CONSEQUENCES OF CHANGE 



zygotes can be produced by the chance fusion ot complementary 

 gametes, such as the first two or the last two, each of which was 

 itself unbalanced. In both the plant and the animal a proportion of 

 the eggs are sterile, but different classes of eggs in the two cases. 



In breeding the simple interchange hybrid, such as has been found 

 wild in Campanula (or produced by crossing in Datura) we are 

 concerned only with those types of progeny which can live. We 

 then find that it breeds like a simple mendclian hybrid with one 

 difference: it is the hybrid which we distinguish and record as dif- 

 ferent from either of the two homozygous types of its parents or 

 of its progeny. Selfmg yields i AB . AB-\-CD . CD : 2 AB . CDj 

 BC . DA : I BC . BC-{-DA . DA; that is to say equal numbers are 

 homozygous (with bivalents only) and heterozygous (with rings 

 of four) ; provided, that is, that the homozygotes are as viable as 

 the heterozygotes, which is not always the case. 



The hybrid for two interchanges has potentially two rings of four 

 chromosomes at meiosis, or where the interchanges involve a 

 common chromosome, a ring of six. Thus, by interchange of 

 A and E:— 



AB CD EF AB CD EF 



/ \ +11 becomes \ / \ / \ 



BC DA EF BC DE FA 



This double interchange hybrid again gives only three viable 

 types of progeny and in the ratio of i : 2 : i. Thus two unit changes 

 in the chromosomes are giving one unit difference in segregation 

 and in heredity. Not only this, but they are at the same time 

 throwing three linkage groups into one. Here we have a first 

 example of how structural change can, by combination, reduce the 

 number of units of heredity : it restricts recombination, partly by 

 directing segregation and partly by killing a selected portion ot the 

 recombinants. 



Interchanges can successively increase the size of the ring, and the 

 linkage group it represents, until they include all the chromosomes 

 in the complement. This condition is found in many wdld species 

 such as Rlioeo discolor with 12, Oenothera muricata with 14, and 

 Hypericum punctatum with 16, chromosomes. With this knowledge 

 it was possible also, by crossing plants hybrid for different single 



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