288 CHROMOSOMES IN HEREDITY: MECHANICAL 



There is a reduction of crossing-over in Drosophila equally on 

 both sides of the break, which indicates that pairing of the chromo- 

 somes may begin either distally or proximally to the break. This 

 rule would probably not apply to organisms with polarised chromo- 

 somes at the zygotene stage. 



In plants having more than two chromosomes associated in a 

 ring, chiasmata are formed in such a way as can be derived only 

 from a radial pachytene association instead of the linear 

 type of association found in ordinary pairing both in diploids 

 and polyploids (Fig. io6). The crossing-over map should therefore 

 be radial or star-shaped (D., 1931 c). This prediction has been 

 made independently and verified by observations on crossing- 

 over between three factors in ring-chromosomes of (Enothera 

 (Emerson, 1931 ; cf. Brink and Cooper, 1932). 



In the progeny of interchange heter ©zygotes in Drosophila, 

 Muller (1930 b) found that the interchanged segments had usually 

 passed to opposite germ-cells at reduction as in a ring in (Enothera, 



The reduction of crossing-over in interchange hybrids is paralleled 

 by the reduction in the pachytene pairing, its replacement in part 

 by non-homologous pairing (cf. Ch. V) and the reduction in the 

 number of chiasmata formed (Gairdner and D., 1931) (Chs. V and 



VI). 



(v) Conditions of Variation. The frequency and distribution of 

 chiasmata and of crossing-over are liable to variation subject to 

 environmental, genetic and developmental conditions. 



(a) Observations of Chiasmata. The chiasmata in the m 

 chromosomes of Vicia Faba (Maeda, 1930 b) show a different 

 frequency variation at metaphase in different preparations. This 

 is to be ascribed to an environmental or developmental difference 

 (Fig. 96). The same conclusion can be derived on the chiasma 

 theory of pairing from the variations of pairing in Triticum hybrids 

 (Kihara, 1929 c), and the parallelism supports this theory (cf. Mather, 

 1935 i> ', Oehlkers, 1935 ; Kattermann, 1933). 



The chiasma frequency differs in different clones of Fritillaria 

 imperialis (Figs. 95 and 96). The highest mean frequency at 

 diplotene is 5-2, the lowest 3*0. This is a regular property of each 

 clone, irrespective of the time of preparation, and is therefore assumed 



