What must occur for a girl to be colorblind?

C. Males only need to inherit one copy to be colorblind, since this gene is on the X chromosome and the smaller Y chromosome doesn't mask it. Females can carry the colorblindness allele, but if they have a dominant normal vision allele on their other X chromosome, they will have normal vision. If they have the colorblindness allele on both X chromosomes they will be colorblind. They only way they can get two copies of it is if each parent had it - a colorblind dad and a carrier or colorblind mom.