A woman trying to get pregnant has a sister whose child has an autosomal recessive disease characterized by dysfunction of mucus-secreting cells. As a result, this child has abnormally thick mucus that tends to precipitate into dense plugs that obstruct the pancreatic ducts, bronchi, and bronchioles, as well as the bile ducts. Which of the following tests could be performed to determine if this woman and her husband are carriers of this disease?