What Did Sutton See?: Thirty Years of Confusion Over the Chromosomal Basis of Mendelism

IN December 1902, 2 years after the rediscovery of Mendel's 1865 article, America's leading cytologist, Edmund Beecher Wilson, announced to the readers of Science that a graduate student of his at Columbia University had discovered the physical basis of the “Mendelian principle,” by which Wilson meant the segregation of Mendelian factors (Wilson 1902). In an article published the following year, which became a classic of genetics, this student, Walter Stanborough Sutton, explained how the behavior of chromosomes during meiosis—as he interpreted it in his observations of spermatogenesis of the grasshopper Brachystola magna—could explain not only Mendelian segregation but also Mendelian assortment. Sutton had done much of the cytological work as a student of Clarence Erwin McClung at the University of Kansas, but his interpretation of his results in light of Mendelism was done at Columbia. Supposing that Mendel's factors were located on chromosomes, he realized that the “association of paternal and maternal chromosomes in pairs and their subsequent separation during the reducing division [of meiosis]” could explain Mendelian segregation (Sutton 1902, p. 39) and that Mendelian factors on different chromosomes would assort independently if, at the division in which paternal and maternal chromosomes separate “any chromosome pair may lie with maternal or paternal chromatid indifferently toward either pole irrespective of the positions of other pairs” (Sutton 1903, p. 234). Although Sutton's conclusions gained general acceptance only gradually (Morgan 1910; Bateson 1922), his brilliant insight brought cytology and genetics together, initiating the long path of discovery leading to the present understanding of the chromosomal basis of inheritance. Although correct in its essentials, Sutton's analysis contained a critical flaw. As did others at the time, Sutton identified the wrong division of meiosis as the reducing division, the division in which paternal and maternal chromosomes separate. Sutton thought that the separation of paternal and maternal chromosomes and their independent assortment take place during the second meiotic division, while actually they (or, more precisely, their centromeres2) separate and independently assort at the first division. Estella Eleanor Carothers, who in 1913 presented the first clear cytological evidence for the independent assortment of chromosomes, nevertheless perpetuated Sutton's misconception. Carothers followed Sutton as a student of McClung at the University of Kansas and, like Sutton, studied spermatogenesis in Brachystola. Examining slides that Sutton had left behind as well as those she and McClung prepared, Carothers correctly described the segregation and independent assortment of certain recognizable chromosomes in the first division of meiosis. Nevertheless, she supposed that segregation and assortment take place in the second division for all other chromosomes. Confusion over which of the two meiotic divisions was reductive would linger for another 20 years. Despite histories of genetics crediting Sutton for explaining the chromosomal basis of Mendelism, no single work marks the clarification of the understanding of the meiotic divisions. Instead, clarification came from the gradual accumulation and integration of cytological and genetic observations over more than a quarter of a century.