156 CREIGHTON AND MCCLINTOCK 



expla'med by the hypothesis. The?! Morgan showed that a specific 

 locus could be assigjied to a specific chro?noso?}ie. Sturtevanfs dem- 

 onstration of a lifiear arrangement of ge?jes added another link to the 

 proof, and his 'work on bar-eye also substantiated the hypothesis 

 (how completely will be see?i in Bridges' paper on page 163). Now 

 we come to afi analysis that puts the final lijik i?i the chai?i, for here 

 we see correlatio?is betwee?i cytological evidence and genetic residts 

 that are so strong and obvious that their validity cannot be denied. 

 This paper has been called a landmark in experijuental genetics. 

 It is more tha?i that— it is a cornerstone. It is not an easy paper to 

 follow, for the iterns that require retentio?i throughout the analysis 

 are many, and it is fatal to one''s understanding to lose track of any 

 of them. Mastery of this paper, however, can give one the strong 

 feeling of being able to master almost ajiythijig else he ?mght have 

 to wrestle with m biology. 



A REQUIREMENT FOR THE GENETICAL 



Study of crossing-over is the hetero- 

 zygous condition of two allelomorphic 

 factors in the same linkage group. The 

 analysis of the behavior of homologous 

 or partially homologous chromosomes, 

 which are morphologically distin- 

 guishable at two points, should show 

 evidence of cytological crossing-over. 

 It is the aim of the present paper to 

 show that cytological crossing-over 

 occurs and that it is accompanied by 

 genetical crossing-over. 



In a certain strain of maize the sec- 

 ond-smallest chromosome (chromo- 

 some 9) possesses a conspicuous knob 

 at the end of the short arm. Its distribu- 

 tion through successive generations is 

 similar to that of a gene. If a plant 

 possessing knobs at the ends of both 

 of its 2nd-smallest chromosomes is 

 crossed to a plant with no knobs, cy- 

 tological observations show that in the 

 resulting Fi individuals only one mem- 

 ber of the homologous pair possesses 

 a knob. When such an individual is 

 back-crossed to one having no knob 

 on either chromosome, half of the off- 

 spring are heterozygous for the knob 

 and half possess no knob at all. The 

 knob, therefore, is a constant feature 

 of the chromosome possessing it. 

 When present on one chromosome and 



not on its homologue, the knob renders 

 the chromosome pair visibly hetero- 

 morphic. 



In a previous report ^ it was shown 

 that in a certain strain of maize an 

 interchange had taken place between 

 chromosome 8 and 9. The inter- 

 changed pieces were unequal in size; 

 the long arm of chromosome 9 was 

 increased in relative length, whereas 

 the long arm of chromosome 8 was 

 correspondingly shortened. When a 

 gamete possessing these two inter- 

 changed chromosomes meets a gamete 

 containing a normal chromosome set, 

 meiosis in the resulting individual is 

 characterized by a side-by-side synap- 

 sis of homologous parts. Therefore, it 

 should be possible to have crossing- 

 over between the knob and the inter- 

 change point. 



In the previous report it was also 

 shown that in such an individual the 

 only functioning gametes are those 

 which possess either the two normal 

 chromosomes (N, n) or the two inter- 

 changed chromosome (/, /'), i.e., the 

 full genom in one or the other ar- 

 rangement. The functional gametes 

 therefore possess either the shorter. 



1 A^IcClintock, B., Proc. Nat. Acad. Sci., 

 16:791-796, 1930. 



