16 CALVIN B. BRIDGES 



CONCLUSIONS 



Only data which have been obtained under hke conditions can 

 be used in dealing with any problem such as comparative link- 

 age. If in the construction of a chromosome diagram the 

 value used for A-B was that calculated from first brood data, 

 and the value B-C was based on the total output of a female, 

 the prediction of new values from the diagram would be in- 

 accurate, unless the Unkage remained unchanged throughout 

 the life of the female. But if the diagram is constructed wholly 

 from first brood values, predictions will be accurate. The 

 practical point to be derived from this study is the breeding of 

 only one brood from each female, especially in the second chro- 

 mosome work. Any other condition as a standard could not 

 be fulfilled with certainty for any large body of data. 



Linkage has been explained by Morgan on a chromosome basis, 

 in accordance with the cytological evidence. It is assumed 

 that gens occupy fixed positions, linearly arranged within the 

 chromosome. In diploid groups each such linear series is 

 represented by two homologous chromosomes, A and a, every 

 locus in the one (A) corresponding to the same locus in its 

 homologue (a). Before maturation homologous chromosomes 

 become paired, side by side, and the members of each pair be- 

 come twisted about each other. At some of the points of con- 

 tact the two strands twist in two, as it were; moreover, the end 

 of A fuses to the other end of a as they lie opposed. Any gens 

 that were in strand A but on different sides of a chiasma point 

 will emerge in different strands because of the crossing-over, 

 and hence will be segregated to different gametes. It is obvious 

 that the closer together in the strand any two given gens lie, 

 the less is the chance that in any given maturation a chiasma 

 will occur between them, the chiasmas being distributed ac- 

 cording to chance. The basis of hnkage is that two gens lie 

 in the same chromosome so close together that in less than half 

 the maturing germ cells a crossing-over takes place between 

 them. 



There are two simple ways in which this scheme could be 

 modified to give the change in linkage here described. We 



