320 INTRODUCTION TO CYTOLOGY 



sequently the organisms carrying transmissible duplications may be 

 expected to show unusual ratios for some of their Mendelian characters if 

 the additional genes concerned are such as to affect the conditions of 

 dominance. Another general effect is sometimes observed in the size or 

 habit of the plant. In Matthiola, for example, it has been observed that 

 the presence of fragments is correlated with small size, the amount of 

 dwarfing being proportional to the degree of unbalance caused in the 

 chromosome complement by the fragments (Lesley and Frost, 1928).^ 



Inversion. — The reversal in position of a portion of a chromosome is 

 called inversion. The portion inverted may include one end, or it may 

 involve an interstitial region only (Fig. 179, 3, 4). Several such inver- 

 sions have been detected in the altered linkage relations of the genes with- 

 out being visible in the chromosomes themselves. In one of the linkage 

 groups of Drosophila, for example, the genetic data for certain genes 

 indicated that a considerable portion of the "right" end of one chromo- 

 some carrying them had been reversed in position. They indicated fur- 

 ther that when crossing-over occurred in the left end, none took place in 

 the right end, presumably because inverted and non-inverted chromosome 

 sections did not have homologous loci opposite each other. In one case it 

 appeared that the two chromosomes must have arranged themselves so 

 as to allow crossing-over in the right end; here none occurred in the left 

 end.'^ 



In Zea it has been possible to observe the effect of an inversion on 

 the synaptic behavior of the chromosomes. In one case, for example 

 (Figs. 181, A to C and 182, a), a long section of a chromosome was inverted 

 as a result of X-ray treatment. When this modified chromosome met its 

 normal synaptic mate, their homologous elements moved together in spite 

 of the abnormal arrangement in one chromosome and so produced the 

 peculiar configuration shown. Similarly, the inversion of a terminal 

 portion resulted in the formation of a loop at synapsis (Fig. 160). Almost 

 nothing is known about the mechanism of such inversions. It is possible, 

 as Serebrovsky (1929) has suggested, that at some stage the chromosomal 

 threads may undergo bending, fusion, and breakage as shown in Fig. 179. 

 It is probable that some other mechanism accounts for inversions 

 induced in metabolic nuclei by X-radiation. 



It will be readily understood that inversions and other aberrations 

 involving portions of chromosomes may often alter the position of the 

 spindle-attachment region. Helwig (1929) believes that in Circotettix 



® For further accounts of chromosome fragments, see Lutz (1916) and Hance 

 (1918a) on (Enolhera; M. Nawaschin (1926, 1931a) on Crepis; Belhng (1925a) on 

 Uvularia; Morgan, Sturtevant, and Bridges (1928) on Drosophila; Goodspeed (1929a6, 

 1930c) and Goodspeed and Avery (1930) on Nicotiana; and Lesley and Lesley (1929) 

 on Lycopersician. 



'' Sturtevant (1926a), Sturtevant and Dobzhansky (1931). 



