Cystic Fibrosis occurs in children who have inherited two abnormal copies of a specific gene, one from each parent. A parent who is a cystic fibrosis carrier but does not have cystic fibrosis has one normal copy of this gene and one abnormal copy. The normal copy dominates, so the parent shows no sign of cystic fibrosis. In some cases, carriers pass on their normal copy of the cystic fibrosis gene to their child. In some cases, they pass on their abnormal copy. A child who receives a normal copy from one parent and an abnormal copy from the other will be a cystic fibrosis carrier. A child who receives an abnormal gene from both parents will develop cystic fibrosis.

When both parents carry an abnormal copy of the gene, each child they produce has a 25-percent chance of inheriting two abnormal copies, a 25-percent chance of inheriting two normal copies and a 50-percent chance of inheriting one abnormal copy and one normal copy.

If one parent has cystic fibrosis and the other parent is a cystic fibrosis carrier, each child has a 50-percent chance of inheriting the disease and a 50-percent chance of being a carrier. If one parent has cystic fibrosis and the other has two normal copies of the gene, the child will be a carrier. Because of the reproductive problems associated with cystic fibrosis, it is highly unlikely a child would be born to two parents with the disease. However, all children born to such a couple would have a 100-percent chance of inheriting cystic fibrosis.

Males and females are equally likely to inherit cystic fibrosis.