Genotypes, phenotypes and alleles

Cystic fibrosis is caused by a mutation in the CF gene, the length of DNA that codes for the CFTR protein. The CF gene occurs in two alternative forms or alleles. First, there is the normal allele which codes for the functioning for the CFTR protein. Secondly there is the mutated allele which produces a non-functional protein. 

The alleles that a person has makes up their genotype. FF, and ff, show a homozygous genotype for the CF gene - there are two identical copies of the allele. Ff is heterozygous for the CF gene.

The characteristic caused by the genotype, is the phenotype.  

The mutations affect CFTR in different ways. In some cases ATP is unable to bind and open the ion channel; in other cases the channel is open but changes in the protein structure lead to reduced movement of chloride ions through the channel. The most common mutation, known as the DF508 mutation, is the deletion of 3 nucleotides. This causes the loss of phenylalanine, the 508th amino acid in the CFTR protein, which is thought to result in misfolding of the protein.