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CHAPTER 20 



of only two combinations). On the other 

 hand, iheflavens.velans hybrid produced four 

 kinds of gametes, the genes for R, m, and P 

 (all still linked to each other) segregating 

 independently of the genes for B and Sp (both 

 still linked to each other), so that half of the 

 gametes contained one of the two parental 

 combinations, the other half carried one of 

 the two recombinations. In this case, there- 

 fore, genes that belonged to a single linkage 

 group in the parent races behaved as two 

 linkage groups during the gametogenesis of 

 their hybrid. (The fact that 50% recombina- 

 tion occurred in the gametogenesis of the 

 hybrid means that we cannot explain these 

 results by postulating i\\dii flavens (or velans) 

 is always a single linkage group, which cannot 

 crossover with the partner gene complex of 

 the parent race, but which can do so when its 

 partner is velans {or flavens)) 



Other tests of the hybrid containing jia- 

 vens.curvans showed m and P still linked but 

 separate from B, which was, in turn, separate 

 from Sp and Cu, so that there were now three 

 linkage groups, and perhaps more would have 

 been found had additional genetic markers 

 been employed. In all cases, however, the 

 same hybrid combination always showed the 

 same linkage groups in its gametogenesis. 



In view of the fact that at least three linkage 

 groups can be identified in certain hybrids 

 (even though under certain conditions these 

 act as one), it is to be expected that the diploid 

 would have at least three pairs of chromo- 

 somes. Cytological examination confirms 

 this genetic expectation, there being seven 

 pairs of chromosomes in all of the Oenothera 

 strains discussed in the present Chapter. 

 (Oenothera gigas, the triploid mentioned in 

 Chapter 18, has 21 chromosomes.) Accord- 

 ing to our assumption that the balanced lethal 

 system is based upon a single pair of genes 

 located on a single pair of homologs, this pair 

 of chromosomes must necessarily remain 

 heterozygous to be viable in Fi. But this 

 would not be expected to be true for the other 



six pairs of chromosomes, which should 

 "segregate independently. So, for example, 

 gametes of biennis which carry the albicans 

 recessive lethal should be equally likely to 

 carry the riibens or the albicans homolog in 

 each of the other six cases of independent 

 segregation. But this is not found. It is 

 theoretically possible, however, to have seven 

 pairs of chromosomes each heterozygous for 

 a different recessive lethal which would, upon 

 self-fertilization, produce only viable Fi like 

 itself. Since this explanation would predict 

 that only about ^2^ of all ovules would develop 

 as seeds, it cannot be the correct one for 

 Oenothera. 



FtGURE 20-6. Circle of 14 c/iromosomes in Oeno- 

 thera. Chromosome number is clear in upper cell 

 where the circle has broken open. {Courtesy of 

 R. E. Cleland.) 



