CHARACTERISTIC TRISOMICS ^2^ 



Zygotes. 



2;t + I (i) Fragments : viability not clearly lowered ; show 

 specialised abnormality, 

 (ii) Whole chromosomes : viability slightly lowered. 

 Show generalised abnormality. 

 2a; + 2 (i) Fragments : accentuation oi 2x -{- i type. Viability 

 lowered, 

 (ii) Whole chromosomes : accentuation of 2a; + i type. 

 Viability greatly lowered. 



(c) Trisomics. Records of fertility and elimination showing the 

 differentiation of the chromosomes are supported by a large series 

 of qualitative observations. Thus in numerous species (Table 50), 

 a whole series of trisomic phenotypes have been identified corre- 

 sponding with the different members of the complement which are 

 present in excess. In Datura and (Enothera there are not only the 

 types of trisomic corresponding in number to the haploid comple- 

 ment, but there are the special types corresponding to viable inter- 

 change combinations and these are much more numerous (Ch. IX). 

 The extra chromosomes are specific in the time of their action as 

 well as in the kind of effect they produce. For example, the 

 trisomic Datura type with the worst pollen at maturity is not the 

 one with the lowest pollen effectiveness in fertilisation. Again, 

 the -extent of the disproportion always has a quantitative effect ; 

 the tetrasomic has the corresponding trisomic characters in an 

 exaggerated degree, while the same number of chromosomes in a 

 tetraploid or hexaploid have a reduced effect (e.g., Triticum vulgare, 

 6x, Kihara, 1919-24, Huskins, 1928 ; Avena sativa, 6x, Huskins, 

 1927 ; Primula keivensis, 4X, Newton and Pellew, 1929), whence the 

 occurrence of only high polyploid species with variable numbers 

 [v. Ch. VIII). 



Simple trisomics do not normally breed true and multiple trisomics 

 never do so. They are therefore found permanently only in clonal 

 plants. Amongst these their special qualities have often led to 

 their being selected for propagation (Table 48 B) . 



{d) Balance and Proportion. Two principles may be inferred 

 from these observations : — 



11— ? 



