FRAGMENTATION AND TRANSLOCATION 



319 



was of three sizes in three flies, and genetic data showed that in each 

 successively larger deletion one more known gene had been eliminated. 

 Moreover, the order of disappearance was in exact accordance with the 

 order previously assigned to these genes in the chromosome map built up 

 from crossover data. This amounts to a direct proof of the essential 

 correctness of the map of this region of the chromosome, so far as the 

 serial order of the genes is concerned. 



Another mode of behavior on the part of free fragments is that in 

 which they act regularly in the somatic divisions but manifest irregular- 

 ities at the time of meiosis, in part because of deficient synaptic reaction. 

 When fragments are present in addition to a fully normal chromosome 

 complement, they may be eliminated altogether or pass to some of the 



a ,. 



Fici. 182. — Synaptic configurations after inversion and deletion in Zea. a, after 

 inversion of median portion of one member; cf. Fig. 181, A-C. b, after deletion of portion 

 of shorter arm of satellited chromosome; cf. Fig. 181, D. c, after deletion of small median 

 region; cf. Fig. 181, F. {After McClintock, 19316.) 



spores (or gametes) only. In this way gametes with or without extra 

 fragments may arise. When there are several fragments in the sporocyte 

 they may sometimes synapse among themselves, as in certain strains of 

 Tradescantia (Darlington, 19296). In other cells they seem to be 

 attached to the normal bivalents in various ways. They may be dis- 

 tributed unequally to the spores, or, because of lagging, they may be 

 partly or completely lost, especially when unsynapsed. Not infrequently, 

 however, one or more are transmitted through the gametes to the next 

 generation. 



Finally, fragments may sometimes pass regularly through the entire 

 life cycle, behaving as extra independent chromosomes. This may be 

 expected when, for example, there are in the somatic cells two fragments 

 which are able to react properly in equational nuclear division because 

 they have spindle-attachment regions and to undergo synapsis regularly 

 because of their homologies. 



The genetic effect of the presence of extra chromosomes or fragments 

 is known as duplication, i.e., certain genes are present in excess. Con- 



