A geneticist is a person — a scientist — whose work is genetics: how living things pass qualities to their children through genes. The -ist ending tells you it is the person, not the subject. A geneticist may look at plants, animals, or people; some work in hospitals, some in labs, some on farms. They may study why an illness runs in a family, how to grow stronger crops, or how two living things are related. Though actually, a short line you sometimes see (“the study of how qualities are passed on in genes”) is the meaning of genetics, not geneticist. If you want the study, say genetics; if you want the expert, say geneticist. It is a job-word, like artist or scientist, not the name of the field itself.