An organism may have more than one reciprocal translocation. The 

 two best-known examples of multiple translocations are Rhoeo discolor 

 with twelve chromosomes in a ring (Figure 8-7) and Oenothera 

 Lamarckiana with a ring of twelve chromosomes and one bivalent. In 

 both cases separation is preferentially of the alternate type and fertility 

 should be high. Little is known of the genetics of Rhoeo, but it is highly 

 sterile. There is no obvious cytological reason, however, why this should 

 be so. Oenothera, on the other hand, is about 50 per cent fertile, and 

 nearly all of the offspring are heterozygous for the translocations, as is 

 the parent. Genetical investigations have shown that the homozygotes 



D : 



A' 



Figure 8-6. Diagrammatic Representation of a Reciprocal Translocation 

 Involving Two Pairs of Chromosomes. A-B. A'-B', and C-D, C'-D'. Pairing 

 of the chromosomes and their subsequent separation during meiosis results 

 in a ring configuration at first metaphase with the chromosomes designated 

 as A-B, A'-D, and C-B', C'-D'. (From Wilson, G. B., 1952. "OuUine of 

 Genetics," Michigan State University Press, East Lansing, Mich., p. 25.) 



of both types usually fail to develop, apparently because of a balanced 

 lethal condhion in which a number of genes prove fatal in the homo- 

 zygous condition. 



Because it may lead to change in the basic number of chromosomes, 

 one kind of translocation is worth special mention. If two chromosomes 

 with subterminal kinetochores are involved in a translocation such that 

 most of the long arm of one exchanges with some of the short arm of 

 the other, the result will be a chromosome with a median or submedian 

 kinetochore and a small chromosome consisting of a kinetochore plus a 

 short region on either side (Figure 8-8). Since in many, if not most 

 cases, the proximal arms of chromosomes are heterochromatic and ge- 

 netically inert, and since minute chromosomes run a high risk of being 

 lost, a translocation of the type described can result in decrease in 

 chromosome number without loss of vital genetic material. Such events 



186 / CHAPTER 8 



