There can be little doubt that the first humans on Earth pondered the observation that children resembled their parents more than other members of the population. Unfortunately, however, we have no record of their ideas as to why this occurred. The Greek philosophers Hippocrates and Aristotle obviously thought extensively about this fact and developed theories to explain resemblances among relatives. Genetics as we know it today, based on the “gene theory of inheritance,” began with the work of Gregor Jobann Mendel.
Gregor Mendel (1822-1884) is appropriately called the “father of genetics” His precedent-setting experiments with garden peas (Pisum sativum), published in 1866, were conducted in the limited space of a monastery garden while he was also employed as a substitute schoolteacher. The conclusions that he drew from his elegant investigations constitute the foundation of today’s science of genetics. Why was Mendel so successful in discovering basic principles of genetics?
Mendel was not the first to perform hybridization experiments, but he was one of the first to consider the results in terms of single traits. Sageret in 1826 had studies the inheritance of contrasting traits. Others of Mendel’s predecessors had considered whole organisms, which incorporate a nebulous complex of traits; thus, they could observe only that similarities and differences occurred among parents and progeny, and so missed the significance of individual differences. Employing the scientific method, Mendel designed the necessary experiments, counted and classified the peas resulting from his crosses, compared the proportions with mathematical models, and formulated a hypothesis for these differences. Although Mendel devised a precise mathematical pattern for the transmission of hereditary units, he had no concept of the biological mechanism involved. Nevertheless, on the basis of his preliminary experiments and hypotheses, he predicted precise patterns of transmission of hereditary units and subsequently verified his predictions with the results of later crosses.
In 1900, Mendel’s paper was discovered simultaneously by three botanists: Hugo de Varies in Holland, known for his mutation theory and studies on the evening primrose and maize; Carl Correns in Germany, who investigated maize, peas, and beans; and Eric von Tschrmak-Seysenegg in Austria, who worked with several plants, including garden peas. Each of these investigators obtained evidence for Mendel’s principles from his own independent studies. They all found Mendel’s report while searching the literature for related work and cited it in their own publications. William Bateson, an Englishman, gave this developing science the name “genetics” in 1905. He coined the term from a Greek word meaning “to generate”.
In addition to naming the science, Bateson actively promoted Mendel’s view of paired genes. He used the word “allelomorph” subsequently shortened to “allete,” to identify members of pairs that control different alternative traits. Also during the early 1900s, a Frenchman, Lucien Cuenot, showed that genes controlled fur color in the mouse; an American, W.E. Castle, related genes to sex and to fur color and pattern in mammals; and a Dane, W.L. Johannsen, studied the influence of heredity and environment in plants. Johannsen began using the word “gene” from the last syllable of Darwin’s term “pangene”. The gene concept, however, had been implicit in Mendel’s visualization of a physical element or factor (Anlage) that acts as the foundation for development of a trait. These men and their peers were able to build on the basic principles of cytology, which were established between 1865 (when Mendel’s work was completed) and 1900 (when it was discovered). Why were Mendel’s important discoveries not recognized for such a long time (35 years) after the studies were completed and reported?
Wilhelm Roux had postulated as early as 1883 that chromosomes within the nucleus of the cell were the bearers of hereditary factors. The only model he was able to devise that would account for his observed genetic results was a row of lined-up objects duplicated exactly. To explain the mechanics of gene transmission from cell to cell, he therefore, suggested that nuclei must have invisible structures held in rows or chain that duplicated themselves when the cell divided. Constituents of the nucleus that seemed best designed to carry genes and fill these requirements were chromosomes. Experiments of T. Boveri and W.S. Sutton in 1902 brought confirming evidence that a gene is part of a chromosome. The theory of the gene as a discrete unit of a chromosome was developed by T.H. Morgan and his associated from studies on the fruit fly, Drosophila melanagaster. H.J. Mullar later promoted the merger of the two sciences that had contributed most to the chromosome theory-cytology (the study of cells) and genetics – as cytogenetics.When Mendel’s work was discovered in 1900, the English physician-biochemist Sir Archibald E. Garrod was studying congenital metabolic diseases in humans. One of these was alkaptonuria, which is caused by a block in the catabolism of the amino acids phenylalanine and tyrosine. Garrod proposed that alkaptonuria was due to a single defective gene that produced a nonfunctional product resulting in the metabolic block. Garrod’s results and his interpretation of these results were described in detail of his book Inborn Errors of Metabolism, published in 1909. Garrod’s concept of one mutant gene-one metabolic block was largely ignored by the scientific community until 1941 when Beadle and Tatum rediscovered this concept during their neoclassical work on Neurospora and refined it to one gene-one enzyme. Subsequently, this concept was revised to the more accurate concept of one gene-one polypeptide. Thus Garrod’s work, like Mendel’s, is an example of a major breakthrough that was sufficiently “ahead of its time” that its significance was not recognized by contemporary scientists and, thus, was not appreciated until its rediscovery many years later.