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CHAPTER 20 



occurred between any chromosome tip in a 

 circle of four and a tip of some other pair of 

 chromosomes, a circle of six chromosomes 

 will form in the heterozygote for both recipro- 

 cal translocations. This is illustrated at the 

 right of Figure 20-10 where the upper dia- 

 gram shows the configuration before tips 4 

 and 5 exchange, and the lower diagram shows 

 the circle of six following this exchange. Still 

 larger circles can be formed by successive 

 interchanges of this type, six being required 

 to form a circle of 14. The presence of 

 reciprocal translocations in heterozygous 

 condition could explain how various sized 

 circles containing even numbers of chromo- 

 somes are produced in Oenothera. 



We have not yet completed our analysis of 

 Oenothera, however. One of the questions 

 remaining is: By what mechanism is it ar- 

 ranged that alternate chromosomes in a circle 

 proceed to the same pole during meiosis? 

 The answer to this is unknown. A second 

 question stems from the fact that almost all 

 the different races of Oenothera found in 

 nature form a circle of 14. Are the six trans- 

 locations involved the same in all races? 

 No, for if they were, viable hybrids between 

 races would form either circles of 14 or seven 

 pairs at meiosis. The very fact that all the 

 configurations in Figure 20-9 are found in 

 such hybrids must mean that different gene 

 complexes must differ from each other in 

 the specific ways that their chromosome ends 

 have become translocated. There are many 

 thousands of ways, theoretically, that 14 

 ends can be arranged in seven groups of 

 two. How can we determine how many of 

 these different arrangements are found in 

 nature? 



Let us start by taking a particular gene 

 complex, calling it the standard, and labeHng 

 its chromosome ends 1-2, 3-4, 5-6, 7-8, 9-10, 

 11-12, 13-14. (Normally, i.e., in nature, this 

 complex would form a circle of 14 with the 

 other gene complex, which would therefore 

 have no chromosome with the same pair of 



ends as has the standard.) Now a series of 

 interracial hybrids is formed with the stand- 

 ard as one of the complexes, and their meiotic 

 arrangements scored. Suppose in one case 

 the hybrid forms 5 pairs and a circle of 4. 

 This must mean that the ends of 5 chromo- 

 somes are in the same order in the complex 

 under test as in the standard, but that they are 

 in a different order in the remaining two chro- 

 mosomes. Until now there was no reason 

 to assign ends 1-2, 3-4 of the standard to any 

 particular chromosomes. But now we can 

 arbitrarily assign these ends to the two stand- 

 ard chromosomes in the circle of 4, and, 

 therefore, the chromosomes in the circle from 

 the complex under test can be called 2-3, 4-1, 

 In this way, the composition of ends of two 

 pairs of chromosomes is specified perma- 

 nently. Figure 20-1 1 (top) shows the standard 

 and tested complexes in our example syn- 

 apsed according to numbers. 



Let us call the complex just tested A. Sup- 

 pose next another complex, B, is made hybrid 

 both with the standard and with A. You can 

 see that an appropriate result will specify 

 other ends of standard. A, and B. Such 

 procedures can be carried out until all of the 

 standard's chromosomes are specified and 

 the complete order of all 14 ends determined 

 for any other complex. In this manner, it is 

 discovered in nature that a circle of 14 is 

 produced in many different ways, a theoretical 

 and an actual example being shown in the 

 central and lower parts of Figure 20-11. In 

 fact, of 350 complexes analyzed, more than 

 160 different segmental arrangements have 

 been found. All these results are consistent 

 with the hypothesis that during the course of 

 evolution the ends of Oenothera chromo- 

 somes have been shuffled many times in dif- 

 ferent ways by reciprocal translocation. 

 Finally, a most convincing test of the recipro- 

 cal translocation interpretation would be the 

 ability to predict the meiotic chromosomal 

 arrangement to be found in a hybrid not yet 

 formed. This has been done many times and 



