342 PERMANENT HYBRIDS 



parental gamete paired at opposite ends with ends of two 

 chromosomes from the other gamete. In this way segregation 

 would yield onl}^ two types of gamete, and these two types would 

 be the same as those that had gone to make up the parent zygote. 

 He also assumed that the type of configuration was a specific 

 property of the two complexes of each species or hybrid m relation 

 to 07ie another. 



This conclusion has been borne out by the extensive observations 

 of recent years. All the complex heterozygote species have 

 multiple chromosome rings. The homozygous forms, whether 

 species or segregates, have seven pairs, or occasionally live pairs 

 and a ring of four. 



These observations showed the mechanism by which the 

 segregation of complexes takes place. They showed the consequence 

 of the chromosomes associating in rings, but owing to some confusion 

 at this time in the interpretation of chromosome pairing and of 

 meiosis in general, they did not at once reveal how the formation 

 of rings and the inheritance of complexes came about and what it 

 meant in genetic terms. In the light of later studies of the mechanism 

 of chromosome pairing it became clear, however, that these large 

 multiple rings were derived by normal processes from numerous 

 interchanges, just as the smaller rings described in simpler structural 

 hybrids are derived from one or two interchanges. As a result of 

 six interchanges a ring of fourteen could be built up by association 

 ot all the interchanged segments, the formation of chiasmata in 

 them and their later complete terminalisation (D., 1929, 1931 d). 



Since the chromosomes are associated only at their ends, it is 

 necessary, in considering their pairing properties, to regard them 

 as made up merely of two " segments " which are effective in 

 pairing. In this way a homozygous (Enothera with seven pairs of 

 chromosomes at meiosis has a constitution as follows (homologous 

 segments bearing the same letters) : 



7(2) (AB (CD (EF [GH [KL [MN [OP 



AB) CD) EF) GH) KL) MN) OP) 



while a complex heterozygote with a ring of 14 chromosomes is 

 made up as follows : 



