CROSSING-OVER IN INVERSIONS 273 



of the crossing-over properties of dyscentric hybrids is important 

 for several reasons. First, it enables us to distinguish the results 

 of different crossing-over relationships between chiasmata — 

 reciprocal, complementary and disparate. Secondly, it explains 

 different kinds of chromosome behaviour found at meiosis in 

 structural hybrids and not hitherto understood. It therefore 

 enables us to detect the presence of these kinds of structural 

 differences in nature. Thirdly, it shows how new chromosome 

 types such as have been found in nature can arise from crossing- 

 over in such hybrids, i.e., by secondary structural change. Cases 

 described earlier as the result of fragmentation are probably often 

 due to this crossing-over, and spontaneous ring chromosomes such 

 as have been found can imdoubtedly arise from the crossing-over 

 observed between inverted and translocated segments in Chorthippus. 

 Fourthly, it provides us with a natural experiment in the behaviour 

 of the centromere, showing its permanence and individuaUty, as 

 well as what happens to chromatids with two centromeres and with 

 none, and hence the vital importance of its position in the chromo- 

 some. 



But crossing-over in inversions is perhaps of most immediate 

 interest in showing the frequency with which inversions occur and 

 give rise to inversion hybrids, not only when races and species are 

 crossed, but also within a normally mating wild population. Their 

 importance in a natural population depends to some extent on the 

 abnormal results of crossing-over in giving new chromosome types, 

 but to a much greater extent, at least in diploids, on their effect in 

 suppressing effective crossing-over and therefore in holding groups 

 of genes together in such a way that they will behave as units of 

 crossing-over although they are not units of mutation (D., 1936 

 a and d). 



Table 42 

 Dyscentric Structural Hybrids 



I. Irradiated Stocks. 



Zea Mays {2x). McClintock, 1931. pach., ana. I. 



Drosophila melanogaster {2x). Painter, 1934 b ; Painter and Stone, 1935 ; 

 Griineberg, 1935 '• Beadle and Sturtevant, 1935 ; Muller and Proko- 

 fieva, 1935 (salivary gland) ; Sturtevant and Plunkett, 1926 ; Sturte- 

 vant, 1926; Sturtevant and Dobzhansky, 1930 (breeding experiments) . 



Viciafaba {2x). Mather, 1934. met. I. 



