46 



CHAPTER 6 



equally frequent types of gametes at the end 

 of meiosis. The same result is also obtained 

 either when there is a chiasma between the 

 centromere and the gene in question in one 

 tetrad but not the other (Case B), or when a 

 chiasma occurs in each of the tetrads (Case 

 C). Therefore, independent segregation of 

 different pairs of chromosomes can serve as 

 the physical basis for independent segregation 

 of different pairs of genes, regardless of 

 chiasma formation. 



Let us examine, now, the consequences of 

 assuming that A and B are on one and the 

 same chromosome while a and b are on the 

 homologous chromosome of the pair (Fig- 

 ure 6-8). When there is no chiasma. Case A, 

 only the old (maternal and paternal) combi- 

 nations are found in the gametes. When 

 there is a chiasma between the two different 

 pairs of genes, Case B, then all four classes 

 occur with equal frequency (two old and two 

 new combinational types). But, unless every 



tetrad has a chiasma in the region between 

 linked genes, the number of old gene combi- 

 nations found among the gametes will exceed 

 the new combinations. Although a tetrad 

 usually contains one or more chiasma, there 

 are numerous points along the chromosome 

 where a chiasma can arise. It would necessi- 

 tate an additional hypothesis to require that 

 each tetrad have a chiasma within a given 

 interval. Moreover, we have no knowledge 

 as to the genie interval, that is, the distance 

 between genes presumed to be on the same 

 chromosome. Accordingly, we shall neglect, 

 for the time being, the possibility that genes 

 on the same chromosome pair could form 

 old and new combinations with equal fre- 

 quency — that is, we shall assume that two 

 pairs of genes which do so, and are therefore 

 segregating independently of each other, must 

 be located on different pairs of chromosomes. 

 Evidence that is at least consistent with this 

 presumption is obtained from studies with 



FIGURE 6-8. Meiotic fate of gene pairs located in the same 

 pair of chromosomes. 



Pole^« Metaphase I ►Pole 



Haploid Meiotic 

 Products at 

 Telophase II 



CASE A 



AB, AB, ab, ab 



CASE B 



After one 

 chiasma 

 between 

 gene pairs 



I1::fl 



AB, Ab, aB, ab 



