Gene Arrangement; Crossover Maps 



133 



the following type respectively: some split, 

 but none cut; some cut, but none split; all 

 normal; some both split and cut. In this 

 way the generation being tested for cross- 

 over frequency is largely protected from 

 differential viability, its genotypes being 

 scored in the next generation. For map- 

 ping and other purposes, the extra labor 

 entailed by this method is often justified. 



Variability in crossover frequency may be 

 due also to factors — such as temperature, 

 nutrition, age of the female, and presence 

 of specific genes — which influence the very 

 process of crossing over. 



To better understand the relationships be- 

 tween crossover maps and chiasmata, con- 

 sider the properties of an over-simplified 

 model (Figure 10-3). Assume that a chro- 

 mosome (ignoring the centromere) is com- 

 posed of five equally long regions, the ends 

 of each marked by a gene; that each tetrad 

 of this type contains one, and only one, 

 chiasma following a crossing over; and that 

 this chiasma can occur in a random posi- 

 tion among these segments. For the hexa- 

 hybrid shown in Figure 10-3, the chance 

 the chiasma will occur in the a-b region is 

 20%; out of each 25 tetrads (producing 

 100 haploid meiotic products), five or 20% 

 will have the chiasma in the a-b region. 

 These five will produce 10 crossover and 10 

 noncrossover strands. Adding the latter 10 

 and the 80 noncrossover strands from the 

 remaining 20 tetrads, gives 90 noncrossover 

 strands. For this region, therefore, 20% of 

 tetrads have a chiasma and 10% of haploid 

 meiotic products are crossovers as explained 

 in Chapter 9. Similarly, in the b-c region 

 10% crossovers would be noted. The 

 chiasma would occur in the a-c region 40% 

 of the time, and 20% of all haploid meiotic 



figure 10-3. Crossover consequences of a 

 single chiasma. 



p i *+l? 9 x ANY O* 



F Sons: spl + / Y 41.5 % 



+ ct / Y 41.5 % 



+ + / Y 8.5 % 



spl ct / Y 8.5 % 



figure 10-2. Crossover frequency for two X- 

 linked genes in Drosophila. 



MEIOTIC 40 

 PRODUCTS 



40 



45 



45 



PERCENT OF ALL PRODUCTS 



