Although the role of hereditary units (factors) in transfer of genetic characters over several generations in organisms was advocated by Gregor John Mendel, yet the mystry of the 'hereditary units was unravelled during early 1900s. In 1909, W. Johanson coined the term 'gene' that acts as hereditary units. However, early work done by several workers proposes various hypotheses to explain the exact nature of genes. In 1906, W. Bateson and R.C. Punnet reported the first case of linkage in sweet pea and proposed the presence or absence theory. According to them the dominant character has a determiner, and the recessive character tacks determiner. In 1926, T.H. Morgan discarded all the previous existing theories and put forth the paniculate gene theory. He thought that genes are arranged in a linear order on the chromosome and look like beads on a string. In 1928, Belling proposed that the chromosome that appeared as granules would be the gene. This theory of gene was well accepted by the cytologists. In 1933, Morgan was awarded Nobel prize for advocating the theory of genes. After the discovery of DNA as carrier of genetic informations, the Morgan's theory was discarded. Therefore, it is necessary to understand both, the classical and modern concepts of gene.
According to the classical concepts a gene is a unit of (i) physiological functions, (ii) transmission or segregation of characters, and (iii) mutation. In 1969, Shapiro and co-workers published the first picture of isolated genes. They purified the lac operon of DNA and took photographs through electron microscope.
In 1908, the British physician Sir E.R. Garrod first proposed one-gene-one product hypothesis. In 1941, G.W. Beadle and E.L. Tatum working at St Standford university clearly demonstrated one-gene-one enzyme hypothesis, based on experiments on Neurospora crassa. They made it clear that genes are the functional units and transmitted to progenies over generations; also they undergo mutations. They treated N. crassa with X-rays and selected for X-ray induced mutations that would have been lethal. Their selection would have been possible when N. crassa was allowed to grow on nutrient medium containing vitamin B6. This explains that X-rays mutated vitamin B6 synthesing genes. They concluded that a gene codes for the synthesis of one enzyme. In 1958, Beadle and Tatum with Lederberg received a Nobel prize for their contribution to physiological genetics.