 While this account of genes was derived from genetic evidence, i.e., Mendelian crosses, the results could be mechanistically explained with reference to chromosomes. Chromosomes had been suggested as containing hereditary material before, but evidence was not forthcoming until the early twentieth century. Cytologists had been able to observe the behavior of chromosomes the formation of gametes and draw analogies to the segregation and independent assortment of Mendelian units. They demonstrated the role of chromosomes in the determination of sex and made observations which suggested an exchange of materials between homologous chromosomes (Brush 1978; Carlson 1966: Ch. 6; Crow &amp; Crow 2002). On this basis, the Morgan group’s account identified linkage groups with chromosomes (Morgan et al. 1915; Vorms 2014). By the 1930s, cytological observations had reached a resolution that enabled researchers to correlate genetic linkage map loci with physical landmarks in the chromosomes, leading to cytological maps, which further facilitated the study of chromosomes in heredity (Falk 2004; Gannett &amp; Griesemer 2004).