CONSEQUENCES OF CHANGE 



segregation. In the homozygotes, pairing and crossing-over being 

 again normal, the super-gene breaks down into its separable units. 

 The unit of heredity may therefore rise or fall from one generation 

 to the next as hybridity rises or falls. To these flexible super-genes 

 we shall have to return later. 



Interchange heterozygotes likewise show a reduced recombination 

 near the interchange. They also show a marked loss of fertility. 

 Belling found such a "gene" in the bean Stizolohium which he inter- 

 preted as due to an interchange. In the heterozygote, interchange, 

 again like inversion, reduces the distances in the recombination, 

 linkage, or crossing-over map. In a diploid with x pairs of chromo- 

 somes, X separate lines make the crossing-over map and they 

 correspond, as they must always do, with the x pachytene threads. 

 The hybrid with a single interchange has two lines intersecting at 

 the point where the interchange has occurred to make a cross; and 

 with two interchanges it has a six-spoke figure just as it has six 

 arms at pachytene. So each new interchange reduces by one the 

 number of linkage groups in the hybrid. 



The spokes of the interchange wheel would all radiate from one 

 point if the interchanges all occurred at corresponding points; but 

 in practice they occur at different points. The ring-of-six has six 

 arms which radiate not strictly as spokes, but as the arms of a cross 

 of Lorraine. The middle piece of this cross is formed by median 

 segments which are homologous and may pair, but have distal arms 

 on both sides which are not homologous (Fig. 32). Crossing-over 

 between these two median segments occurs in Campanula rings 

 and gives gametes with only one of the two interchanges and 

 consequently progeny with a ring of four. In other words the 

 combination of chromosomes brought about by two interchanges 

 can be broken down in the hybrid by crossing-over between 

 them, just as the combination of two gene differences within one 

 chromosome can be broken down in the hybrid by crossing-over 

 between them. 



Allopolyploidy 



When a diploid plant or animal, as is usual with species hybrids, 

 is heterozygous for numerous structural changes, their effect is less 

 noticeable in distorting the pairing at the first metaphase of meiosis, 



134 



