CREIGHTON AND MCCLINTOCK 



dence that gene mutation may be to 

 any extent controllable, but the mag- 

 nitude of the heat effect, being similar 

 to that found for chemical reactions 

 in general, is too small, in connection 

 with the almost imperceptible "nat- 

 ural" mutation rate, for it, by itself, to 

 provide a powerful tool in the muta- 

 tion study. The result, however, is 

 enough to indicate that various factors 

 besides X-rays probably do affect the 

 composition of the gene, and that the 

 measurement of their effects, at least 

 when in combination with X-rays, will 

 be practicable. Thus we may hope that 

 problems of the composition and be- 

 havior of the gene can shortly be 

 approached from various new angles, 

 and new handles found for their in- 

 vestigation, so that it will be legitimate 

 to speak of the subject of "gene physi- 

 olog\%" at least, if not of gene physics 

 and chemistry. 



In conclusion, the attention of those 

 working along classical genetic lines 

 may be drawn to the opportunity, 



155 



afforded them by the use of X-rays, of 

 creating in their chosen organisms a 

 series of artificial races for use in the 

 study of genetic and "phaenogenetic" 

 phenomena. If, as seems likely on gen- 

 eral considerations, the effect is com- 

 mon to most organisms, it should be 

 possible to produce, "to order," 

 enough mutations to furnish respec- 

 table genetic maps, in their selected 

 species, and by the use of the mapped 

 genes, to analyze the aberrant chro- 

 mosome phenomena simultaneously 

 obtained. Similarly, for the practical 

 breeder, it is hoped that the method 

 will ultimately prove useful. The time 

 is not ripe to discuss here such pos- 

 sibilities with reference to the human 

 species. 



The writer takes pleasure in ac- 

 knowledging his sincere appreciation 

 of the cooperation of Dr. Dalton 

 Richardson, Roentgenologist, of Aus- 

 tin, Texas, in the work of administer- 

 ing the X-ray treatments. 



\^ 



A Correlation of Cytological and Genetical 

 Crossing-Over in Zea mays 



HARRIET B. CREIGHTON a?ld BARBARA MCCLINTOCK 



Reprinted by authors' and publisher's f>ermis- 

 sion from Proceedings of the National Academy 

 of Sciences, vol. 17, 1931, pp. 492-497. 



The Siitton-Boveri Hypothesis (page 21) has beeji successively 

 stre?igthejied by several of the papers in this collection. First Bateson 

 afid Punnett unknoivitigly demoiistrated linkage, a phenome?wn best 



