CF is caused by a recessive allele of an autosomal gene for a cell membrane protein. John and Helen ...

Cystic fibrosis (CF) is a rare metabolic disorder that mostly affects the lungs. People with CF have a high risk of death from respiratory complications.

CF is caused by a recessive allele of an autosomal gene for a cell membrane protein. John and Helen would like to have a baby. They are, however, concerned about the occurrence of cystic fibrosis in both of their families. John has a child who has CF with his ex-wife, and Helen’s sister died from CF. John and Helen do not have CF, and neither do their parents.

a) What are the probabilities of John being totally healthy and him being a carrier?

b) What are the probabilities of Helen being totally healthy and her being a carrier?

c) Which one or ones of the following three alternatives are possible: John and Helen’s baby is totally healthy/carrier of CF/has CF disease?

Justify your answers.