Molecular biology and molecular genetics are closely related fields of study that focus on the structure, function, and behavior of molecules within living organisms. Molecular biology is a broad field that encompasses many different sub-disciplines, including molecular genetics. Molecular genetics is a sub-discipline of molecular biology that focuses specifically on the role of genes and how they are expressed and inherited within living organisms. In other words, molecular genetics is a more specialized field within molecular biology that focuses on the genetic makeup of cells and how genes are passed down from one generation to the next.