If an individual is heterozygous for two different CFTR alleles, what does this imply about their phenotype with respect to CF?

• A. They will definitely have cystic fibrosis
• B. They are at increased risk for severe symptoms
• C. They are likely to be asymptomatic
• D. They are guaranteed to be a carrier

Answer: (C)

Heterozygosity for two different CFTR alleles implies that the individual has one normal allele and one mutated allele associated with cystic fibrosis (CF). In the case of CF, the condition typically arises from being homozygous for certain pathogenic mutations in the CFTR gene, meaning an individual needs two copies of the mutated allele to manifest the disease fully.

When an individual has one normal allele (wild type) and one mutated allele (which could be recessive), the normal allele can often compensate for the function of the mutated one. Therefore, these individuals usually do not exhibit symptoms of cystic fibrosis and can often be classified as asymptomatic, despite being carriers of the disease allele. Carrier status itself does not guarantee the presence of symptoms, as the normal CFTR function from the healthy allele is sufficient to maintain proper physiological functioning.

Thus, the individual's phenotype regarding cystic fibrosis can be characterized as likely asymptomatic, affirming the understanding that heterozygous individuals typically do not obtain the full phenotype associated with the disease.