TRUE-BREEDING TRISOMICS 353 



This occurs in (E. grandiflora normally ; in CE. suaveolens only when 

 the flavens complex is associated with a different complex in the 

 hybrid with (E. biennis. Such a difference in behaviour depending 

 on a difference in partner is to be expected if crossing-over is the 

 basis of these mutations, for crossing-over is conditioned by the 

 presence of a partner for the region in which it occurs, and what are 

 interstitial segments in one combination may be differential seg- 

 ments in another, 



(v) Trisomic Mutation. The mutations involving changes in 

 chromosome number are to be explained as the result of irregularities 

 in chromosome distribution at meiosis, such as those occurring in 

 other organisms. The fact that they occur in much greater numbers 

 than in species with simple pairing is due to the mechanical con- 

 ditions of separation in a ring. Normally a ring in (Enothera 

 segregates " disjunctionally," i.e., so that the chromosomes that 

 are paired pass to opposite poles. It will be seen that if this 

 orientation takes place almost simultaneously in different parts of 

 the ring, sometimes chromosomes separated by an even number 

 will come to lie to one side of the equator and two of the intervening 

 chromosomes must then pass to the same pole, or one be left on the 

 equator to divide equationally at the first division. These 

 occurrences have been observed both in (Enothera (Cleland, 1926 

 et al. ; D., 1931 d) and in Rhoeo (D., 1929 c) where a ring of 12 

 chromosomes is formed and conditions of segregation can be more 

 easily studied than in (Enothera. Irregularities must necessarily 

 occur in the intervening chromosomes in both directions round the 

 ring, so that non-disjunction is always double. If the ring is 

 replaced by a chain the non-disjunctional result occurs, although 

 the two chromosomes which pass to the same pole may not have 

 been joined, but only have been capable of having been joined. 

 Where the non-disjunctions occur on the same side of the equator 

 two gametes with eight and six chromosomes are formed instead of 

 seven and seven. The high frequency of this occurrence is 

 responsible for the high frequency of trisomies (1-5 per cent, in 

 (E. Lamarckiana) . The lagging of chromosomes of undecided 

 orientation is probably responsible for the failure of separation of 

 two daughter-nuclei and the formation of unreduced gametes 



R.A. CYTOLOGY. 12 



