Question about Autosomal Recessive Alleles 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.

I asked my Biology teacher and she said that the if the father is not a carrier, he must have the disease. So the father is aa.