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CHAPTER 1 1 



otherwise diploid, that arc XO, XXX, and 

 XXY. Nondisjunction of the small fourth 

 chromosome can load to the production of 

 individuals with one fourth chromosome 

 (huplo-IY individuals) or three (triplo-IY 

 individuals) (Figure 11-7) being in this 

 respect monosomic and trisomic, respec- 

 tivelj : instead of disomic as is normal. Even 

 though addition or subtraction o\' a chromo- 

 some IV makes visible phenotypic changes 

 from the disomic condition as can be seen 

 from the phenotypes, both aneusomic 

 changes are viable. On the other hand, 

 individuals monosomic or trisomic for cither 

 of the two large autosomes die before com- 

 pleting the egg stage. 



When triploid Drosophila females — with 

 all chromosomes trisomic — undergo meiosis, 

 bundles of three homologous chromosomes 

 (trivalents) may be formed at synapsis. 

 This is because, at one place along the length 

 of a chromosome, the pairing is between two 

 homologs, and at another place it is between 

 one of these two and the third homolog. 

 In this way, although pairing is two-by-two 



at all levels, all three homologs are held to- 

 gether as a divalent. At the first mciotic 

 division the two homologs that are synapscd 

 at their ccntromeric regions, separate, and 

 go u> opposite poles, while the third homolog 

 goes to either one of the poles. At the end 

 o\' the second mciotic division, two nuclei 

 each have one homolog of the trivalent. and 

 two nuclei each have two homologs. The 

 same result is obtained when synapsis is 

 entirely between two homologs and excludes 

 the third. Since each of the four trisomies 

 present at metaphase I segregates independ- 

 ently, eggs are produced which have one of 

 the following: 



1 . Each chromosome type singly and, 

 therefore, contain one complete ge- 

 nome (being haploid) 



2. Two chromosomes of each type and, 

 therefore, contain two genomes (be- 

 ing diploid) 



3. Any combination in which some chro- 

 mosomes are represented once and 

 others twice (being aneusomic). 



figure 11-7. Haplo-IV (left) and triplo-IV (right) females of D. melano- 

 gaster. The haplo-IV is smaller than the wild-type female shown in Fig. 2-6. 

 (Drawn by E. M. Wallace.) 



