Genetics and CF
What is a gene?
A gene is the basic unit of heredity. Genes are responsible for the physical characteristics that each person has (like eye color, facial features, and many health conditions). Each gene occupies a certain location on a chromosome (a thread-like material that is located in the nucleus of every single cell in the body). Chromosomes come in 23 pairs, and each chromosome carries thousands of genes.
Each gene has a specific role in determining how a person's body is put together and how it functions. The role of a gene is determined by its individual DNA code (deoxyribonucleic acid, the chemical coding for a gene). DNA is made up of four building blocks called bases. These bases are joined in a specific order for each gene. When a change occurs in the arrangement of the bases, it can cause the gene not to work properly.
What are genetic disorders?
A structural gene change which can cause a disease or a birth defect is called a mutation. Genes are inherited in pairs, with one gene inherited from each parent to make the pair. Cystic fibrosis occurs when both genes in the pair have a mutation. A person with cystic fibrosis inherits one CF gene from each parent. Cystic fibrosis is a genetic disorder caused by inheriting a pair of genes that are mutated or not working properly.
The Cystic Fibrosis Gene
Everyone inherits two copies of the CFTR (cystic fibrosis transmembrane conductance regulator) gene. However, some of the inherited copies are mutations. To date, over 700 mutations of the CFTR gene have been identified. A person with CF inherits two mutated copies of the CFTR gene. These mutations can either be homozygous, the same, or heterozygous, different mutations. The most common mutation is delta F508, accounting for approximately 70% of all mutations. Those homozygous for this mutation tend to be pancreatic insufficient.
What Does the Mutation Do?
The CFTR gene is a protein that functions as a chloride channel. A chloride channel helps maintain the proper balance of salt and water within a cell. A mutation in CFTR causes a dysfunction of the salt and water balance. This causes dehydration of the secretions (thick mucous) and excessive loss of salt in sweat.
What is a carrier?
A carrier is a person who only has one copy of the mutated gene. The parents of a child with CF each carry one CF gene and one normal gene. They have no symptoms and no disease.
How does CF occur?
When each of the parents contributes a gene to their child, they could pass on either their CF gene or their non-CF gene. Each pregnancy could result in one of three outcomes:
- A one in four (25%) chance that the child will have CF
- A two in four (50%) chance that the child will be a carrier
- A one in four (25%) chance that the child will not carry the CF gene
Can I find out if I have a CF gene?
At the present time, carrier testing is available through a DNA test. If a family member has CF and the gene mutation is known, discovery of the CF gene in other family members can be made with great accuracy. If the specific mutation is not known, the test will be done on the 70%-90% of the CF genes that are most commonly found, but the test won't be 100% accurate. The screening test for people without a family history of CF will also be done on the most common gene mutations, and so cannot be said to be 100% accurate.