Two broad categories are genetic tests and biochemical analysis. Genetic tests examine chromosomal DNA directly, hunting for everything from large-scale abnormalities to oddities in gene sequences. Some tests for breast cancer susceptibility stemming from mutations in BRCA1 or BRCA2, for instance, dissect the DNA itself in a process called sequencing. Other tests for breast cancer risk search for the extra-short proteins made by mutated versions of BRCA1 and BRCA2. If you are deciding whether to seek genetic testing, it's a good idea to consider the technology's limitations.
Some tests are more ambiguous than others.
For some diseases or conditions, the causative genes are known, act in a predictable way, and can be identified through direct tests. With Huntington's disease, for example, if you possess a specific mutation, y ou'll eventually have the disease. Other diseases vary widely in their expression and geneticists have not been able to pair the severity of disease with a measurable genetic configuration. With sickle cell, for example, you know you'll have the disease, but you won't know with what severity it will strike. Some people with the disease may have no hint of that particular gene variation.