Colour blindness is a sex-linked recessive disorder located on the X chromosome .

1. Woman's Genotype: Her father was colour-blind (), meaning he passed his X chromosome with the colour-blind gene to her. Since she is phenotypically normal, she is a carrier (heterozygous). Her genotype is .
2. Man's Genotype: The husband is a normal man, so his genotype is (he has a normal X chromosome).
3. The Cross: (Carrier Mother) (Normal Father).
4. Offspring (Sons): Sons receive their Y chromosome from the father and X chromosome from the mother. The mother produces two types of gametes: 50% carrying the normal and 50% carrying the affected .
5. Result: 50% of the sons will be normal () and 50% will be colour-blind (). The son of a woman who carries the gene has a 50 per cent chance of being colour blind .