Fundamentally, a gene is the unit of inheritance that passes information from one generation to the next. This is the gene of Mendel’s experiments in which the characteristics of pea plants (Pisum sativum) were reliably and systematically transported from the parent generation to filial generations and so on. Mendel’s findings on pea plants that genes carry information from generation to generation in all organisms have been generalized to other organisms. In humans, genes carry information about, for example, blood types, hair lines, personality, intelligence, and psychopathology.
As research and thinking about genes continued after Mendel, locating genes on specific chromosomes became possible. In some pioneering work in the early 1900s, T.H. Morgan found that the white eye mutation in Drosophila melanogaster (Drosophila melanogasternormally have red eyes) was located on the X chromosome. In making that discovery, Morgan used mating...