Chapter *16 



CROSSING OVER AND CHIASMA 



Yi 



"ou HAVE already seen how the 

 study of cross-fertihzing species 

 demonstrated that the genetic 

 material was divisible first into a single pair 

 of genes (because of segregation), then into 

 a number of gene pairs (because of inde- 

 pendent segregation), and then into still more 

 genes (because those in the same chromosome 

 are not forever bound together, or completely 

 linked, but become separated and may pro- 

 duce new combinations). Linked genes may 

 form these new combinations when the genes 

 of a pair mutually switch their position in a 

 pair of homologous chromosomes by a proc- 

 ess named crossing over (Chapter 15). The 

 strength of linkage was found to vary in 

 different cases studied in different organisms. 

 Let us continue to study linkage and crossing 

 over by means of a number of examples taken 

 from the same organism, Drosophila melano- 

 gaster. 



Two mutants b (black body color) and vg 

 (vestigial wings) are studied simultaneously. 

 A Pi cross between vg +/vg + (vestigial ^) 99 

 and + b/-\- b (black) cf cf produces all normal 

 Fi, vg+/-\-b. As shown in Figure 16-1 A, 

 a backcross of the Fi female {vg -\- / -\- b 9 X 

 vg b/vg b cf ) produces only 20% of F2 with 

 recombinant chromosomes (all F2 carry vg b 

 from the father, their maternal chromosome 

 being 40% of the time vg +, 40%o + b, \0% 

 -f- +, 10% vg b). Since these results are in- 

 dependent of sex, we conclude that b and vg 



are linked autosomally. (For comparison, 

 recall from the last Chapter that the X-linked 

 genes m and w showed 33%, crossing over.) 



When, however, the reciprocal cross is 

 made with the Fi, the dihybrid being the male 

 (vg +/+ ^ c^ X vg b/vg b 9), 50% of off- 

 spring are vg -\-/vgb (vestigial) and 50% 

 + b/vg b (black) (Figure 16-1^8). This dem- 

 onstrates no offspring with crossovers, so 

 that linkage is complete for these genes in 

 the male Drosophila. Of course, had linkage 

 been complete in the female also, we should 

 not have had any evidence that vg and b are 

 separable and are therefore two genes instead 

 of one. It develops, moreover, that in Dro- 

 sophila any genes which show incomplete link- 

 age in the female show complete linkage in the 

 male, so this sex, therefore, does not undergo 

 the process of crossing over.^ It may be 

 noted that in animals, in general, the hetero- 

 gametic sex has reduced or no crossing over. 



What will be the consequence of crossing 

 two Drosophila which are dihybrid vg +/+ b"? 

 The female will produce gametes in the follow- 

 ing frequencies: .4 vg +, A -\- b, A -\ — \-, 

 .1 vg b; the male .5 vg +, .5 + b. Combin- 

 ing these by means of a branched track pro- 

 duces the results shown in Figure 16-2. The 

 2:1:1:0 phenotypic ratio obtained is 

 easy to distinguish experimentally from a 

 9:3:3:1 ratio, which would be expected 

 according to independent segregation. We 

 can generalize the results to be expected in 

 Drosophila. Let the two Hnked mutants be 

 a and b and each dihybrid a +/+ b. Let 2p 

 equal the frequency of noncrossover eggs 

 (a + plus + b), and 1 — 2p the frequency of 

 crossover eggs (a b plus + +). When linkage 

 is incomplete, p < .5. The results are shown 

 in Figure 16-3. We see there that no matter 

 what value, below .5, p is permitted to have, 

 the phenotypic ratio will be 2 : 1 : 1 : 0. If 



^ The convention used here, and usually on subsequent 

 occasions, is to describe the phenotypes of individuals 

 only with respect to the appearance of mutant traits, 

 all traits not mentioned being of the normal type. 

 116 



2 A special kind of "crossing over" does occur on 

 very rare occasions in the germ line (see p. 118) of male 

 Drosophila, but is not of the kind that typically occurs 

 in females. 



