Colour blindness is an X-linked disorder. For colour blindness to be effective, all X chromosomes within an individual must be affected, and since men only have one X chromosome and women have two, the chances of men being impacted are exponentially higher.
However, in this case, the mother is colour-blind and hence she carries XCXC genotype. Father’s mother is also colour-blind and hence even she will be having the genotype as XCXC, and she will be transferring her XC chromosome to her son and hence, even the son will be colour-blind (XCY). So, by this, we can tell all the X chromosomes are coding for colour-blind gene, and they inherit this gene to all their children, and hence, the chances of their child being colour-blind would be 100%.
