I had a homework question that I could not figure out.
It states:
A woman has cystic fibrosis in her family and did not want to have a child that suffered from the disease. She and her spouse went for genetic testing and counselling. She tested positive as a carrier and her spouse tested negative, and their first child was born with cystic fibrosis. Explain how this could happen.
This doesn't make sense because the punnet square would be:
$$ \begin{array}{c|c|c} & A & A \\ \hline A & AA & AA \\ \hline a & Aa & Aa \end{array} $$
This says there are no two recessive alleles which can cause cystic fibrosis.