The definition of Molecular Genetics is the subfield of biology that studies the molecular structure and function of genes.
Humans traits are influenced by the genes we have in our body, which is why heredity influences things like body weight. However, humans don't have "obesity genes" but it's just that some genes might be less efficient in telling the brain that they're full or they might not efficiently dictate how the body makes extra calories into fat.
These days, scientists and researchers have been doing research of genes that put people at risk for genetically influenced disorders through finding families and drawing out their blood. They examine their DNA and find differences, then try to extract the chromosomes that have relationships to these genes.
Due to such research, we are now able to reveal who is at risk for at least a dozen diseases. With more research, we will soon be able to find out genes that influence depression, alcoholism, obesity and etc. Not only this, we'll be able to give soon-to-be parents a read-out on how their fetus' genes differ from the normal pattern and what this means. This is dangerous, however, because if we label people since the moment they're born as "at risk for a learning disorder" or some other name, they might face discrimination later. Furthermore, research is showing that we might be able to choose the baby's sex before conception with reasonable chances of success.
This is causing a lot of controversy, because although molecular genetics and development might be convenient, in that we get to design our babies, it still has ethical issues, and as a result, it will face much opposition and we also might lose future Tolstoys or Van Goghs by changing the babies' genetics.