402 



BREAKDOWN OF GENETIC SYSTEMS 



cells vary in the number of their bivalents from none to the full 

 sixteen under environmental and genotypic control {cf. Saccharum, 

 Bremer, 1930 ; Kniphofia, Moffett, 1932). The same is true of 

 differences of behaviour in different cycles of cyclical parthenogenesis 

 as in Phylloxera (Morgan, 1915, Ch. IX). Particular treatment 

 with heat and other agents has produced effects on meiosis analogous 

 to those resulting from abnormal genotypic conditions, but less 

 precise and less significant (De Mol, 1928, 1933 ; Stow, 1930). 



The occasional failure of pairing between partners that normally 

 pair in approximately true-breeding organisms (Table 61) may 

 arise from many causes. In some cases the shortest chromosome is 

 affected, and we may then suppose that the margin of safety in 

 chiasma-formation is too small and environmental conditions are 

 directly responsible. The particular mechanism will probably be 

 associated with the high variance in chiasma frequency between 

 bivalents and the low variance between nuclei which Mather has 

 ascribed to " competition " (Ch. VII). Where long chromosomes 

 are affected, or the nucleus is polyploid, we may suppose that 

 exceptionally awkward distribution in the zygotene nucleus has 

 prevented the chromosomes from pairing in time. 



Table 61 



Exceptional Failure of Pairing not due to Hybridity 



Diploids. 



Nicotiana alata {2n = 18) 



Zea Mays {2n = 20) . 

 Ranunculus acris {2n = 14) 

 Cycas revoluta {2n =22) 

 Dorstenia plumaricafolia {2n = 26) 

 Crepis capillaris {2n = 6) 

 Trimerotropis citnna ^ {2n = 23) 

 Uvularia grandiflora {2n = 14) 

 Secale cereale {2n = 14) 

 Scilla italica {2n = 16) 

 Fritillaria pontica {2n =24) 

 Chorthippus parallelus [S chromo- 

 some) .... 

 Mecostethus grossus {2n = 23) 

 Stauroderus bicolor {L chromosome) 

 Triploid 



Narcissus poetarum [jx = 21) 



Ruttle, 1927 ; Goodspeed and 



Avery, 1929 a. 



Randolph, 1928. 



Sorokin, 1927 b ; Larter, 1932. 



Nakamura, 1929. 



Krause, 1931 (? fragments). 



Hollingshead, 1930 b {v. supra). 



Carothers, 1931 (unequal bivalent). 



Belling, 1925 c. 



Mather and Lamm, 1935. 



Dark (1932 c, Fig. 12 3) ^ ^, 



\ ^j ' o J/ Chromosomes 



'!■ 



D-, 1935^ 



D., 1936 

 White, 1936 

 D., 1936. 



Nagao, 1929 (61" -f y). 



with lowest 



chiasma- 



frequency. 



