Genetics 49 



ping chromosomes but of unknown significance in populations. 

 Duplications are of importance because they represent a possible 

 cytogenetic mechanism for an increase in the total amount of genetic 

 material. How the total amount of genetic material has changed in 

 the course of the evolution of life is not known. Nor is it possible at 

 present to decide whether it is necessary that the amount of genetic 

 material increase concomitantly with the increase in complexity that 

 has taken place. 



Inversions and Translocations. Inversions and reciprocal trans- 

 locations are more conspicuous and better-understood chromosomal 

 alterations, and their effects are well known. Organisms in which 

 these changes have become a regular feature of the genetic system 

 are discussed in Chap. 9. Here only the cytogenetic aspects of such 

 changes are considered. Unless pairing behavior is studied, an in- 

 verted region of a chromosome ordinarily cannot be detected visu- 

 ally except in organisms with polytene chromosomes or conspicuous 

 chromomeres. During the first meiotic prophase, when homologous 

 chromosomes pair, heterozygosity for an inversion is revealed bv the 

 fact that one chromosome must twist in order for svnapsis to be 

 accomplished (Fig. 3.2). This characteristic inversion loop is also 

 seen in the salivary-gland chromosomes of DrosophiJu and other 

 Diptera where the polvtene chromosomes are somatically paired. If 

 the inversion is a short one, crossing-over may not take place within 

 the inversion loop. If the inversion is sufficiently long and has the 

 proper relation to the centromere, crossing-over will occur and the 

 result will be the formation of a dicentric chromosome and an 

 acentric fragment. The acentric fragment will not behave properly 

 on the spindle at anaphase, and the dicentric fragment will be 

 broken. Gametes that are formed will include, in addition to bal- 

 anced ones, those in which whole chromosome regions are lacking, 

 and these will be nonfunctional. 



Looking at the genetic results, it will appear as though crossing- 

 over had been suppressed. However, only the products of crossing- 

 over have been lost. The number of crossovers within the inversion 

 loop and their distribution among the chromatids of the bi\alent 

 determine what effects there will be. Because the genes within an 

 inversion loop are effectively linked in a heterozygote under certain 

 conditions, inversions are discussed in more detail in relation to re- 

 combination in Chap. 9. Organisms that are homozygous for a 

 chromosomal inversion can be recognized as such only by matmg 

 them with a different type and then observing the pairing beha\ior 

 of the chromosomes in the hybrid (except in organisms with polv- 

 tene chromosomes, where they can be detected by careful examina- 



