GENETIC BASIS 35 



suppose), it is far from being pulverized. If each gene were 

 on a tiny separate chromosome and the germplasm was com- 

 posed of hundreds or thousands of such units, then we might 

 get complete recombination of specific differences in hybrid 

 populations. Germplasms, however, are not constructed in 

 that way or in anything like that way. The genes are car- 

 ried in long, protein, threadlike units, the chromosomes. 

 Within each differing chromosome pair in a hybrid nucleus, 

 only a very limited amount of exchange is possible. WTien 

 crossing over does take place between sister chromosomes, 

 leading to new intrachromosomal recombinations, the sister 

 chromosomes are each longitudinally bipartite. At each 

 point of exchange (chiasma) one thread (chromatid) of each 

 exchanges with one thread of the other, leaving the other 

 two threads in their original conditions. Gene exchange is, 

 therefore, only half of what had been supposed on cruder 

 h>T)otheses of crossing over. 



The effects of basic chromosome structure upon specific 

 and racial cohesion are of importance because they are uni- 

 versal and because in the aggregate they are powerful, much 

 more powerful than might be expected without precise cal- 

 culations. They are universal in that, with the exception of 

 such organisms as bacteria (for which the e^ddence is still 

 inconclusive), all germplasms in both plant and animal king- 

 doms have their genes in chromosomes, which (molecularly) 

 are long, threadlike structures. The cohesive effects of a 

 germplasm organized in this fashion are therefore always at 

 work. From the very beginnings of differentiation between 

 two varieties to the point where distinct genera may very 

 occasionally cross with each other, these inherent forces of 

 germinal cohesion are active, generation after generation. 

 When two species hybridize, in each successive hybrid gen- 

 eration and in each successive backcross these forces come 

 into play in every reduction division. 



The aggregate magnitude of the specific and racial co- 

 hesion resulting from linkage is based on the fact that specific 

 differences are the sum of all the differences between the 



