HETEROPLOIDS 177 



dant cytological and genetic evidence that these flies owe 

 their peculiarities to the absence of one chromosome. 



Flies lacking both IV-chromosomes have not been 

 found and the ratio obtained when two haplo-IV's are 

 bred together (giving 130 haplo-IV's to 100 normals) 

 shows that the nullo-IV's die. 



If a diploid fly that is eyeless is mated to a haplo-IV 

 fly carrying wild type genes in its single chromosome-IV, 

 some of the F^ offspring will be eyeless and these will be 

 haplo-IV. Theoretically, half of the offspring should be 

 eyeless, but the presence of the eyeless gene in the single 

 fourth chromosome lowers the viability of the haploid 

 98 per cent of expectation, and this relation holds when 

 the other recessive mutant types (bent and shaven) are 

 present in the single IV-chromosome. According to 

 Bridges, bent lowers survival by 95 per cent and shaven, 

 100 per cent, i.e., haplo-shaven does not develop. 



The Jimson weed. Datura stramonium, has 24 chromo- 

 somes. A number of types under cultivation have been 

 detected by Blakeslee and Belling with 25 chromosomes 

 (2n-f-l). It is probable that there are 12 such types, each 

 of which has a different extra chromosome. The slight 

 but constant differences shown by these 12 triplo-types 

 (2n-|-l) involve all parts of the plant. These differences 

 are well shown in the capsules (Fig. 101). In two of these, 

 at least (triplo-globe and triplo-poinsettia), in which Men- 

 delian factors are present in the extra-chromosome 

 group, it has been shown by Blakeslee, Avery, Farnham, 

 and Belling, that the twenty-fifth chromosome involved is 

 a different one in the two cases. In one of these in particu- 

 lar, namely, the trisomic type poinsettia, involving a chro- 

 mosome that carries the gene for purple stem pigment 

 and white flower color, the effects on the inheritance due 

 to one extra chromosome have given the clearest results. 

 These show that those germ-cells carrying the extra chro- 



