Biology for IGCSE & O level - Inheritance (Section 8 - No. 37)

In an X-linked recessive disorder, which parent(s) must contribute the affected allele for a daughter to express the disorder?

Only the father.

Only the mother.

Both the mother and the father.

Either the mother or the father.

A daughter must inherit two copies of the affected allele, one from each parent, to express an X-linked recessive disorder.