Sarah has chronic bronchitis and frequently ends up hospitalized with pneumonia. She has no family history of cystic fibrosis, a serious recessive genetic disorder which affects the respiratory and digestive tracts. She and her husband, Greg, have one child after undergoing fertility treatments to overcome Greg's infertility. They are surprised when the child is diagnosed with cystic fibrosis and the genetic counselor tells them that they each also have the condition, which causes their bronchitis and infertility. Which of the following explanations would be a reasonable explanation of their situation?