Final answer:
Sara and Alex, with Sara being homozygous normal and Alex being a carrier, have a 0% risk of having a child with cystic fibrosis. However, there is a 50% chance that their child will be a carrier of the cystic fibrosis allele without showing any symptoms. A Punnett square would show that all potential offspring are either healthy or carriers.
Step-by-step explanation:
Cystic fibrosis is a genetic noninfectious disease inherited as an autosomal recessive trait, resulting from a mutation in the CFTR gene. For a child to inherit this condition, they would need to receive two copies of the defective gene, one from each parent. Considering that Sara is homozygous for the normal allele and Alex is a carrier of the cystic fibrosis allele, we would use a Punnett square to determine the chance of them having a child with cystic fibrosis.
In this scenario, since Sara (denoted as 'FF') has two normal alleles and Alex (denoted as 'Ff') is a carrier, their offspring could either be normal (F) or carriers (f), but they cannot have cystic fibrosis since they would need two recessive alleles (ff) to express the disease. The possible genotypes of the offspring would be 'FF' or 'Ff'. The 'FF' offspring would be completely free of the cystic fibrosis allele, while 'Ff' offspring would be carriers without showing symptoms of the disease.
Creating a Punnett square would show that all the potential offspring have a 0% chance of having cystic fibrosis and a 100% chance of being either healthy or carriers. Therefore, Sara and Alex have no risk of having a child with cystic fibrosis, but a 50% chance of having a carrier child.