IIS 



CHAPTER 9 



Wrinkled, no tendrils x Round, tendrils 

 wt + + 



wt 



F ] Round, tendrils ^z^ 



wt 



P, F, Round, tendrils (self-fertilized) 



w t w t 



+ + 

 F Round, tendrils 



Round, no tendrils 



Wrinkled, tendrils 



Wrinkled, no tendrils 



mgurf. 9-2 (above). Linkage between non- 

 allelic genes in the garden pea. 



figure 9-3 (below). Linkage between non- 

 allelic genes in the garden pea. The dihybrid 

 parent is the same as the F, in Fig. 9-2. 



P^ F | Round, tendrils x Wrinkled, no tendrils 



+ + w t 



w t 



Round, tendrils 



Round, no tendrils 



Wrinkled, tendrils 



Wrinkled, no tendrils 



In a crossing-over process that produces 

 genetic recombinations called crossovers. 



What other characteristics can we estab- 

 lish lor the crossing-over process and the 

 crossovers it produces? Among the prog- 

 eny obtained from backcrossing the dihy- 

 brid ( Figure 9-3 ) . 16 received crossovers 

 in the gametes contributed by the dihybrid. 

 1008 did not. Again, the reciprocal cross- 

 over classes are about equal in frequency. So 

 approximately one crossover was produced 

 for each 63 noncrossovers. A simple cal- 

 culation will show that the F_. results in Fig- 

 ure 9-2 are consistent with this proportion. 



These genes can also make a dihybrid 

 which receives one mutant (recessive) and 

 one normal (dominant) gene from one par- 

 ent (w+) and one normal and one mu- 

 tant gene from the other parent (-M). 

 When such a dihybrid is test crossed, the 

 crossovers (w(or -j- +) and noncrossovers 

 (w -\- or -f O also occur in the proportion 

 1:63. Crossing over, apparently, occurs 

 with the same frequency whether the two 

 mutant genes enter the dihybrid from the 

 same parent or from different parents. 

 Crossovers, therefore, occur in the gametes 

 of an individual with a frequency that is 

 constant and independent of the specific 

 combination in which the nonalleles were 

 received. If this is the normal behavior, it 

 must follow that even in -| — \-/-\ — |- and 

 w t/w t individuals, one gamete in each 64 

 produced is a crossover for these genes but 

 undetected because it carries no new com- 

 bination of nonalleles. Notice that the 

 crossover progeny are fewer than the non- 

 crossover progeny. This must mean that 

 when two linked mutants enter a dihybrid 

 in the same gamete, the mutants tend to be 

 transmitted together to the gametes made 

 by this dihybrid (coupling); if, on the other 

 hand, the mutants enter the dihybrid sepa- 

 rately, they tend to be transmitted separately 

 to the next generation (repulsion). 



