Genetic Systems II | 183 



change the position of the centromere and thus alter the relative 

 length of the arms. Unequal reciprocal translocations also may 

 change the chromosome length. Since the number of crossovers is 

 proportional to the length of a chromosome arm, there may be a 

 change in the amount of recombination simply as a result of size 

 changes. As is discussed below, if a chromosome is very small, for 

 example, because of translocation, and does not pair properly, it 

 may be lost and the basic chromosome number changed. 



Changes in Chromosome Number 



One way of expressing the amount of recombination afforded by the 

 genetic system is to use the recombination index of Darlington. This 

 simple measure is equal to the haploid number of chromosomes plus 

 the average number of chiasmata. The larger the recombination index 

 (R.I.), the greater the number of new gene combinations formed 

 by recombination and segregation. The index does not take into 

 account the existence of supergenes formed as a result of structural 

 changes or apomixis, but its use leads to interesting comparisons. 

 An increase in the basic number will increase the R.I.; for this 

 reason, chromosome-number change may be regarded as an aspect 

 of the genetic system related to the balance between immediate fit- 

 ness and long-range flexibility. Stebbins has made an analysis of 

 the distribution of R.I. as indicated by chromosome number in 

 flowering plants. In woody plants (trees and shrubs) the basic 

 gametic number is significantly higher than that of herbaceous 

 genera. This suggests that among long-lived plants a high recom- 

 bination index has been favored by selection. In short-lived 

 rapidly reproducing organisms, the genetic system usually seems 

 to favor immediate fitness at the expense of flexibility. There 

 is, however, a certain amount of flexibility inherent in a short life 

 cycle with its rapid turnover of genes. Within strictly cross-pollinated 

 herbaceous flowering plants, Stebbins finds that annual species tend 

 to have a lower recombination index than perennials. Most of these 

 annual species are either cross-pollinated with a low R.I. or pre- 

 dominantly self-pollinated, with a high R.I. They appear to be 

 specialized for rapid occupation of uniform habitats. 



POLYPLOIDY 



The basic chromosome number clearly appears to have had selective 

 value. How are changes in the basic number accomplished? The 

 minimum number of chromosomes, though all different, that func- 

 tion as a harmonious and integrated unit is a genome. For purposes 

 of discussion, the genomic number may be symbolized as x. The 



