150 STRUCTURAL HYBRIDS 



variation as is observed is due solely to the failure of chiasma 

 formation between some of the homologous pairs of segments. 



The simplest condition which can lead to this behaviour is that 

 of a heterozygote whose gametes contain chromosomes that have 

 exchanged segments. Thus if such an exchange occurs in an 

 organism of constitution AB, AB, CD, CD (each chromosome can 

 be considered as made up of two segments when only a single 

 structural change is involved) two new chromosome types will 

 result : BC and DA (or BD and AC). The heterozygote derived 

 from this interchange will have the constitution AB, BC, CD, DA, 



B 



Fig. 51. — The development of a ring-of-six in a double interchange- 

 heterozygote in Campanula (of the constitution, AB-BC-CD- 

 DE-EF-FA). A, diplotene (with numbers of chiasmata given). 

 B, diakinesis, (6) + 5 (2) = 16. X 2500. (From Gairdner and 

 D., 1931.) 



and pairing between all four pairs of homologous segments will give 

 a ring of four when the chiasmata are terminal. Pairing between 

 three of them will give a chain of four, and between two of them 

 either two pairs or a chain of three and a univalent. In this way 

 variations of chiasma formation will lead to variations in the size 

 of the association, even though pairing is regular in the corresponding 

 homozygotes, because the unit of pairing in this heterozygote is no 

 longer a whole chromosome but a segment equal to about half a 

 chromosome. The occurrence of an interchange of segments 

 between non-homologous chromosomes was first inferred in trisomic 

 forms of Datura, which will be considered later. In a number of 



