Explain why individuals with cystic fibrosis have different levels of severity of the disease. ...

Both the types and the numbers of mutations in the single gene responsible for cystic fibrosis, called the CFTR gene, can produce different disease phenotype. More than 1,900 mutations of several types have been identified in this gene. People with cystic fibrosis have a wide range of clinical symptoms. The relationship between the type and location of the CFTR mutation and the clinical phenotype has been investigated for a number of mutant alleles. Some mutations totally disrupt protein synthesis and severe clinical symptoms result. In other mutations, proteins may be synthesized but only partially functional. These types of mutations are associated with milder forms of the disease. It is possible for someone with cystic fibrosis to carry two different mutant alleles and be a compound heterozygote. This genetic variability further contributes to the range of phenotypes seen in cystic fibrosis.