33° EVOLUTION, GENETICS, AND EUGENICS 



from 0. lamarckiana they give the two parental types of offspring hav- 

 ing presumably 14 and 15 chromosomes. Since there are only seven 

 pairs of chromosomes these trisomic types can not all be accounted 

 for by duplication of a different chromosome in each case. 



The genetic relationships between these trisomic mutants are also 

 interesting and peculiar. Thus lata can give rise to scintillans in its 

 offspring, and scintillans can similarly produce lata. Also lata (polli- 

 nated from lamarckiana) can give rise to several other trisomic muta- 

 tions. Many of these relationships can be explained by the assump- 

 tion of double non-disjunction, i.e., both members of one chromosome 

 pair entering one germ cell while both members of another pair enter 

 the opposite cell. It is highly probable that such cases occur. Thus 

 germ cells would be produced with the chromosome content 

 AACDEFG and BBCDEFG. It can be shown that by such a process 

 one trisomic mutation could give rise to another in its offspring. It 

 is very probable that lata is a primary trisomic mutation, e.g., 



AABCDEFG . A A A , 



— .nmrrn i 1<e -> W1 th three A chromosomes, while some other 



, • • ( _i_ ^ «. + - u w a AABC DEFG 



trisomic (2W+1) mutations are probably secondary, e.g., — rrir , rr w , 



ACCJJJbr G 



i.e., with for example three A and three C chromosomes but only one 

 B chromosome. All such irregular chromosome distributions are 

 probably enabled to occur through a weakness in the attraction which 

 normally leads to close pairing of homologous chromosomes during 

 synapsis or on the heterotypic spindle. There is much evidence of 

 variation in the strength of this attraction in Oenothera. 



Cleland has recently found that the chromosomes in various 

 Oenothera species retain their end-to-end connections even on the 

 heterotypic spindle. He has also shown that in several species the 

 arrangement of the chromosomes is more or less constant and char- 

 acteristic during the stages immediately preceding the reduction 

 division. Thus in 0. franciscana four of the chromosomes form a ring 

 while the other ten are arranged in five ring pairs which are at first 

 linked to the circle of four in a definite way. In a form called 0. 

 franciscana sulphur ea, which is derived from O. bicnnisXfranciscana, 

 12 chromosomes end-to-end form a circle and the other two form a 

 pair which is at first linked round the larger chain of 12, and this 

 arrangement is said to be constant. Thus in the derived form a re- 

 arrangement of the chromosomes with relation to each other has taken 

 place. Cleland finds similar fixed arrangements in other species. 



