Mutations in the CFTR gene lead to the homozygous recessive disease CF.

Couple #1: baby is homozygous for a missense mutation, causing a serine to be made instead of a threonine. They are similar, but the change reduces the function of the CFTR protein.

Couple #2: baby is heterozygous for a nonsense mutation in the CFTR gene. The other allele is wild-type.

Couple #3: baby with missense mutation identified in couple #1’s baby and the nonsense mutation seen identified in couple 2’s baby.

Phenotype severity from least to most severe:

a) [LS] couple #2’s baby à couple #1’s baby à couple #3’s baby [MS]

b) [LS] couple #1’s baby à couple #2’s baby à couple #3’s baby [MS]

c) [LS] couple #3’s baby à couple #2’s baby à couple #1’s baby [MS]

d) [LS] couple #2’s baby à couple #3’s baby à couple #1’s baby [MS]

Learn: Genetics Exam 3