The early decades of the twentieth century were a crucial period for genetics. By 1900, Mendel’s findings had been rediscovered but the physical nature of hereditary remained unclear until 1902–1904, when Sutton and Boveri independently proposed that chromosomes were the bearers of heredity based on their observations of chromosome behaviour during meiosis. This pioneering work laid the foundation of modern genetics.

A subsequent period of intense study into these newly discovered ‘units of Mendelian inheritance’, mostly by the labs of Wilson (who supervised Sutton) and Morgan, built on this work and demonstrated that the ‘fundamental’ laws of Mendel are sometimes broken. In some cases, the non-random segregation of just one or two chromosomes was observed, and this was later explained as the co-segregation of several sex chromosomes. However, it also led to the discovery of an extreme form of non-Mendelian inheritance — the non-random segregation of an entire haploid genome — and the phenomenon of germline-specific DNA.