Cystic fibrosis (CF) is a serious genetic disease that primarily affects the lungs and digestive system by causing the production of thick, sticky mucus. This abnormal mucus is caused by mutations in the CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) gene. For a child to develop CF, they must inherit a faulty copy of the CFTR gene from each parent, meaning both parents must be carriers.
Understanding Autosomal Recessive Inheritance
Cystic fibrosis follows a pattern known as autosomal recessive inheritance. Every person inherits two copies of the CFTR gene, which provides instructions for the CFTR protein regulating the flow of salt and fluids in and out of cells. A person with one working copy and one faulty copy is known as a carrier. Carriers typically do not show symptoms because the single healthy gene copy produces enough functional CFTR protein. The disease only manifests when an individual inherits two non-working copies of the CFTR gene.
Using ‘A’ for a healthy gene copy and ‘a’ for a faulty copy, a person with CF has the genetic makeup ‘aa.’ A carrier has the makeup ‘Aa,’ and a non-carrier has two healthy copies, ‘AA.’
Calculating Risk When Both Parents Are Carriers
When both parents are carriers (Aa), they each have an equal chance of passing either copy to their child. This combination of genes leads to four possible outcomes for each pregnancy. These outcomes are independent, meaning the result of one pregnancy does not influence the next.
There is a 25% chance (one in four) the child will inherit a faulty copy from both parents (‘aa’) and have cystic fibrosis. There is a 50% chance (two in four) the child will inherit one healthy and one faulty copy (‘Aa’), making them an asymptomatic carrier. The remaining 25% chance results in the child inheriting a healthy copy from both parents (‘AA’), meaning they will neither have the disease nor be a carrier.
Carrier Screening and Genetic Testing
Carrier screening is the only way to know for certain if you are a carrier of a CFTR gene mutation. This testing is often offered to couples planning a pregnancy and usually involves a simple blood draw or a saliva sample. While over 1,800 different mutations can cause CF, standard screening panels typically test for the most common ones.
A negative screening result significantly lowers the chance of being a carrier. However, it does not completely eliminate the possibility of carrying a rarer mutation not included in the panel.
If testing reveals that both partners are carriers, they should seek consultation with a genetic counselor. Counseling helps the couple understand the specific risks, discuss reproductive options, and navigate the results. For couples with a known high risk, prenatal testing options like chorionic villus sampling (CVS) or amniocentesis are also available during pregnancy to determine the baby’s genetic status.