One of the people who shaped the new field of research was the Danish genetic scientist Wilhelm Johannsen (1857–1927). It is virtually impossible to tell the history of genetics without him, since he came up with many of the concepts we now take for granted.

Prior to Johannsen’s work, people looked to Charles Darwin (1809–1882) to understand how species change over time. Darwin’s theory of evolution was groundbreaking, but at the time, scientists had yet to crack the code for the mechanisms of heredity and, thus, evolution. 

Around 1900, the mutation theory was proposed. It described how new species could appear as a result of sudden hereditary changes – mutations – and not just through the slow process of natural selection that Darwin had proposed. Now scientists had two different explanations for evolution, which led to intense debate.

Today we know that evolution is driven by the interaction of mutations and selection, but around 1900, scientists lacked a vocabulary for many of the mechanisms they were debating. Johannsen was critical of the Darwinian concept of incremental changes and argued that heredity should be understood as the result of distinct hereditary units. In this debate, he was more aligned with the mutation theory and its focus on sudden hereditary changes.

Through experiments with legumes, Johannsen arrived at an insight that continues to shape the science of biology to this day. Growing plants from a single type of bean, he consistently selected the largest and the smallest beans for new generations.

After multiple iterations, he was surprised to find that the largest beans did not necessarily produce larger plants. He concluded that the size of the plants was determined by hereditary factors.

Johannsen’s results could be explained by the concepts of genotype and phenotype, which he introduced in 1903. The genotype reflects the hereditary content, which remains constant, while the phenotype comprises the manifest properties, which may be affected by the environment. Significant differences in phenotype do not necessarily reflect differences in genotype. In other words, looks may be deceiving, since the environment may alter the visible manifestation without altering the hereditary material.

Later Johannsen gave biologists – and everyone else – a new, impactful word: gene. He used this term to describe the hereditary unit that transfers certain properties from one generation to the next – albeit without describing what this unit, the gene, actually is.

It was a fruitful scientific neologism, and to this day biologists are still debating how the term should be understood. Some researchers view the gene as a DNA unit with a clear molecular biological function, whilst others argue that the complexity of heredity makes it impossible to capture the meaning of the term in a single definition.

Johannsen also engaged in one of the most controversial debates of his time. During the early 20th century, genetics became closely associated with eugenics: the idea of improving human genetics through selective reproduction.

Many countries, including the United States, Germany and the Nordic countries, passed laws to allow the involuntary sterilisation of people who were thought to possess undesirable traits, often due to poverty, mental illness or disabilities. These measures were legitimised with reference to genetic science, which was believed to have the ability to identify hereditary flaws.

Johannsen was sceptical of this view. While not entirely rejecting the notion of government regulation of reproduction, he was sharply opposed to involuntary sterilisation.

In his opinion, the scientific knowledge of heredity was much too uncertain to justify such drastic intervention. He argued that society had no firm basis for determining who should be allowed to reproduce. His sceptical stance was an important scientific counterpoint at a time when many were seduced by the potential of eugenics. Today, it stands as an early testimony to the need for the ethically responsible use of genetic knowledge.

The chapter is written by Kristian Sjøgren.