Trisomies 419 



The question of the viability of the gametophytes that bear 

 the extra chromosome is very interesting, for decidedly the pres- 

 ence of an extra chromosome upsets the balance of the genes. It 

 is true that in a trisomic there is no incomplete genome as there 

 is in a monosomic type, but it is conceivable that the genie bal- 

 ance will be so disturbed that the gametophyte generation will 

 not be normal. One of the most complete studies of the effect of 

 the extra chromosome has been in the genus Datura. The Jimson 

 weed, D. stramonium, has normally twelve pairs of chromosomes. 

 It is conceivable, therefore, that twelve different trisomic types 

 might be found, each one of which is trisomic for a different 

 one of the twelve pairs. In the normal diploid each chromosome 

 is represented twice; therefore each gene in the plant is repre- 

 sented twice. Supposedly, the plant has existed in that condition 

 for a long time, and any variation from it is in the direction of 

 an unbalance. Thus if a certain chromosome is trivalent while 

 all the others are bivalent, its genes will each be represented three 

 times, and the characters controlled by those genes will be exag- 

 gerated. Since each of the twelve pairs of chromosomes bears 

 different genes, it might well be possible to identify from their 

 phenotypes twelve morphological types which differ from the 

 normal and also from each other, each of which is trisomic for 

 one of the twelve pairs of chromosomes. Blakeslee and his co- 

 workers have identified twelve such morphological types and 

 have determined cytologically that they are trisomic forms. Fig- 

 ure 115 shows the capsules of each of these twelve types and 

 below them the chromosome with respect to which the types are 

 trisomic. 



Such an unbalance, however, may affect not only the pheno- 

 type of the plants but also the viability of the gametophyte 

 generation. It is more reasonable to expect an unbalance in 

 the gametophyte generation than in the sporophyte. The chro- 

 mosome number of trisomic sporophytes is 2n + 1? which means 

 that the genes in the trisomic chromosome stand in the relation- 

 ship of 3:2 to those in the other chromosomes. However, 

 gametophytes which bear the extra chromosome have n + 1 

 chromosomes. In those gametophytes the genes in the trisomic 

 chromosome are twice as numerous as those in the other chro- 

 mosomes, and, therefore, there is a greater unbalance in the 

 gametophyte than in the sporophyte. Specifically, however, only 



