Question

Two phenotypically normal parents have a child with CF. i. What are the genotypes of each parent? ii. What is the probability that their next child has cystic fibrosis? Explain and complete a Punnett square.

Answer 1

To first dive into your question, here are some vocabulary terms that I will be using and will be helpful to you as well.

• Homozygous dominant (BB): Is not affected by cystic fibrosis.
• Homozygous recessive (bb): Is affected by cystic fibrosis.
• Heterozygous (Bb): A carrier for cystic fibrosis.
• Phenotype (Ex: Having cystic fibrosis): The appearance of an allele pair.
• Genotype (Ex: bb): The genetic makeup of an allele pair.

We know that if both of the parents are phenotypically normal, there is no way that they can be homozygous recessive (bb), or have cystic fibrosis. We also know that since they have a child with this disease, they can't be homozygous dominant (BB) either. This means that both parents have to be carriers (Bb) in order for them to have a child with cystic fibrosis.

Below I have attached a Punnett square with both of the heterozygous parents.

Each child they have will have a 1/4 or 25% chance of having cystic fibrosis.