There are three characteristics of genetic tests which make them different. Firstly, genetic tests by and large are about chances and risk. They rarely give absolutes. Mostly a genetic test can be interpreted as giving an individual, say, a 50 per cent chance of developing a certain disease over his or her lifetime. Of course many medical tests deal in uncertainties.
Genetic tests are just more uncertain than most.
Second, and perhaps most importantly, the results of genetic tests are applicable not just to the individual tested, but they have implications for the other members of the family as well. These implications also apply to future generations within that family. By and large, in order to accept the need for a medical test or a medical course of treatment, it is important for patients to understand all the implications of that test or treatment. Such informed consent may be difficult to obtain from an individual if we do not know all of the future implications. It is impossible to obtain consent from all family members who might be affected, especially those yet unborn.