48 



CHAPTER 4 



Pole-* Metaphase I 



Pole 



Haploid Meiotic 

 Products at 

 Telophase II 



CASE A 



No chiasma 

 between 

 gene pairs 



n 



A a 



B b 



AB, AB, ab, ab 



CASE B 



After one 

 chiasma 

 between 

 gene pairs 



It 



A a 

 b B 



I 



AB, Ab, aB, ab 



figure 4-8. Meiotic fate of gene pairs presumably located in the same pair of 

 chromosomes. 



the physical basis for independent segrega- 

 tion of different pairs of genes, regardless of 

 chiasma formation. 



Let us examine next the consequences of 

 assuming that A and B are on the same chro- 

 mosome, and a and b are on the homologous 

 chromosome of the pair (Figure 4-8). 

 When no chiasma occurs between the two 

 different pairs of genes, Case A, only the 

 old (maternal and paternal) combinations 

 are found in the gametes. When such a 

 chiasma occurs. Case B, four gametic classes 

 are produced with equal frequency (two old 

 and two new combinational types ) . But, un- 

 less every tetrad has a chiasma in the region 

 between the nonalleles, the number of old 

 gene combinations found among the gametes 

 will exceed the new combinations. Al- 

 though a tetrad usually contains one or 

 several chiasmata, there are numerous points 

 along the chromosome where an exchange 



leading to a chiasma might occur. An addi- 

 tional hypothesis would be needed if each 

 tetrad were required to form a chiasma 

 within a given interval, such as between A 

 and B. Moreover, we have no knowledge 

 as to the genie interval, or the distance be- 

 tween nonalleles presumed to be in the same 

 chromosome. Accordingly, we shall neg- 

 lect, for the time being, the possibility that 

 nonalleles in the same chromosome pair 

 can form old and new combinations with 

 equal frequency — that is, we shall assume 

 that two pairs of genes which do so, and are 

 therefore segregating independently of each 

 other, are located in different pairs of chro- 

 mosomes. Evidence consistent with this as- 

 sumption is obtained from studies with gar- 

 den peas. From the breeding behavior of 

 hybrids for two or more gene pairs, it is 

 possible to establish the existence of seven 

 different pairs of genes (each happening to 



