour blindness have pigment genes that are missing, or not working properly. Genes on the X-chromosome normally make these chemical pigments. As females have two X-chromosomes, they usually have at least one set of working pigment genes. But males have only one X-chromosome (and a Y-chromosome, which does not have any pigment genes), so if they have a single altered pigment gene they will be colour blind.