Cystic fibrosis is a human genetic disorder with a single-gene inheritance pattern. A normal couple has a child with cystic fibrosis. Explain how this is possible using the following terms: allele, dominant, gene, mutation, and recessive. You can also include additional terms.

My answer: Cystic fibrosis is a recessive trait so it will only show when a person carries two copies of this recessive allele. Neither of the parents have two copies of the recessive allele because we're given the information that they are normal. In addition, both parents have one copy of the dominant allele and one copy of the recessive allele, which means they are carriers for this genetic disorder. If a parent only has the dominant and, in this case, normal allele, then the child will at least inherit a copy of the dominant allele. Thus he will display the normal phenotype and will not have this disorder. 

I'm not sure what to include with the terms "gene" and "mutation."