146 









^ it ^ 



I. 



FIGURE 18-9. The chromosomal complement of a normal human female. Cell was in mitotic 

 metaphase {hence chromosomes appear double) when squashed and photographed. {Courtesy of 

 T. C. Hsu.) 



mosomes in the former than in the latter case. 

 Thus, polyploids can stand whole chromo- 

 some additions and subtractions better than 

 can diploids. 



We have already mentioned in Chapter 13 

 some of the phenotypic consequences of whole 

 chromosome subtraction and addition in 

 human beings — otherwise diploid indi- 

 viduals may be XO (Turner's type female) or 

 XXY (Klinefelter's type male). Mongolian 

 idiocy is known to be sometimes the result 

 of a trisomic diploid chromosomal constitu- 

 tion. In this case, the trisomic is the third 

 smallest of all human chromosomes (the 

 smallest being the Y) (Figures 18-9, 18-10). 

 Trisomies for seven other autosomes are also 

 known, each producing its own characteristic 



set of congenital abnormalities. It is a reason- 

 able expectation that the haploid condition 

 of these or of any other autosome would be 

 lethal before birth, in line with the view that 

 chromosome subtraction is even more detri- 

 mental than chromosome addition. 



Although we have not always specified the 

 particular manner in which each of the 

 genomes aneuploid for whole chromosomes 

 originated, you will recall that we have already 

 indicated two general mechanisms by which 

 such aneuploidy may arise in the germ line. 

 It should also be mentioned that nondis- 

 junction may also occur in somatic cells, as 

 occurs when both daughter chromosomes 

 proceed to the same pole at mitotic anaphase. 

 This produces, in a diploid, one daughter cell 



