Cystic Fibrosis and Your Genes
by Callan Russell
This article examines the contribution that your genes have on your health. More specifically, how your genetic makeup affects whether or not a certain genetic disease is expressed. Genetic inheritance is explained through a concrete example of a single-gene genetic disease, Cystic Fibrosis. Additionally, this article expresses the importance of knowledge of your genetic makeup and the role that genetic testing can play in an individual’s life.
Keywords: Cystic Fibrosis, Evolution, Genetics, Health, Genetic Counseling, Family Planning
Human disease can alter lives in permanent, and often heartbreaking, ways. Most people have a story about how they have been affected by disease. In a world where tragedy is at the forefront of our personal lives via news stories and GoFundMe pages, it is almost impossible not to be touched by disease. In the harsh reality where many diseases end in an individual’s death, how does disease persist?
While there are various causes that lead to disease, one important contributing factor may be your genes. A gene is made up of DNA which is transferred from each parent to their offspring to determine some characteristic.  Therefore, genetic diseases can be passed down from parent to offspring because each parent gives a copy of his/her genes to his/her child.  If the copies from both parents are identical for a given gene, then the child is considered homozygous for that gene; but if the two copies are different, then the child is considered heterozygous. This concept is better known as genetic inheritance and is the process by which genetic diseases are passed on to offspring.
Cystic Fibrosis (CF) is an ideal model for studying genetic inheritance as it is associated with a single gene, which makes it a more straightforward example than diseases associated with multiple genes.  CF causes damage to the lungs and digestive system via mucous secretions that cause obstructions in these organ systems leading to inflammation, tissue damage, and disseminated destruction throughout the body. This mucous that causes physical damage of the airway also predisposes patients to developing secondary bacterial infections, which can result in respiratory failure. 
There are an estimated 20,000 genes in the human genome, and just one of those genes is associated with CF.  The gene associated with CF codes the cystic fibrosis transmembrane conductance regulator (CFTR) protein.  This protein helps maintain a balance of salt and water throughout the body. When this protein is not working correctly because its gene has a mistake in its sequence, a mucous buildup is experienced all over the body in many different cells and CF results.
CF is considered a recessive genetic disease. In order to develop this disease, a person must receive two copies of the gene associated with the disease.  This means that affected individuals are homozygous because they inherited the same copy from both parents. An individual is heterozygous if they only have one mutant copy of the gene. Heterozygous individuals are called carriers because they carry one copy of the bad gene but do not show symptoms. 
Changes in the DNA sequence can be both inherited and acquired. Whether or not that mutation persists in the population is then determined by mechanisms of evolution.  Natural selection is a process of evolution in which individuals better adapted to their environment have higher reproductive success.  Due to the possession of advantageous traits, these individuals have a higher rate of survival, resulting in their offspring having a similar, high rate of survival. More of these offspring survive and pass on their advantageous traits. CF results in traits that are not advantageous for an individual’s survival, therefore, natural selection acts against CF.
How can genetic diseases like CF persist in the population if they are selected against by natural selection? The answer may be hidden within carriers of the disease who do not develop symptoms because they do not develop CF. These individuals experience no negative effects from carrying a bad copy of this gene and assist in the “survival” of this genetic disease. They risk passing on their single copy of the bad gene to any children that they have. This pattern continues until a homozygous individual is born.
While carriers are asymptomatic, there are still ways to determine if you are a carrier for the CF gene. Genetic testing is an accessible screening process that tests your genes for the presence of the copy that causes disease.  These tests are sometimes covered by insurance and sequence the specific genes in question in the patient’s DNA. Genetic testing serves as the greatest preventative tool to allow individuals to make informed decisions when planning for their family’s future. A person with a family history of CF should strongly consider undergoing a genetic test to screen for CF when planning to have children. If a positive genetic test is received, other options can always be considered in the process of family planning such as adoption, surrogacy, or donors.
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The author thanks Dr. Tessa Andrews, Margot Popecki, and Lydia Anderson for helpful comments throughout the writing process.