Work Step by Step
Since Charles is unaffected but had a child with cystic fibrosis, requiring him to have a cystic fibrosis allele to pass on, if N=normal and n=cystic fibrosis, Charles is Nn. Since Elaine's brother had cystic fibrosis (nn) but his parents did not, they were both Nn. Elaine is unaffected, so she could be NN (1/3 chance, based on Punnett square analysis) or Nn (2/3 chance). The chance that their child would be nn is the chance Elaine is Nn (2/3) times the chance that Charles will pass on his n allele (1/2) times the chance that Elaine will pass on her n allele (1/2).