326 INTRODUCTION TO CYTOLOGY 



extra elements are present, forms abnormal synaptic configurations. 

 These often lead to marked irregularities in chromosome distribution 

 in the meiotic mitoses and hence to sterility among the spores or gametes. 

 Thus a complement ordinarily requires a certain structural "symmetry" 

 as well as numerical " balance " if the organism is to retain its full fertility.. 



Since it is known that translocations take place from time to time 

 under natural conditions, and also that some of these, when homozygous, 

 do not impair the developmental and reproductive capacities of the 

 organism, one of the reasons why related organisms have chromosome 

 sets differing in minor details of their morphology should be evident. 

 Such differences are very common among related species (p. 126). 

 Even within the same species one may expect to find races which differ 

 less in the outfit of genes they carry, i.e., in genotype, than they do in the 

 arrangement of these genes and in chromosome morphology. The 

 individuals of two such races might look alike, yet show different linkage 

 relations among their characters when bred. Furthermore, it seems 

 altogether probable that losses, duplications, and translocations have 

 played a considerable role in the production of widely different chromo- 

 some complements and genotypes and therefore in the origin of stable and 

 fertile types to which specific rank has been assigned. Hence such 

 cytogenetic phenomena are of great importance to students of phylogeny 

 and evolution. 



Finally, the discovery that certain agencies, such as X-rays and heat, 

 can be used to increase the frequency with which such chromosomal 

 changes occur has placed in the hands of the investigator a tool with 

 which he will be able not only to analyze the functions of chromosomes 

 in greater detail but also to bring about the production of new types of 

 plants and animals more rapidly than they would otherwise appear in 

 nature. 



