Cystic fibrosis is a genetic disease. This means that people inherit it from their parents through genes (or DNA), which also determine a lot of other characteristics including height, hair color and eye color. Genes, found in the nucleus of all the body's cells, control cell function by serving as the blueprint for the production of proteins.
To have cystic fibrosis, a person must inherit two copies of the defective CF gene — one copy from each parent. If both parents are carriers of the CF gene (i.e., they each have one copy of the defective gene, but do not have the disease themselves), their child will have a 25% chance of inheriting both defective copies and having cystic fibrosis, a 50% chance of inheriting one defective copy and being a carrier, and a 25% chance of not having CF or carrying the gene.
There have been over 1500 CF genes identified. DeltaF508 is the most common one, some people have two copies of this, called double deltaF508. It has been difficult to establish a correlation between severity of the desease and inherited CF genotype, because there are other genes that also can affect CF disease. For example, CF siblings could carry exactly the same genotype from the same parents, one may die in childhood, the other might live well into adulthood. So it's complicated, other factors enter in. My genes are deltaF508 and 3601-17T>C.