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CHAPTER 17 



Variability in crossover rates may be due 

 also to factors influencing the process of 

 crossing over itself. Such factors include 

 temperature, nutrition, age of the female, 

 and the presence of specific genes. 



In order to understand better the relation- 

 ships between crossing over and chiasmata, 

 consider the properties of the following, over- 

 simplified model (Figure 17-2). Assume that 

 a chromosome (ignoring the centromere) is 

 divided into five equal segments marked by 



^ Hybrid Studied 



MEIOTIC 40 

 PRODUCTS 



40 



FIGURE 17-2. Crossover con- 

 sequences following a single 

 chiasma. 



PER CENT OF ALL PRODUCTS 



45 



45 



six genes. Assume further that each tetrad 

 with this chromosome contains one, and only 

 one, chiasma which can occur at random 

 among these segments. What would be ob- 

 served if only the region between a and b was 

 followed in the hexahybrid shown in the 

 Figure? The chance that the chiasma will 

 occur in the a-b region is 20%. From each 

 25 tetrads 100 haploid meiotic products are 

 produced. Of each 25 tetrads, 20%, or five 

 tetrads, will have the chiasma in the a-b 

 region. These will produce 10 crossover and 

 10 noncrossover strands. The latter 10, when 

 added to the 80 noncrossover strands from 

 the other 20 tetrads, give 90 noncrossover 

 strands. So, in this region, 20% chiasmata 

 gives 10% crossovers, as explained in Chap- 

 ter 16. Similarly, were only the b-c region 

 followed, 10% crossovers would be observed. 



If now only the a-c region is followed, the 

 chiasma will fall within it 40% of the time 

 and 20% of all haploid meiotic products will 

 be crossovers between a and c. Note that 

 in this model the sum of the distances a-b and 

 b-c equals the distance as measured between 

 a and c directly. So the model aligns the 

 genes linearly, just as was observed in the 

 experiment described near the beginning of 

 this Chapter. 



Note also, from the way the model was 

 described, that the occurrence of a chiasma 

 in one region automatically excludes its 

 occurrence in some other region. We find, 

 then, that the chance for a chiasma in the a-c 

 region is equal to the sum of the separate 

 chances for a chiasma in the a-b and b-c re- 

 gions. It is a general rule that the total 

 probability that any one of a series of mutually 

 exclusive events will occur is equal to the sum 

 of their separate probabilities of occurrence. 

 Accordingly, the chance of a chiasma occur- 

 ring between a and /is, of course, 



100 (20 + 20 + 20 + 20 + 20)%, 



so that recombination is 50%, and the num- 

 ber of map units in our model is fifty. 



