Around 1900 the study of genetics really took off. One pioneering group was that of the noted biologist Thomas Hunt Morgan, a faculty member of Columbia University in New York City. Around this time the fruit fly, Drosophila melanogaster, became the geneticist’s research animal of choice. These little flies have many characteristics that are easy to observe under a magnifier or a low-power microscope and that are inherited in simple ways. Fruit flies are fairly easy to maintain in the laboratory, have short reproductive cycles, and produce many offspring. These features are ideal for doing genetic research.
Morgan attracted some very gifted students—A. H. Sturtevant, Calvin Bridges, and H. J. Muller—who would go on to become first-rate geneticists. Morgan was awarded the Nobel Prize in 1933, and Muller received it in 1946, for discoveries in genetics. Along with others these pioneers established the physical basis for inheritance: Genes are carried in structures called chromosomes, of which humans have 46. They confirmed and extended Mendel’s basic rules and also found a number of new genetic phenomena, including the process of crossing over, how mutations can be induced, and the mechanism of sex-linked inheritance.
Molecular genetics is the study of the molecular mechanisms within DNA and RNA that explain the underlying basis of inheritance. One could argue that the era of biochemical genetics, or molecular genetics, began with the studies of George Beadle and E. L. Tatum using the bread mold Neurospora crassa as a research organism beginning in 1941. They were interested in establishing that a gene (within the DNA) was responsible for a specific protein or enzyme, a concept known as the “one gene—one enzyme” hypothesis. The methods to do that kind of detailed biochemical work in the fruit fly were not available at the time, so Beadle and Tatum chose bread mold.
Bread mold has several useful features for genetic research. It can be grown in simple, laboratory-prepared media in petri dishes. Mutations can be induced in the mold that change its nutritional requirements in simple, straightforward ways. This information can then be used to figure out that a gene controlling a particular enzyme in a nutritional pathway has been altered. Their work allowed them to formulate the one gene–one enzyme hypothesis, and in 1958 they, along with Joshua Lederberg, were awarded the Nobel Prize for their discoveries in molecular genetics.
Finally, one of the most famous experiments in the history of biology was conducted by Oswald T. Avery, Colin M. MacLeod, and Maclyn McCarty. In 1944 they showed conclusively that deoxyribonucleic acid, or DNA, is the genetic material. Their cleverly designed protocol, described in the sidebar “DNA Is the Genetic Material,” involved infection of bacterial cells by a virus that normally infected them.
