CHI ASM A INTERFERENCE 291 



showing no evidence of crossing-over. As to the sex chromosomes, 

 the cytological and genetical observations agree in showing the 

 occurrence of crossing-over, the method of which we shall consider 

 later (Ch. IX). 



The genetic suppression of crossing-over necessarily means the 

 suppression of chiasma formation, pairing and fertile segregation. 



Thus Gowen (1928 et alibi) and Bridges (1929) found that with 

 suppression and reduction of crossing-over by autosomal factors 

 in the female the fertility of the female was reduced, and that this 

 was associated with irregularities in the pairing of the chromosomes. 

 The factor had no effect on the male. This means that the male 

 character of localisation or suppression of chiasmata inhibited the 

 effect of this factor for crossing-over suppression. 



Beadle (1933) has discovered that in " asynaptic " maize which 

 has as a rule no pairing of chromosomes at meiosis, the progeny 

 nevertheless show the results of normal crossing-over. This is 

 because the progeny are derived from selected germ-cells resulting 

 from regular segregation, and therefore from a complete pairing. 

 Beadle's observations are particularly significant in showing that 

 the minimum frequency of chiasmata and crossing-over compatible 

 with this complete pairing is the same as the normal frequency of 

 chiasmata and crossing-over. 



The asynaptic maize and female Drosophila therefore show the 

 necessity of crossing-over and chiasma-formation for pairing and 

 segregation in these two widely separated organisms. The chiasma- 

 type hypothesis and the chiasma theory of metaphase pairing are 

 equally confirmed. 



(vi) Linear Distribution, (a) In Relation to one Another 

 (Interference). A statistical examination of chiasma frequencies 

 in bivalent chromosomes of Vicia, Fritillaria, Rosa and Tiilipa 

 shows that their distribution is too great near the mode and too 

 little near the extremes on an expectation of randomness (Haldane, 

 1931 h). 



This means that the occurrence of one chiasma reduces the chance 

 of occurrence of another in its neighbourhood. Chiasmata show 

 " interference " like that shown by cross-overs. It should be noted 

 that the distribution in position in the chromosome of chiasmata 



10—2 



