230 



( II M'TER 17 



GAMETES ((o)) G 



GAMETES (O)) G 



/ \ 



FIGURE 17-4 {above). Balanced lethal gene 

 complexes in O. biennis and O. Lamarckiana. 



figure 17-5 (below). Linkage groups in hy- 

 brids from interracial crosses. 



m P B Sp Cu 



FLAVENS-CURVANS 



FLAVENS-PERCURVANS 



FLAVENS-FLECTENS 



FLAVENS-VELANS 



RUBENS-FLAVENS 



RUBENS-CURVANS 



CURVANS-VELANS 



RUBENS-VELANS 



gene complexes is albicans. rubens. Figure 



17-4 shows how these balanced lethal gene 

 complexes are distributed generation after 

 generation in biennis and Lamarckiana. All 

 the recessive lethal alleles in either of the 

 two strains cannot be identical to those in 

 the other; if they were the F, from crossing 

 them would consist of only two different 

 phenotypes, whereas three types are actually 

 obtained. We can conclude, therefore, that 

 the balanced lethal system generally found 

 in Oenothera involves either a multiple 

 allelic series or several pairs of genes or both. 

 Each of the three different Fi hybrids 

 obtained from crossing Lamarckiana with 

 biennis breeds true upon self-fertilization, 

 showing that each hybrid contains two com- 

 pletely linked gene complexes. This con- 

 clusion may or may not be true, however, 

 of the breeding behavior of other hybrids 

 obtained from interracial crosses. This 

 ambivalence is illustrated in Figure 1 7-5 

 with the gene complexes present in the dif- 

 ferent hybrids shown at the left. The dis- 

 tribution of the various genetic markers (top 

 of Figure 1 7-5 ) in the gametes of these 

 hybrids was determined from breeding tests. 

 For example, the curvans. velans hybrid pro- 

 duced only two kinds of gametes though 

 heterozygous for all these marker genes, the 

 markers behaving as if they were all com- 

 pletely linked. On the other hand, the 

 Havens. velans hybrid produced four kinds 

 of gametes. The genes R, m, and P (all still 

 linked to each other) segregated independ- 

 ently of the genes B and Sp (both still linked 

 to each other), so that half of the gametes 

 contained one of the two parental combina- 

 tions, the other half carried one of the two 

 recombinations. In this case, therefore, 

 genes which belonged to a single linkage 

 group in the parent races behaved as two 

 linkage groups during the gametogenesis of 

 their hybrid. Since 50% recombination oc- 

 curred in gametogenesis of the hybrid, these 

 results are not really explained by postulat- 



