Chromosomal Rearrangements in Nature 



233 



phase I, moreover, adjacent chromosomes 

 in the circle go to opposite poles of the 

 spindle, so that at the start of the separation 

 the chromosomes assume a zigzag arrange- 

 ment (Figure 17-7). If we assume that 

 paternal and maternal chromosomes alter- 

 nate in the circle, then all paternal chromo- 

 somes would go to one pole and all ma- 

 ternal chromosomes to the other. The com- 

 plete linkage of all genes in a complex would 

 be explained by such chromosome segrega- 

 tion (if crossing over is rare), and the gam- 

 etes produced by an individual would be 

 identical to those which united to form it 

 (Figure 17-8). 



If in an alternate segregation procedure 

 maternal and paternal genomes separate, a 

 circle should always contain an even number 

 of chromosomes. Moreover, we could pre- 

 dict that when one gene complex no longer 

 behaves as a single linkage group, it will 

 also no longer form a single circle of four- 

 teen chromosomes with the other gene com- 

 plex. Fourteen chromosomes can be ar- 

 ranged fifteen different ways in circles (com- 

 posed of even numbers of chromosomes) 

 and pairs as shown in Figure 17-9. Indeed, 

 when various race hybrids are made, all fif- 

 teen types and no others are found at meta- 

 phase I — any particular hybrid always form- 

 ing the same meiotic configuration. (The 

 top cell in Figure 17-7 shows an inner 

 circle of four and an outer circle of ten 

 chromosomes.) If what has been supposed 

 about alternate segregation is true, it should 

 also follow that even though alternate chro- 

 mosomes within a circle show complete link- 

 age with each other, such linkage groups 

 should segregate independently of other link- 

 age groups consisting of chromosomes either 

 in separate circles or in separate pairs. This 

 expectation can be tested by comparing the 

 number of genetically determined linkage 

 groups in the different hybrids of Figure 

 17-5 with the chromosome arrangements 



Q 14 Q 10, 2 Po.rs 



O 10 ' O 4 O 6 - O 4 - 2Pairi 



(7) 8, (JJ) 6 Q 8, 3 Pairs 



O •• O <• O 4 O 4 - O 4 - 3 *- 



Q\ 12, 1 Pair Q 6, 4 Pairs 



8, Q4, 1 Pair O 4 ' 5 Pairs 



6, (~) 6, 1 Pair 7 Pairs 



O "' O 4 ' O 4 ' } Pair Q=CIRCIE 



figure 17-9. Circle and pair arrangements 

 possible for Oenothera chromosomes. 



seen cytologically during their meiosis. Such 

 a comparison reveals that the number of 

 separate groups of chromosomes observed 

 in meiosis is always equal to, or greater than, 

 the number of linkage groups detected ge- 

 netically. In fact, whenever a sufficient 

 number of genetic markers are used, the 

 number of linkage groups always equals the 

 number of chromosome groups. 



Although the preceding discussion indi- 

 cates that a rather unique segregation of 

 alternate chromosomes in a circle and the 

 presence of balanced lethal systems can ex- 

 plain most of the unusual genetic behavior 

 of Oenothera, other matters still need ex- 

 planation. What causes these chromosomes 

 to form circles in the first place? A clue to 

 this, contained in the observation made on 

 p. 172 (see also Figure 12-6), is that two 

 pairs of nonhomologs will be associated as 

 a double tetrad during synapsis if a recip- 

 rocal translocation involving them is present 

 in heterozygous condition. Figure 17-10 

 illustrates this situation in Oenothera. All 

 Oenothera chromosomes are small, are 

 roughly the same size, and have median 

 centromeres. To help us identify homol- 



