ments (translocations and inversions). The cytological significance of 

 the more important of these intrachromosomal alterations are, briefly, 

 as follows: 



Duplication and Deletion 



A piece of a chromosome may be duplicated or deleted to give the 

 intrachromosomal equivalent of aneuploidy. In principle, much of what 

 we had to say concerning aneuploids applies here as well. In general 

 either change reduces viability, the reduction being more marked in the 

 case of a deletion and less effective in a polyploid than in a diploid. 

 Small deletions and duplications cannot usually be detected cytologically 

 and may become confused genetically with gene mutations in the strict 

 sense, for example, bar eye in Drosophila. Larger ones may show up in 

 the heterozygous state as a difference in length between homologues or 

 an unpaired loop in pachytene. 



As noted by Swanson (1957), duplications may be tandem, reverse 

 tandem, or displaced. How any particular duplication may arise poses a 

 question the answer to which is not generally clear. Tandem duplications 

 may be formed by a process of unequal crossing over or other exchange 

 between homologues. 



A deletion may occur in many ways as spontaneous or induced akinetic 

 fragments or as the complementary result of processes leading to duplica- 

 tion. Deletions — no matter how they occur — are frequently lethal, es- 

 pecially when homozygous or hemizygous, and also often have marked 

 phenotypic effects. 



Inversions 



When a segment of a chromosome is turned end for end, it is said to 

 be inverted relative to the original order. Thus a chromosome in which 

 the gene order is designated as ABC DEFG HI may be changed to the 

 order ABC GFED HI in which the underlined segment has been inverted. 

 If the homologous chromosome retains the original order, then at 

 meiotic pairing it is subject to a certain degree of difficulty because of 

 the fact that when the homologues are lying side by side homologous 

 regions are not contiguous in all places. If the region involved is short, 

 no pairing will occur within it and this is usually reflected genetically 

 by a decrease in crossing over for the inverted region. If the region is 

 relatively long, pairing may occur through the formation of a loop which 

 involves the inverted region in one or the other of the homologues. It 

 is this form of pairing which leads to cytological difficulties. Two kinds 

 of inversions may be recognized, i.e., paracentric, which is confined to 



182 / CHAPTER 8 



