What is a hereditary disorder? Taking haemophilia as the example, show with the sex chromosomes how a healthy mother can pass it to a son while her daughters stay healthy, and name one other disorder inherited this way.

1. 1 · Define. A hereditary disorder is caused by a defective or missing gene passed on from parents.
2. 2 · Name two. Haemophilia and colour blindness; their genes are on the X chromosome.
3. 3 · Explain for boys. A boy has only one X, which comes from his mother, so a defective gene on it is not masked.
4. 4 · Explain for girls. A girl has two X chromosomes; a normal gene on the other X usually masks the defect, so she is only a carrier.
5. 5 · Real life. Genetic counselling helps such families know the chances before having children.
6. 6 · Close in one line. So X-linked disorders pass from carrier mothers to their sons.

Scoring tip: Say clearly where the son's X comes from; that one line carries most of the marks.