|My results for Canavan disease||My results for cystic fibrosis|
Most of the reports will have tables similar to the one on the left for Canavan Disease: you either have no copies of any genetic variants tested that are associated with Canavan Disease, one copy, or multiple copies (and therefore have Canavan Disease). Other diseases are slightly more complicated in that some genetic variants might not result in the same severity of disease. In the case of Cystic Fibrosis, shown on the right, there are some genetic variants that do not always cause disease, and some that cause more severe disease than others. One variant in particular is well-known to cause “classic” cystic fibrosis in people with two copies of it.
Types of Mendelian diseases
- Autosomal recessive – The gene involved resides on one of the 22 non-sex chromosomes (known as “autosomes). Two mutations impairing the gene (one on each copy of the chromosome) are required to cause the disease. Examples are cystic fibrosis, Tay-Sachs disease, and sickle cell anemia. Sometimes, carriers may have a mild form of the disease, with or without overt symptoms; this is the case with sickle cell anemia.
- Autosomal dominant – The gene involved resides on one of the 22 non-sex chromosomes. One mutation impairing the gene may be sufficient to cause disease. Examples are familial hypercholesterolemia and Marfan syndrome.
- X-linked recessive – The gene involved resides on the X chromosome. Two mutations impairing the gene (one on each copy of the chromosome) are required to cause the disease in women. Because men only have one copy of the X chromosome, one mutation is sufficient to cause the disease in men. X-linked recessive diseases are more common in men than in women for this reason. A well-known example is hemophilia.
X-linked dominant, Y-linked, and mitochondrial diseases also exist but they are fewer and more rare.Online Mendelian Inheritance in Man (OMIM) is a publicly available catalog of Mendelian disease information.
One special case in the Carrier Status category is Hemochromatosis, a disease that results from mutations in a gene that regulates iron absorption in the body. This is a fairly common genetic disease and many people with European ancestry will be carriers of one of the two mutations that 23andMe tests. It turns out that of those who have two copies of a disease-associated variant, few (30% of men and about 1% of women) will actually develop iron overload, the primary symptom of the disease.Note that while the Carrier Status reports do tell you if you have two copies of a disease-associated variant, it is very rare for someone to receive that result for most of the diseases due to their extreme rarity. (In these cases, it is even more unlikely for someone to be surprised by such a result, since you would almost certainly already know that you have the disease.) The main function of these reports, therefore, is to inform you of your status as a carrier or non-carrier for the genetic variants tested.For some populations, this information is especially useful. People with Ashkenazi Jewish ancestry have a higher chance of carrying genetic variants associated with certain diseases, including Tay-Sachs and Canavan disease. Sickle cell anemia is more common in those with African ancestry. And cystic fibrosis is one of the more common genetic diseases in populations with European ancestry. If you do turn out to be a carrier for something, you can learn more about which variant you carry in the Technical Report. Informed Medical Decisions, Inc., to give customers direct access to independent, board-certified genetic counselors that have been specifically trained in 23andMe’s reports.
Next post: Red pill, or blue pill? Learn about your likely reactions to certain drugs and medications with 23andMe’s Drug Response reports – When One Size Doesn’t Fit All.Check out our companion series, Ancestry at 23andMe, and other articles in 23andMe How-To.* Based on current offerings. Note that a few of the diseases included in the Carrier Status category are not strictly recessive (Alpha-1 Antitrypsin Deficiency, Familial Hypercholesterolemia Type B, Hypertrophic Cardiomyopathy (MYBPC3 25bp-deletion), Torsion Dystonia, and BRCA Cancer Mutations); in these cases, having one copy of a mutation may be associated with increased risk for the disease or related conditions.