RING-FORM A TION 153 



ring with terminal chiasmata although they are powerless to distort 

 a configuration held together by numerous interstitial chiasmata. 



It appears that the proportion of non-disjunction is usually about 

 30 per cent, in rings of 12 or 14 chromosomes, just as it is in a ring 

 of four or six {cf. Gairdner and D., 1931). This suggests that 

 orientation of successive chromosomes in the ring usually begins 

 at one part of the ring, and if begun disjunctionally will proceed 

 regularly along the ring {v. Ch. XII). 



(v) The genetical effects of non-disjunction are important. A 

 ring of four segregates disjunctionally to give gametes AB, CD and 

 BC, DA, non-disjunctionally to give AB, BC and CD, DA. Although 

 in Zea and Pisiim all the disjunctional products of segregation in 

 the ring are viable, interchange heterozygotes are nevertheless 

 semi-sterile. This is due to the non-viability of the 50 per cent, of 

 non-disjunctional gametes, which lack one of the segments present 

 in the parental complement. In Campanula, where there is less 

 non-disjunction, partial sterility of the heterozygote is due to a 

 second cause as well, viz., the non-viability or lower viability of 

 homozygous segregates. This is the probable explanation of the 

 occurrence of the ring-forming heterozygote in nature, in Campanula 

 as well as in (Enothera : the heterozygote breeds true because the 

 homozygotes are not viable. 



(vi) In all interchange heterozygotes so far found in plants the 

 chiasmata are usually formed in both arms of the interchanged 

 chromosomes. In Trimerotropis citrina (Carothers, 1931) a ring 

 of four has been found at first metaphase, which is evidently due to 

 the individual being an interchange heterozygote. But here the 

 centromeres are near the ends, and when the ring of four is formed 

 each chromosome has chiasmata with two other chromosomes on 

 the same side of the centromere. The ring therefore lies in the 

 equatorial plane and is never disjunctional. The significance of 

 this will be considered later (Fig. 89, Ch. VII). 



(vii) The fact that interchange heterozygotes, having four, six or 

 more chromosomes in a ring at meiosis, occur in nature gives a great 

 interest to the origin and inheritance of the condition. They are 

 on our hypothesis hybrids, produced by the union of dissimilar 

 gametes. This dissimilarity is due to interchange taking place in 



