Written by Single-gene disorders are caused by a mutation in a single gene. These disorders are often rare, but some examples of common single-gene disorders include sickle cell anemia, cystic fibrosis, and Tay-Sachs disease.
Single-gene disorders can be inherited in one of two ways:
- Autosomal dominant disorders: In autosomal dominant disorders, only one copy of the mutated gene is needed to cause the disorder. If a parent has an autosomal dominant disorder, there is a 50% chance that each of their children will inherit the disorder.
- Autosomal recessive disorders: In autosomal recessive disorders, both copies of the gene need to be mutated in order to cause the disorder. If a parent has an autosomal recessive disorder, they are a carrier of the disorder and there is a 25% chance that each of their children will inherit the disorder.
There are also X-linked disorders, which are caused by mutations in genes on the X chromosome. X-linked disorders can be dominant or recessive. In X-linked dominant disorders, a female with the mutation will have the disorder, while a male with the mutation will be affected more severely. In X-linked recessive disorders, a female who is a carrier of the mutation will have a 50% chance of passing the mutation to each of her sons.
The symptoms of a single-gene disorder can vary depending on the type of disorder and the person affected. Some single-gene disorders may cause only mild symptoms, while others can be life-threatening.
There is no cure for most single-gene disorders, but there are treatments that can help manage the symptoms. The goal of treatment is to improve the quality of life for the person affected.
Here are some of the most common single-gene disorders:
- Cystic fibrosis (CF) is a genetic disorder that affects the lungs, pancreas, and other organs. People with CF have thick, sticky mucus that can clog their airways and block the flow of pancreatic enzymes. CF is caused by a mutation in the CFTR gene, which codes for a protein that helps control the movement of salt and water in and out of cells.
- Sickle cell anemia (SCA) is a blood disorder that causes red blood cells to become misshapen and sticky. This can lead to pain, infections, and other complications. SCA is caused by a mutation in the HBB gene, which codes for a protein called hemoglobin. Hemoglobin is the protein that carries oxygen in the blood.
- Tay-Sachs disease (TSD) is a rare genetic disorder that causes progressive damage to the nervous system. People with TSD typically die in early childhood. TSD is caused by a mutation in the HEXA gene, which codes for an enzyme called hexosaminidase A. Hexosaminidase A helps break down a fatty substance called GM2 ganglioside. When this enzyme is not working properly, GM2 ganglioside builds up in the brain, causing damage.
- Huntington’s disease (HD) is a progressive neurodegenerative disorder that causes uncontrolled movements, emotional problems, and cognitive decline. HD is caused by a mutation in the HTT gene, which codes for a protein called huntingtin. Huntingtin is a protein that helps regulate the function of nerve cells. When the HTT gene is mutated, huntingtin becomes abnormally long and clumpy. These clumps of huntingtin damage nerve cells and eventually lead to death of the cells.
- Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability. FXS is caused by a mutation in the FMR1 gene, which codes for a protein called fragile X mental retardation protein (FMRP). FMRP is important for the development of the brain. When the FMR1 gene is mutated, FMRP is not produced or is produced in low levels. This can lead to intellectual disability, learning disabilities, and other problems.
If you are concerned that you or your child may have a single-gene disorder, talk to your doctor. They can help you determine if genetic testing is right for you and can refer you to a genetic counselor if needed.