Different alleles produce variations in inherited characteristics such as eye color or blood type.
Not all cases are inherited, but genes have been found for familial forms of Alzheimer's disease. Human cells have 22 pairs of autosomes. Two strands of DNA are held together in the shape of a double helix by the bonds between their base pairs. Carriers do not usually develop disease but can pass the mutated gene on to their children. Carrier testing is designed for healthy people who have no symptoms of disease, but who are known to be at high risk because of family history.