 The early “Mendelians” employed and expanded Mendel’s experimental system, notation, and terminology (Shan 2020). They codified Mendel’s regularities as “laws” of heredity, while at the same time investigating exceptions to uniformity or dominance (e.g., “blending” characters), to the ratios of segregation (e.g., due to “composite” characters, which came to be seen as influenced by several factors), and to independent assortment in crosses involving several characters (e.g., because characters appeared to be “coupled”). Mendel’s experiments, perceived as exemplars for the study of variation and heredity, named “genetics” by William Bateson (1861–1926; 1907), played an important role for the formation of both the field and its central concept, the gene. Bateson emphasized the idea that discrete “unit-characters” could be identified in organisms, which occurred in alternative forms or “allelomorphs”. In crosses these characters segregated in Mendelian ratios. This indicated and was explained by assuming the “purity of gametes”, i.e., the fact that reproductive cells can transmit only one form of a character. When two gametes that were alike with respect to one character united, the resulting zygote was “homozygous” for this character, otherwise it was “heterozygous”. While these observations were initially discussed mainly in terms of characters and types of gametes or the gametes’ ability to transmit characters, Bateson came to think of the dominant allelomorph as being determined by some “factor” present in the gamete, while the recessive character resulted when the factor was absent in both gametes (Darden 1991: Ch. 6; Schwartz 2002). “Factor” thus came to be used by many Mendelians similarly to Mendel’s “elements”. The term referred to whatever made two gametes different regarding the inheritance of characters, but it was not connected to any specific theory regarding the physical nature of the hereditary material.