74 CELL HEREDITY 



bination. The unexpected phenomenon of genetic recombination be- 

 tween hnked allelic pairs can be accounted for as shown in Figure 3.5. 

 In the prophase of the first meiotic division, two of the four chromatids 

 may exchange parts. As a result, one exchange forms two recombination 

 chromatids. Each of these, together with the parental chromatid, still 

 joined to the same centromere, passes to an opposite pole of the spindle. 

 Thus two recombinant and two parental chromatids are assorted among 

 the four gametes. This process of exchange is called crossing over. 

 In the remainder of this chapter, we shall proceed on the assumption 

 that recombination between linked markers results from chromosome 

 exchange or crossing over, as shown in Figure 3.5. This model provides 

 a basis for predicting the result of crosses, and can be tested experi- 

 mentally. We will see that, in general, this model does satisfy the data 

 for recombination between different genes. More rigorous proofs of the 

 cytological basis of crossing over will be presented in Chapter 4. The 

 molecular basis of recombination will be discussed in Chapter 7. 



A single crossover event, occurring between the chromosomal loci of 

 two allelic pairs of genes, recombines them. It is characteristic of 

 crossing over that only two of the meiotic products are recombinant and 

 that these are reciprocal in nature, e.g., mt'*' alb~ and mt~ aW^ . An- 

 other consequence of crossing over is that the segregation of genes lo- 

 cated distal to the point of its occurrence takes place at the second 

 rather than at the first division. The order of genes within the ascus 

 reflects this. Thus, when there is no crossing over between the locus 

 alb and the centromere, half the time the order will be one of these two 

 possibilities: 



alh^ alb^ alb~ alb" 

 alb- alb- alb^ alb+ 



But when a crossover event occurs, there is an equal chance that the 

 order will be any one of these four: 



alb "^ alb- alb^ alb- 

 alb- alb^ alb- alb^ 

 alb^ alb- alb- alb^ 

 alb- alb^ alb^ alb' 



depending on how the chromatids segregate in the second division. 



It is clear that second-division segregation changes the order only of 

 genes that are distal to the point of crossing over when the centromere 

 is used as a landmark. Thus, in the example in Figure 3.5, the mt gene 

 shows the order mt^ mt^ mt- mt- because no crossover event occurred 

 between it and the centromere, but one did occur proximal to the locus 



