Cystic fibrosis is a hereditary disease which affects around 1 in every 2500 babies born in Australia. It affects several organs in the body, causing thick, sticky mucous secretions in the lungs and pancreas. This leads to respiratory problems, including recurrent infections, and difficulty digesting food.

For every child affected by cystic fibrosis, there are around 100 people who carry the faulty gene. Everyone has two copies of the gene which, when faulty, causes cystic fibrosis. Being a carrier means that one copy of the gene is faulty, and the other is normal. Carriers are not affected by cystic fibrosis because the normal gene compensates for the faulty one.

This diagram illustrates how genes for diseases like cystic fibrosis can be passed from parents to their children. If two carriers of the CF gene have a child, there is a 1 in 4 chance that the child will have cystic fibrosis, a 2 in 4 chance that it will be a carrier like its parents, and a 1 in 4 chance that it will not carry the gene at all.

Where there is a risk that genes for diseases may be passed on, it is important to note that the figures above apply to every child. For example, if a couple has three healthy children, it does not mean that the fourth child will have cystic fibrosis.