236 ESSENTIALS OF BIOLOGY 



Let ahcdefghht such a row in one chromosome and let 

 another chromosome carry, or be composed of an analogous 

 series, iklmnop q. When synapsis occurs homologous chromo- 

 somes pair, so that the conjoined structure may thus be repre- 

 sented, .77 ■^ ^ . Now disjunction occurs in the reducing 

 I k L m n p q -' 



division and the two rows of genes come apart again, but before 

 this occurs it may happen (and there is said to be evidence that 

 it does happen) that the two mating chromosomes become 

 partially twisted round each other (i). 



2 



Fig. 32. — Diagram of the Events in a Simple " Crossing-over " of the 



Chromosomes. 



In coming apart, in the disjunction, (2) the chromosomes may 

 break where they cross over each other and the result will be as 

 in 2, Fig. 32. Apparently the twisting of the chromosomes, and 

 their subsequent breaking apart are events that occur at random 

 and it will be seen that the linkages of groups of genes are events 

 that occur at random. It will be seen also that by regarding the 

 chromosomes as linear arrangements of genes, each of the latter 

 being responsible for the development of a single character, the 

 number of possible reassortments of Mendelian characters may be 

 greatly increased. This will also be a consequence of the crossing- 

 over of the chromosomes. And if the very simple scheme of 

 crossing-over just indicated should prove insufficient to account 

 for the reassortments we can have a double-twisting, a " 4-fold 

 chiasma," and so on. 



81^. The Genes. Itwill be seen that the cytological investiga- 

 tions enable geneticists to make a correlation between (i) events 

 that occur in the nuclei of the germ-cells and (2) the events that 

 occur when organisms that belong to different races (or show slight 

 differences in their morphology), and that are mutually and 

 indefinitely fertile with each other are crossed by sexual mating. 

 It is inferred from this correlation that entities that are called 

 genes are in the nuclei and that these entities are causal agencies 



