 Mendelian research soon showed the independent assortment of factors in Mendel’s experiments to be a special case, not a law. Departures from independent assortment of traits allowed the identification of linkage groups, in which variants of two or more traits co-occur, which eventually were shown to correspond to the proximity of their place or locus on distinct chromosomes. (Indeed, Mendelian research helped expose properties of the chromosomes, such as their role in sex determination, and investigate many other biological issues. Waters [2004] criticizes philosophers who interpret Mendelism solely in terms of establishing a theory of inheritance: “Posing and solving carefully orchestrated pedigree problems was the means, not the ends, of classical genetics”.) Johannsen’s resistance to the idea that “discrete particles of the chromosomes are ‘bearers’ of special parts of the whole inheritance” (1911: 131–2) was shared by others (Deichmann 2014), but such reservations did not hold back the rise of Mendelianism to a dominant position in research into heredity well before the material make-up and functioning of those particles—the genes—was revealed in the 1950s. The particulate view was affirmed by producing heritable alterations in phenotypes after bombarding organisms with high-energy ionizing radiation. 