It depends on the study that you have planned to work on. Eg if you are willing to study some gene responsive to a particular stress then you need to plan accordingly. It will be more clear if you explain your experiment a little bit.

The gene to isolation depends greately on goal of your work, What you mean under genomic studies? It is very wide concept, please note more particular goal of your research.

Could you rephrase your question, please - I have absolutely no idea what you are asking.

Yes, your question is way to vague. There could be countless genes responsible for a plant's medicinal qualities. Morphology of any plant/animal is not based on the expression of a single gene.

Thanks for editing and clarifying your question. This sounds to be a big challenge. To start such a project you should make sure FIRST that there really is a significant medical effect - traditional herbs may often just have a major placebo effect, or being used as psychosomatic therapy working via personal Interactions or via an the immense Price (so the Patient Never would damit it would not work); some Time ago Pepper was probably such a big medicine. But other plants, Salix or Digitalis lead to the development of real medicine. I could think of many Methods to find such a gene. It would be good to have a simple test System, like for finding antiinflammatory compounds some cell Culture of Human or Animal cells, but it dependa on what you would like To find. One way would be to try to make antibodies, or differential subtractive Expression libraries of expressed genes - comparing similar plants, or different Parts of the same plant, with and without the medical effect.