The human genome consists of approximately three billion base pairs. Though very similar, each person has small variations referred to as single nucleotide polymorphisms (SNPs).
These variations are thought to be responsible for individual differences, including the predisposition to certain diseases. By having your family’s DNA to compare, doctors and scientists are able to map the shared mutations that are linked to disease in a family.
The more generations that can be compared the more accurate the predictions become. In the near future some minor DNA mutations can be corrected before they are able to cause disease. Knowing which ones are a risk then also becomes very important.