CROSSING-OVER 



559 



evolutionary consequences, and ones that can be tested. First, 

 any organism that has more chromosomes than any particular 

 ancestor must have had a centromere and has probably had two 

 ends reduplicated. Secondly, these centromeres and ends will be 

 associated with groups of genes in the same linear order up to the 

 point at which the first structural or gene changes have occurred. 

 In other words, the chance of finding homologous series of genes 

 within a haploid set is greatest next to the ends and next to the 



Before Change 



nfler Chanoi 



Ur\e.cfUQl Interchange 



/y (A/c(j7}/Des) 



/2 (h/omoxyQo/e s) 



(^ a t n 



Fig. i6o. — Diagram to show how " fusion " and " fragmentation " 

 can arise by unequal interchange followed by loss and gain 

 respectively of the smaller product. 



centromeres and diminishes as we proceed away from them. 

 Catcheside (1933) in haploid (Enothera and Nishiyama (1934) in 

 triploid Avena have found pairing within the haploid set, but 

 only at or near the ends. Incidentally we may notice that if by the 

 chances of breakage a centromere becomes an end it will remain 

 an end. 



If the centromeres are permanent structures not arising de novo 

 and are also necessary for the perpetuation of chromosomes at 

 mitosis, an important conclusion follows. Increase and decrease 

 of chromosome numbers, such as has been loosely attributed to 



