Genetics and cell biology is the science of heredity. It is studied at the level of whole organisms (classical or transmission genetics), the DNA itself (molecular genetics), or whole populations (population and evolutionary genetics). Geneticists use molecular approaches to investigate how genes determine physical traits, including susceptibility to diseases and inherited disorders. Cell biology focuses on the structure and function of the components of living cells (such as the cell membrane, the flagella responsible for cell movements, cell differentiation in developing organisms, abnormal cell division, and metabolic interactions between cell compartments).