From Abstract Unit to Molecular Structure
A journey through the intellectual revolutions that shaped our understanding of heredity
The gene is one of the most central and yet most mutable concepts in modern biology. Anyone learning about heredity, disease, or evolution today inevitably encounters this concept. Yet what a gene is has radically changed throughout the history of science—from an abstract unit without physical attributes to a concrete, sequenced DNA structure. This conceptual history is not a dry chronicle but an exciting journey through the intellectual upheavals of the 20th century. It shows how new technologies and experimental brilliance have repeatedly redefined our understanding of life itself. The history of the gene is ultimately our own history, the search for an answer to the age-old question: What makes us who we are?
The story of the gene begins not with the word itself, but with the groundbreaking work of the Augustinian monk Gregor Mendel in the 1860s. In his monastery garden in Brno, Mendel investigated the laws of heredity through meticulous cross-breeding experiments with pea plants. His observations—uniformity in the first generation, the segregation of traits in a 3:1 ratio in the second generation, and the independent assortment of different trait pairs—led him to assume invisible, discrete "factors" responsible for the transmission of characteristics 8 .
Mendel thought of these factors as abstract units; he made no statements about their material nature. His work, which received little attention during his lifetime, was rediscovered around 1900 by the botanists Hugo de Vries, Carl Correns, and Erich von Tschermak and formed the basis for a new science 1 3 .
The Danish botanist Wilhelm Johannsen finally introduced the term "gene" in 1909 2 3 . He derived the word from the German "Pangens," a term coined by de Vries, which in turn traced back to Darwin's pangenesis theory. Johannsen deliberately chose the term to be concise and without preconceived notions about the physical nature of the underlying element 2 . For him, the gene was initially a "calculation unit," an abstract concept for describing inheritance phenomena that he separated from its concrete physiological effect.
Along with the gene, Johannsen also introduced the fundamental term pairs "genotype" and "phenotype." This distinguished for the first time between an organism's genetic makeup (genotype) and its observable appearance (phenotype), which is shaped by the interaction of genetics and environmental factors 2 3 .
| Scientist | Time Period | Contribution | Concept of Hereditary Factor |
|---|---|---|---|
| Gregor Mendel | 1860s | Formulation of inheritance rules through pea experiments | Abstract "factors" or "elements" |
| Hugo de Vries | Around 1900 | Rediscovery of Mendel; term "Pangens" | Physiological units of heredity |
| Wilhelm Johannsen | 1909 | Coined the terms "gene", "genotype" and "phenotype" | Abstract "calculation unit", separated from physiological effect |
| William Bateson | 1905 | Coined the term "genetics" for the new field of research | - |
In the early 20th century, Thomas Hunt Morgan and his colleagues working with fruit flies (Drosophila) connected Mendelian genetics with cytology. They were able to show that genes are arranged linearly on chromosomes and created the first genetic maps by analyzing linkage groups 3 . In this "classical period," the gene was no longer viewed as an abstract unit but as a dimensionless point on a chromosome 3 . The work of Morgan and his team gave the gene a physical location, thus making it accessible to a mechanistic understanding.
In the phase described as "neoclassical," which began in the 1930s, the gene concept was further refined. Through the work of George Beadle and Edward Tatum, the "One Gene - One Enzyme" hypothesis crystallized 1 . This stated that each gene is responsible for the production of a specific enzyme. The gene was no longer just a point but a linear sequence, an instruction for a functional building block of the cell.
The crucial turning point was the discovery that DNA (deoxyribonucleic acid)—and not protein—is the carrier of genetic information 1 . This led directly to the burning question: How is DNA structured to fulfill this function?
| Period | Time Frame | Gene Concept | Key Discovery |
|---|---|---|---|
| Classical | ~1900-1930s | Gene as dimensionless point on a chromosome | Linking Mendelian genetics with chromosome theory |
| Neoclassical | ~1930s-1960s | Gene as linear DNA sequence coding for a protein ("One Gene - One Enzyme") | DNA is carrier of genetic information |
| Modern | From ~1970s | Recognition of more complex structures (introns, exons, non-coding genes) | The gene is not an autonomous unit but part of complex networks |
The elucidation of the three-dimensional structure of DNA by James Watson and Francis Crick in 1953 was a milestone that forever changed the concept of the gene. This success was preceded by years of intensive research and a productive, albeit conflict-ridden, scientific race.
