CHROMOSOMES AND GENES 



By RICHARD B. GOLDSCHMIDT 



DEPARTMENT OP ZOOLOGY, UNIVERSITY OF CALIFORNIA, BERKELEY, CALIF. 



Cytologicaii work during the first dec- 

 ades of the cell theory up to about 1875 was 

 mostly concerned with establishing the gen- 

 eral ideas regarding the cell and its role in 

 the organization of living beings. Modern 

 cytology begins with the discovery of cell 

 division, fertilization, and the chromosomes 

 and their role in cell division and fertiliza- 

 tion (Biitschli, Strasburger, 0. Hertwig, 

 Flemming, Van Beneden, 1875-81). These 

 basic discoveries led immediately to the 

 theory, most ingeniously derived by Roux 

 (1883) and elaborated in detail by Weis- 

 mann (1886), that the chromosomes are the 

 bearers of the materials which are respon- 

 sible for the transmission of hereditary 

 traits and that these materials, or deter- 

 miners, are arranged in the chromosomes in 

 linear order. Though 30 more years were 

 needed for the experimental proof of these 

 conceptions, they have ever since been fore- 

 most in the minds of cytologists, who actu- 

 ally worked mostly with the purpose of 

 elucidating the role of the chromosomes in 

 heredity. This work culminated in Boveri 's 

 first experimental demonstration of the cor- 

 rectness of the chromosome theory of hered- 

 ity through his brilliant analysis of the fate 

 of dispermic sea-urchin eggs. With the be- 

 ginning of the century the period of Men- 

 delian heredity set in. It is true that the 

 first development of Mendelism after its 

 rediscovery was almost completely in- 

 fluenced by geneticists who were opposed to 

 the chromosome-theory (Bateson, Johann- 

 sen). But those investigators who had a 

 proper cytological background remained 

 true to the Roux-Weismann theory, as they 

 had every reason for doing after Sutton had 

 shown the complete parallelism between 

 Mendelian behavior and chromosomal dis- 

 tribution; and still more so, when the dis- 

 covery of the sex-chromosomes and the cor- 

 rect understanding of their behavior had 



shown that the genetic behavior of sex- 

 distribution which follows the type of a 

 Mendelian backcross, as first derived by 

 Bateson and Castle, is paralleled by the 

 special behavior of definite chromosomes. 

 Thus the idea of determiners, or genes, as 

 Johannsen proposed to call them, arranged 

 in linear order in the chromosomes and re- 

 sponsible for mendelizing traits, became 

 firmly established during the first years of 

 this century (except for the resistance of 

 Bateson and Johannsen, which broke down 

 only later under the pressure of the discov- 

 eries of the Morgan school). It was even 

 discussed (Boveri, Goldschmidt) under 

 which conditions a larger number of genes 

 than the haploid number of chromosomes 

 might mendelize, the solution being an ex- 

 change between homologous chromosomes. 

 It is generally known that the modern ex- 

 perimental foundation for Roux and Weis- 

 mann's classic theory of chromosomes and 

 genes was finally added by Morgan and his 

 group after the discovery of crossing over 

 and the consequent brilliant analysis (since 

 1910). The final proof of the chromosome 

 theory of Mendelian inheritance was fur- 

 nished (Bridges), the classic idea of the 

 linear order of the determiners in the chro- 

 mosome was put upon an experimental basis 

 (Morgan, Bridges, Muller, Sturtevant), 

 and a definite locus in the chromosome was 

 assigned to tne genes on the basis of their 

 cross-over values. Combining the genetical 

 facts with those found by the cytologists 

 regarding the structure of the chromosome 

 (Eisen, Montgomery, v. Winiwarter, Mc- 

 Clung, and many others), the chromosome 

 was conceived as a string of corpuscular 

 genes, arranged bead-like in a definite order. 

 It is this concept which we shall call here 

 the classic theory as developed from Roux 

 and Weismann to Morgan. If we look for 

 a comparable stage in the development of 



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