These companies don’t sequence your entire DNA, they instead look for genetic markers or specific genes that are associated with particular diseases. However, it should be pointed out that just because you might have a particular genetic marker associated with a disease, does not mean that you *will* get that disease. It is for this reason that these companies are the subject of much scrutiny.
These companies are increasingly improving the amount of ‘genetic counselling’, which means that you will be given information so that you can understand what it is the evidence from your DNA profile can actually tell you – and if you had a particular marker that was very strongly associated with a disease, and puts you at high risk, they would only release this information to you after speaking with you.
It won’t be long before anyone can get their entire DNA genome determined, and at an affordable price; you’d be able to keep it on a memory stick in your pocket. But just knowing the sequence of bases that make up your DNA is really only the first of many steps required to understand how that sequence results in *you*. There are many other environmental factors that can govern how the information in the DNA is used by cells.
We have now sequenced the human genome but we don’t know what half of it means so although you can can your genome sequenced at the companies Jim mentioned it probably won’t give you an awful lot of information. Knowing your code is only one bit of the puzzle, there are lots of other factors that go into making you who you are.