The allele for colorblindness is located on the X chromosome, so it is sex-linked. Normally, colorblindness is a recessive trait, and requires two copies of the allele. Sincefemales_ have two X chromosomes, they do not exhibit any symptoms if they have only one colorblindness allele. However, sincemales_ have only one X chromosome, they need to inherit the allele only once to express the trait.