RECIPROCAL TRANSLOCATION 329 



plant was of the former interchange type, the plant was a secondary; 

 when it was of the latter interchange type, the plant was the other 

 secondary; when it was of the normal type, the plant was a primary. 

 The trivalent configurations were explained on the assumption that only 

 homologous ends are paired in the late prophase and metaphase. As 

 would be expected, these abnormalities led to a certain amount of sterility 

 in trisomic plants because of the abnormal chromosome distribution in 

 meiosis. 



In addition to the primaries and secondaries certain "tertiary" 

 types were discovered. The breeding behavior of the tertiary known 

 as "Wiry" suggested that both chromosome I (giving "Rolled" as a 

 primary) and chromosome IX (giving "Poinsettia" as a primary) were 

 somehow involved. Cytological observations showed that these two 

 chromosomes actually were connected with each other in meiosis. This 

 led to the definite formulation of the theory that non-homologous chromo- 

 somes occasionally undergo reciprocal translocation (Belling and Blakes- 

 lee, 1926). On the theory that only homologous portions undergo 

 synapsis, it was found possible to explain the curious configurations seen 

 at meiosis after various crosses and to account for the unusual genetic 

 behavior of the plants concerned. The continuation of these investiga- 

 tions led to results of great importance in the field of cytogenetics. 

 Reciprocal translocation has since been reported for other organisms also.^ 

 It has received definite cytological proof and promises to explain many 

 hitherto puzzling phenomena in both plants and animals. 



Reciprocal Translocation in Zea. — For a fuller description of recipro- 

 cal translocation in a diploid plant we may select a particularly clear case 

 from among several known in Zea (Figs. 187 to 189). This case,^ more- 

 over, is the one in which the peculiar morphology of the chromosomes 

 concerned permitted the first complete cytological proof of the correct- 

 ness of Belling's hypothesis of reciprocal translocation. 



In a certain strain of maize known as "semisterile-2" it was observed 

 that approximately one half of the pollen grains and one half of the ovules 

 were functionless. Cytological examination of the sporocytes of semister- 

 ile plants revealed the presence of eight bivalents and a ring of four instead 

 of the 10 bivalents seen in normal plants. It followed that certain non- 

 viable chromosome combinations produced by the disjunction of the four 



^E.g., Zea (Burnham, 1930; McClintock, 1930; Cooper and Brink, 1931; Brink 

 and Cooper, 1931); Pisum (H&kansson, 1929a, 1931a, 1932; Hammarlund and 

 H§,kansson, 1930; E. Richardson, 1929; Pellew and E. Sansome, 1931; E. Sansome, 

 1932); Campanula (Gairdner and Darlington, 1930); Rhoeo, Tradescantia, and Zebrina 

 (Darlington, 1929afe; Sax, 1931e; Nebel, 19326); Aucuba (Meurman, 1929a); Droso- 

 phila (Sturtevant and Dobzhansky, 1930; Muller, 1930c; Dobzhansky, 1933); 

 (Enothera (see later in chapter); Godetia and Clarkia (H&kansson, 1925, 19316); 

 Hypericum (Hoar, 1931); and Circoteitix (Helwig, 1932). 



3 Burnham (1930), McClintock (1930a6). 



