28o CHROMOSOMES IN HEREDITY : MECHANICAL 



corresponding diploids, for in the triploid it results not only from 

 structural hybridity, but also from intra-haploid reduplication. 



We have seen that one of the essential properties of secondary, 

 as opposed to primary structural change, is that it is exactly 

 repeatable. All exactly repeated or exactly reversed changes that 

 have been described as primary must therefore be under suspicion. 



Table 43 



Secondary Structural Changes due to Crossing-Over after Primary Changes 



Primary Change. 



1. Dyscentric inversion. 



2. Dyscentric transloca- 



tion to same arm. 



3. Dyscentric transloca- 



tion to opposite arm 

 of same chromosome. 



4. Dyscentric interchange . 



5. Simple external inter- 



change. 



6. Compound external 



interchange. 



7. External transloca- 



tion. 



8. External dyscentric 



translocation. 



9. Translocation to same 



arm. 



10. Internal interchange. 



1 1 . Translocation to oppo- 

 site arm. 



Effect on Meiosis. 



Dicentric and acentric 



chromatids. 

 Dicentric and acentric 



chromatids. 

 Asymmetrical bivalent 



and as above. 



(At mitosis) dicentric 

 and acentric chroma- 

 tids. 



Ring of four. 



Ring of six, etc., and 



" figure-of-eight." 

 Association of four. 



Association of four. 



Dicentric and acentric 



chromatids. 

 Asymmetrical bivalent. 



Asymmetrical bivalent. 



Asymmetrical bivalent. 



Secondary Change. 



Terminal deficiency. 



Terminal deficiency. 



Terminal deficiency and 

 duplication. 



Terminal deficiency and 

 later breakage. 



None. 



Reverse interchange. 



External interchange. 



Dyscentric interchange, 

 and hence deficiency. 



Intercalary reduplica- 

 tion and deficiency. 



Terminal reduplication 

 and deficiency. 



Terminal reduplication 

 and deficiency. 



Important examples of these are the exact reversals of an inversion 

 found in Drosophila to be accompanied by reversal of a correlated 

 mutation believed to be due to position effect (Griineberg, unpub- 

 lished). For such reversals to occur by random structural change 

 is out of the question. But if a small segment is translocated, 

 reduplicated and inverted within the chromosome, as found by 



