Colour blindness is a sex-linked recessive disorder carried on the X chromosome. Father’s genotype: Colour blind (XcY). Mother’s genotype: Since her father was colour blind (XcY), she must have inherited the Xc allele from him. As she is phenotypically normal, she is a carrier (XCXc). The cross: XcY (Father) × XCXc (Mother) Offspring analysis: Daughters: All daughters receive Xc from the father. One daughter receives XC from the mother (XCXc – carrier), while the other receives Xc from the mother (XcXc – colour blind). Hence, 50% of daughters are colour blind. Sons: All sons receive Y from the father. One son receives XC from the mother (normal), while the other receives Xc from the mother (colour blind). Hence, 50% of sons are colour blind