Chapter 25 

 ANEUPLOIDS AND NONDISJUNCTION 



We discussed in the last chapter such chromosomal aberrations 

 as deficiencies, duplications, inversions, and translocations. All 

 these aberrations involved segments of chromosomes but not 

 whole chromosomes. We shall consider in this chapter abnormal 

 situations in which one or more wt\ole chromosomes will be de- 

 ficient from a genome or will be present as extra chromosomes. 

 Some of these whole-chromosome aberrations are similar to some 

 of the intrachromosomal abnormalities except that they involve 

 a complete chromosome instead of a piece. For example, a chro- 

 mosome may be missing, just as a chromosomal segment may be 

 deficient. An extra chromosome may be present in the same 

 manner that a segment may be duplicated. Of course a chromo- 

 some could not be inverted because a chromosome has no fixed 

 position in the sense that an inverted segment is fixed in its 

 inverted position by its attachment to the remainder of the 

 chromosome. Finally, attached-X strains are examples of the 

 translocation of one whole chromosome to another. Whether an 

 aberration involves a whole chromosome or only a segment, the 

 behavior at zygotene is determined by the rule that the parts of a 

 chromosome pair only with homologous chromosomal segments. 

 Chromosomal aberrations which include whole chromosomes but 

 not chromosomal sets are termed aneuploids. 



Monosomies 



A monosomic type is an individual which is deficient for one 

 whole chromosome. Since, in a diploid animal, the chromosome 

 number in the somatic cells is 2n, that in a monosomic form of 

 the same species would be 2n — 1. The characteristics of a 

 monosomic are essentially the same as those of a deficiency. If 

 the lost chromosome in an animal is small, the animal may sur- 

 vive but the genes in the missing chromosome will be absent; if 

 the lost chromosome is large, the animal will not survive; n — 1 



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