Genes are the individual building blocks that provide information for how the body is built. When a gene is damaged or mutated, information may be lost, and the result may be a genetic disease. An analogous situation would be baking from a recipe that had a portion damaged: How would a cake turn out if the text calling for sugar was unreadable? Because there are thousands of genes in the body, there are thousands of possible genetic diseases.
Single-Gene Diseases
Some diseases are caused by mutations in single genes. When this is the case, doctors can screen for this disease and definitively diagnose the disease if the mutation is present. Examples of single-gene diseases, according to the National Human Genome Research Institute (NHGRI), are Tay-Sachs, sickle cell disease and cystic fibrosis. In each of these diseases, the mutation of one gene causes detrimental effects in cells that require that gene. For example, the Mayo Clinic describes how people with Tay-Sachs disease are deficient in one enzyme that may ultimately lead to brain damage and even death.
Chromosomal Disorders
Every time an egg or a sperm is created, the DNA that makes up genes is replicated and divided. If this replication and division occurs incorrectly, some or all of a chromosome may be duplicated or lost. For example, according to NHGRI, Down syndrome is a disease in which there is a duplication of chromosome 21. The proteins encoded by the extra genes causes the deformities that are associated with Down syndrome. The mental and physical development of people with Down syndrome is slowed, according to MedlinPlus; they may also have heart disease, hearing problems and problems with the intestines, eyes, thyroid and/or skeleton. Another example of a disease caused by the misplacement of chromosomes is Turner syndrome. MedlinePlus reports that an incomplete or missing X chromosome causes physical development in girls to be impaired, and is called Turner syndrome. Females affected by Turner syndrome are characteristically short and are at risk for developing osteoporosis, cataracts and thyroid problems.
Cancer
The American Cancer Society (ACS) provides information about how mutations in certain genes important for the growth of cells can cause cancer. Genes that often cause cancer in a mutated form are called oncogenes, meaning cancer-causing genes. In their normal form, they help the body to function; in their mutated form, they cause cells to grow uncontrollably, causing cancer. Thus, cancer is a genetic disease because the basis for the disease is in the genes. For example, the ACS reports that, in some breast cancers, a gene that codes for a protein called HER2/Neu is mutated, and too much of the protein is made in the cell. The excess HER2/Neu causes the cells to uncontrollably divide, causing the cancer to grow and spread. Some types of cancers are chronic myeloid leukemia (CML), colon cancer, pancreatic cancer and lung cancer.
