What Are Single-Gene Disorders And How Can I Get Tested?
If you’ve ever heard of disorders like cystic fibrosis, hemochromatosis, Tay-Sachs, or sickle cell anemia, you’ve heard of what is known as single-gene disorders. This occurs when DNA changes in one particular gene.
Having one affected gene means these traits are easily traceable through families. The good news is, through tracing, the risk of passing them on to future generations can be predicted.
Testing for a single gene disorder can have a major impact on your health, as well as your children or future children. Single gene disorders affect about one percent of the population. Therefore, understanding your predisposition to one can empower you to take proactive measures toward protecting your health.
Single gene disorders are divided into three categories: dominant, recessive, and X-linked. Keep reading to learn more about each and how it can affect your health.
One category of single-gene conditions is referred to as dominant diseases. These occur when an individual possesses one mutant copy of a gene, known as an allele, along with one healthy copy. The allele overpowers the healthy copy of the gene and causes a disease's symptoms, despite the healthy gene's presence. In rare cases, an individual can get two copies of the dominant mutant gene, resulting in more extreme symptoms.
Dominant diseases are spread from the parent to the child and can pass throughout generations. An example of a dominant single-gene disorder is Huntington’s disease. This is a serious disease that causes the progressive breakdown of nerve cells in the brain. Remember, an individual only has to inherit one dominant allele to get the disease.
Recessive diseases are also single-gene disorders. These conditions can occur when an individual carries two mutant versions of a gene. Recessive gene problems only occur when an individual has two copies of a mutant gene. This is known as a homozygous state.
How does this occur? Both parents carry a single mutated gene (allele) and pass that mutated gene on to the child. Thus, the child receives two copies of the mutated gene, one from each parent. This can be more challenging to trace than dominant diseases because not all carriers of mutant alleles necessarily demonstrate symptoms of the disease. This can cause the disease to appear to skip generations.
To understand X-linked disorders, we might have to go back to your high school science class for a moment. Females possess two copies of the X chromosome (XX), while males have only one copy (XY). Typically X-linked problems are more common in men since they don’t possess another X chromosome to rely on for healthy function.
X-linked conditions, like other single-gene disorders, can be both recessive and dominant. Men will always pass their X chromosomes to the daughters, never their sons. This makes for a more simplified tracing of X-linked diseases since there is no male-to-male transmission.
X-Linked Dominant Disorders
To review, an X-linked dominant disorder indicates that a gene responsible for a genetic disorder is located on the X chromosome. These conditions don’t always affect males more than females. Additionally, the inheritance pattern will depend on whether the father or mother possesses the gene of interest. Any father with an affected X-linked dominant problem will pass it on to his daughters, but not to sons. A mother can pass it on to any of her children.
The good news is that X-linked dominant disorders are rare. Examples of these conditions include Rett syndrome, Alport syndrome, Fragile X syndrome, Aicardi syndrome, and more. The symptoms and effects of these diseases can be serious and life-threatening, so, genetic testing is recommended to understand if you or your children are at risk.
X-Linked Recessive Disorders
X-linked recessive problems are a single gene disorder. Two copies of the mutant gene are needed for the conditions to occur in females. Males only need one copy, making x-linked recessive problems more common in males.
The most common X-linked recessive disorders include red-green color blindness, Hemophilia A, Hemophilia B, and muscular dystrophy diseases like Duchenne and Becker. Additional diseases may include X-linked ichthyosis, X-linked agammaglobulinemia (XLA), and Glucose-6-phosphate dehydrogenase deficiency (G6PD).
It’s important to test and be aware of any X-linked recessive disorders. Having awareness will allow you to proactively test your children once they’re born. It’ll also help you pass information about these conditions to their children or other family members.