The Effects of Unequal Crossing Over at the Bar 

 Locus in Drosophila 



A. H. STURTEVANT 



Reprinted by author's and publisher's permission 

 from Genetics, vol. 10, 1925, pp. 117-147. 



Very often m biological research the greatest co?itributio?i to our 

 knowledge of a process comes from the discovery of events resulting 

 in its disruptio?!. Thus, the discovery of mutations gave us co?isider- 

 able knowledge co?icer?ji?ig normal gefie actio?!, a?id the discovery 

 of lethal mutatio?is provided ijiformation about the process of muta- 

 tion itself. 



In Sturtevajifs paper we see this kind of event. Although the 

 author set out to study mutatio7i, he e?icoimtered a serious disruptio?! 

 of a?i ordi?mry process, a?id thereby made a discovery that has led 

 to co?isiderable alteration i?i the theory of the gene, hi fact, so?ne 

 authors have gone so far as to suggest that the classical idea of the 

 gene ?ieeded to be junked co?jipletely as a co?iseque?ice of the events 

 described i?i this paper. The disrupted process was that of ''crossing- 

 over,^'' which is the ?necha?2is?n whereby equal parts of ho?nologous 

 chromosomes are exchanged during meiosis. Sturteva?it found that 

 the parts exchanged were ?iot always equal, and that the co?ise- 

 que?ices of the i?iequality of exchange were striking indeed. 



I?i Sturteva?ifs simimary you will fl?id the stateme?it that'''. . . two 

 ge?ies lying i?i the sa?ne chro?noso?ne are more effective on develop- 

 ment tha?i the same two genes whe?i they lie i?i differe?it chromo- 

 so?iiesr This concept, a?id exte?isions of it, is now known as ''posi- 

 tio?i effecf (a tenn used by Sturtevant in this paper, page 141), and 

 it has destroyed the older concept of genes as "beads on a string,''' 

 which implies that where the gene is makes no difference i?i the way 

 it works. Sturteva?7fs work puts a new e?7Tphasis on the role of the 

 chro???osome i?i ?niclear co?itrol of celhdar ?netabolis?7i, a role perhaps 

 as important as that of siipervis'mg the per7nane?ice of equ-ality i?i 

 distributio?i of genes at the time of ?}ieiosis. 



INTRODUCTION comes necessary to distinguish changes 



that involve whole chromosomes (e.g., 

 If one thinks of mutations as non-disjunction or tetraploidy), 

 being simply inherited changes, it be- changes that involve several adjacent 

 124 



