100 years ago: Walter Sutton and the chromosome theory of heredity.

EVERY student of elementary genetics learns of Walcould not be disregarded and stand today as essentially ter Sutton (1877–1916). Sutton was the first to correct. At last, cytology and genetics were brought into point out that chromosomes obey Mendel’s rules—the intimate relation, and the results in each field began to first clear argument for the chromosome theory of have strong effects on the other.” It was not until a heredity. This year marks the centennial of Sutton’s decade later, however, that independent assortment was (1902) historic paper, surely the most important genetic definitively proven. Another McClung student, Eleanor event in that year. Sutton worked with grasshopper chroCarothers (1913), found a pair of heteromorphic aumosomes, and it was in this paper that he showed that tosomal homologs in the grasshopper Brachystola, in chromosomes occur in distinct pairs, which segregate which one homolog was larger than its mate. She at meiosis. His concluding statement reads: “I may fishowed that these segregated independently of the X nally call attention to the probability that the association chromosome in the meiotic spindle; the large member of paternal and maternal chromosomes in pairs and of the pair went with the X in 154 cases (51.3%) and their subsequent separation during the reducing divithe small one in 146 cases (48.6%). Later she extended sion . . . may constitute the physical basis of the Mendethis to more than two chromosomes (Carothers 1917). lian law of heredity” (p. 39). In his sophomore year, Sutton became a student of Sutton, a Kansas farm boy, had been a student of C. E. McClung at the University of Kansas. McClung C. E. McClung (1870–1946), a prairie pioneer cytologist (1901) had discovered the “accessory chromosome” and at the University of Kansas. McClung took advantage of correctly concluded that it was related to sex. The X the great abundance of grasshoppers in that state to chromosome designation that is now standard grew out make them pivotal for cytological study and to found a of his custom of labeling this chromosome, X, in drawschool of grasshopper cytologists. In 1912 he moved to ings. But he wrongly concluded that it was male dethe University of Pennsylvania to continue his distintermining. This was a reasonable conclusion, because of guished career. One of the best known of his students an earlier error by none other than his student, Sutton. was W. R. B. Robertson, of “Robertsonian translocaSutton had found a species of grasshopper, Brachystola tions.” magna, in which the male meiotic chromosomes are In a subsequent article, Sutton (1903) gave a much particularly large and clear. But oogenesis in the female fuller development of the hypothesis, including the view was another story. Cytology was a primitive art in those that the different chromosome pairs orient at random days and chromosome counts were very uncertain. Conon the spindle, thus giving rise to Mendelian indepensequently, Sutton erroneously reported 22 chromosomes dent assortment. The paper was characterized by clear in the female, rather than the correct number, 24. Grasslogic and by beautiful chromosome drawings. As Sturhoppers do not have a Y chromosome and, since the tevant (1965, p. 38) says of Sutton’s work: “With this male clearly had 23, McClung’s incorrect conclusion paper, this phase of the history is finished. The concluwas logical enough. The issue was eventually settled by sions were not at once generally accepted, but they Nettie Stevens (1905), who studied Tenebrio, a beetle.