Following the 'rediscovery' of Mendel's work around 1900 the study of genetics grew rapidly and multiple new inheritance theories quickly emerged such as Hugo de Vries' "Mutation Theory" (1901) and the "Boveri-Sutton Chromosome Theory" (1902). Mendel's work also caught the attention of the German geneticist Valentin Haecker, yet he was generally dissatisfied the simplicity of Mendelian genetics as he believed that inheritance and the expression of various characteristics appeared to be much more complex than the proposed "on-off hypotheses". Haecker's primary objection was that Mendelian-based theories still failed to bridge the gap between hereditary units and phenotypic traits. Haecker thus set out to bridge this gap in his research program, which he called Phänogenetik ("phenogenetics"). He outlined his work in a special study "Entwicklungsgeschichtliche Eigenschaftsanalyse (Phänogenetik)" in 1918. 2018 thus marks the 100th anniversary of Haecker's seminal publication, which was devoted to the analysis of the phenotype and highlighted the true complexity of heredity. This article takes a specific look at Haecker and his work, while also illustrating how this often forgotten scientist influenced the field of genetics and other scientists.
Keywords: Genotype; History of genetics; Phenogenetics; Phenotype; Pluripotency; Valentin Haecker.