Changes Involving Genomes and Chromosomes 



145 



possible to obtain 12 different kinds of indi- 

 viduals, each having a different one of the 12 

 chromosomes in addition to the diploid num- 

 ber. Each of these kinds is said to be trisomic 

 for a different chromosome (the triplo-IV 

 Drosophila being trisomic for chromosome 

 IV), and is given a different name. The 

 normal seed capsule and those produced by 

 these 12 trisomies are shown in Figure 18-7. 

 It is also possible to obtain viable plants that 

 are diploid but missing one chromosome of a 

 pair; these are monosomies or haplosomics 

 (just as is a haplo-IV Drosophila). Individuals 

 that have two extra chromosomes of the same 

 type (tetrasomics) or have two extra chro- 

 mosomes of different types (doubly trisomic) 

 are also found. 



Datura permits us to test again our ideas 

 relative to the phenotypic consequences of 

 disturbing the normal balance among chro- 

 mosomes. Compare, by means of Figure 

 18-8, the seed capsules produced by the nor- 

 mal diploid (2N), with those of dip- 

 loids having either one extra chromosome 

 (2N + 1), of the type producing Globe 

 (Figure 18-7), or two of these (2N + 2). 



The latter two are polysomics, which can be 

 called trisomic diploid and tetrasomic diploid, 

 respectively. Although the tetrasomic is 

 more stable chromosomally (each chromo- 

 some having a partner at meiosis) than is the 

 trisomic, the tetrasomic phenotype is too ab- 

 normal to establish a race, since it has a still 

 greater genetic imbalance than the trisomic; 

 and produces a still greater deviation from 

 the normal diploid phenotype. 



In comparison, the tetraploid (4N) indi- 

 vidual is almost like the diploid pheno- 

 typically, as might be expected since chro- 

 mosomal balance is undisturbed. The tetra- 

 ploid which has the "Globe" chromosome 

 extra once (4N + 1, making it a pentasomic 

 tetraploid) deviates from the tetraploid in the 

 same direction as the 2N + 1 already men- 

 tioned deviates from 2N, but does so less 

 extremely. Hexasomic tetraploids (4N + 2) 

 deviate from 4N just about as much as 

 2N + 1 deviates from 2N. You see, then, 

 that addition of a single chromosome to a 

 tetraploid has less phenotypic effect than its 

 addition to a diploid, since there is a rela- 

 tively smaller shift in balance between chro- 



FIGURE 1 8-8. Effect upon the capsule of Datura of the presence of one or more extra ''Globe' 

 {Figure 18-7) chromosomes. 



DIPLOID 2N 



2N + 1 



(Globe) 



2N + 2 



^^^ 



TETRAPLOID 4N 



Jd iH # 



4N + 1 



4N + 2 4N + 3 



