Genetics is about likelihood. Like a weather forecast which conveys that the probability of rain is 60%, which means that given the evidence, chances are higher that it will rain, though there is a 40% chance that it may not rain. Not all genetics is like that. In certain genetic conditions called monogenic conditions, the likelihood is 100%. In certain others, it varies all over the place between 99% to 1% .
Scientists often combine the likelihood of multiple genes to increase the certainty of the likelihood of a certain event. The proper way to interpret genetic likelihood is to become aware of your likelihood of various things, then verify through other pieces of information, if you can, and act on the side of caution.
For example, if genetic tests reveal that certain individuals have a higher likelihood of diabetes, he/she should look for a family history of diabetes and also monitor their blood glucose periodically. They should also take preventive action with regard to diabetes.
Not all conditions are easy to spot through family history, like diabetes, this is where genetic testing on a wide range of parameters such as nutrigenomic profile, fitness profile, and profile of other health conditions is highly helpful. In summary, genetic testings most useful to individuals who are knowledgeable, who are curious about themselves, who are proactive about their health, and who know how to understand and use this information in the correct way.