The crucial experimental method was X-ray crystallography. In this process, a purified DNA preparation is formed into a crystal and bombarded with X-rays. The rays are deflected (diffracted) as they pass through the crystal and produce a characteristic diffraction pattern on a film. From the arrangement and intensity of the spots in this pattern, experts can deduce the three-dimensional arrangement of atoms in the molecule .
Rosalind Franklin, a brilliant physical chemist, worked in Maurice Wilkins' group at King's College London. She had specialized in X-ray diffraction techniques and produced exceptionally clear images of DNA fibers. Her masterpiece, the image known as "Photo 51", unmistakably showed the pattern of a helix . Without Franklin's knowledge, Wilkins showed this photo to James Watson. For Watson and Crick, who were building physical models of DNA in Cambridge, Photo 51 was the crucial clue. It not only confirmed the helical structure but also allowed precise measurements of diameter and pitch .
Based on Franklin's data as well as Erwin Chargaff's realization that in DNA the bases adenine and thymine as well as guanine and cytosine always occur in equal amounts, Watson and Crick built their famous model. It showed DNA as a double helix, with two strands of sugar and phosphate winding around each other, connected by rungs of complementary base pairs (A-T and G-C) 1 .
This elegant structure immediately explained how genetic information can be stored and copied during cell division: The two strands can separate, and each serves as a template for the synthesis of a new, complementary strand. The gene had now finally become a molecular building block—a specific sequence of nucleotides in a DNA molecule.
Visual representation of the DNA double helix structure with complementary base pairs
| Base | Abbreviation | Complementary Base | Bond Type |
|---|---|---|---|
| Adenine | A | Thymine (T) | Two hydrogen bonds |
| Guanine | G | Cytosine (C) | Three hydrogen bonds |
| Thymine | T | Adenine (A) | Two hydrogen bonds |
| Cytosine | C | Guanin (G) | Three hydrogen bonds |
| Reagent/Material | Function in the Experiment |
|---|---|
| Signer DNA | Highly purified DNA preparation from calf thymus, provided particularly clear X-ray diffraction patterns . |
| X-ray Crystallography | Main method for determining the spatial arrangement of atoms in a crystal . |
| Physical Molecular Models | Metal parts and wire constructions used by Watson and Crick to spatially test possible DNA structures . |
| Highly Hydrated DNA Fiber ("B-Form") | State of DNA mimicking the natural environment, which first revealed the characteristic helical structure (Photo 51) . |
The discovery of the double helix did not mark the end but a new beginning. In the following decades, the simple picture of a gene as a continuous protein-coding sequence became increasingly complex.
It was discovered that the coding regions (exons) in eukaryotic genes are interrupted by non-coding sections (introns) 1 .
A single "gene" can produce different proteins by differently joining together the exons 1 .
The modern, molecular understanding of genes is therefore more multifaceted. A gene is now often defined as "a union of genomic sequences encoding a coherent set of potentially overlapping functional products" 1 . It is no longer viewed as an isolated, autonomous unit but as a node in a dense, interacting network whose expression is finely tuned by complex regulatory systems 3 8 .
The journey of the gene concept from Johannsen's abstract unit to today's network-like understanding is an impressive example of the dynamics of scientific progress. Each new definition was not the end of the search but opened the door to new, more profound questions. The gene has proven to be a "concept in flux" that constantly adapts to new findings 8 .
Even today, the history of the gene is not complete. Research in epigenetics, the question of the function of large parts of "junk DNA," and the discovery of ever new regulatory RNA molecules force us to continuously rethink our understanding of what a gene is and does. The biological term 'gene' remains one of the greatest and most fascinating ideas in science—a constantly changing key to understanding life itself.