394 Intrachromosomal Aberrations 



go to one of the poles during mitosis and therefore usually fails 

 to become included in the new nucleus. It generally remains in 

 the cytoplasm, often rounding up in certain types of cells into a 

 very small replica of a nucleus, often termed a ^'micronucleus," 



or 



Fig. 102. Sax's explanation for the production of small deficiencies and 

 inversions by an X-ray hit that breaks two adjacent gyres of a chromonema 

 coiled in early prophase. If the breaks are followed by a criss-cross re- 

 union, a ring deletion will result; if by a reunion of adjacent ends, a small 

 inversion will be produced. (Redrawn from Sax in Genetics.) 



but it soon becomes digested and lost to view. Such a broken 

 piece of chromosome is an acentric jragment. If the deleted 

 piece includes the centromere, it is a centric jragment and be- 

 haves like a normal chromosome; the remainder of the original 

 chromosome, which is now acentric, usually becomes lost very 

 soon. 



Duplication 



When, in a normal diploid organism, one or more loci, but not 

 so many as to constitute a whole chromosome, are present three 

 times or more instead of twice, the reduplicated segment is known 

 as a duplication. The reduplicated segment may be a centric 

 fragment or it may be a chromosomal segment attached to or 

 inserted in either one of the chromosomes with wliich it is ho- 

 mologous or in one of those with which it is not homologous. 

 If it happens to be inserted in a homologous chromosome next 

 to the segment which is identical with it, the situation is one 

 that we have described in Chapters 13 and 17 when we discussed 

 the Bar and the Hairy wing duplications in Drosophila melano- 

 gaster. 



Because the genes located in the duplicated segment are pres- 

 ent three times instead of twice, duplications can give disturbed 



