14S HAROLD H. PLOUGH 



VII. Failure of temperature to affect crossingover in first and third chromo- 

 some 190 



VIII. Acknowledgments 191 



IX. Summary 191 



X. Appendix 194 



I. INTRODUCTION 



The chromosome theory of linkage 



The phenomenon of linkage between Mendelian characters 

 was discovered in 1906 in the sweet pea. Since that time it 

 has been recognized in a number of plants and in a few animals. 

 By far the most detailed analysis of the phenomenon has been 

 made in a long series of studies, on linked mutant characters in 

 the pomace fly, Drosophila. It has been shown that in this 

 animal there are four groups of characters, the members of each 

 group showing linkage to each other, but not to the members of 

 any other group. Further than this it has been shown that the 

 relation between the factors in at least three of the four groups, 

 as shown by the percentages of crossingover between them, 

 can be expressed in the form of a linear series. In such a series 

 the factors are represented as being separated by the same number 

 of linear units as the observed percent of crossingover between 

 the characters. Recent work has accumulated a large body of 

 evidence which makes it appear that this is the only system 

 which will fit the observed results. The very obvious similarity 

 between the mechanism which these genetic results require 

 and the chromosomal mechanism makes the identity of these 

 two extremely probable. 



Variation in linkage intensity 



If the percentage of crossingover observed between any two 

 characters depends on the position of the factors in the chromo- 

 some, then the crossover values for those characters should be 

 the same in any experiment done under the same conditions. 

 It is never possible to say that the conditions are even approxi- 

 mately the same unless the parents whose crossingover ratios 

 are to be tested, are sibs, and are mated at the same time, and 



