In 1919 American geneticist Thomas Hunt Morgan of Columbia University published The Physical Basis of Heredity. In this book he first used the word gene. Previously he had used the term "Mendelian unit"or "factor".
"On the basis of genetic analysis, Morgan could present a number of characteristics of genes.
1. A gene could have more than one effect. For instance, insects that had the white-eye gene not only had white eyes, but also grew slower and had a lower viability.
2. The effects of the gene could be modified by external conditions, but these modifications were not transmitted to future generations. The gene itself was stable; only the character that the gene controlled varied.
3. Characters that were indistinguishable phenotypically could be the product of different genes.
4. At the same time, each character was the product of many genes. For instance, 50 different genes were known to afect eye color, 15 affected body color, and 10 affected length of wing.
5, Heredity was therefore not some property of the 'organism as a whole', but rather of the genes.
6. Genes of the pair did not ump out of one chromosome into another, but changed when the chromosome thread broke as a piece in front of or else behind them. Thus, crossing-over affected linked genes as groups and was a product of the behavior of the chromosome as an entity.
"Morgan's studies were based, to a great extent, on the availabity of a large number of mutants, bu the nature of the mutation process itself remained a mystery...." (Brock p. 15).