Chapter 4 



INDEPENDENT RECOMBINATION 

 BY NONALLELES 



I 



n i hi: preceding chapter we dis- 

 cussed the transmission genetics 

 of alternatives tor a single trait 

 and found that a single pair of genes could 

 explain the data in each case. But what is 

 the genetic unit of transmission when two 

 or more different traits are followed simul- 

 taneously in breeding experiments? The 

 answer may be found in the results of some 

 additional experiments performed with the 

 garden pea. 1 Other studies show that seed 

 shape and seed color, like the flower color 

 trait described in Chapter 3, are each due 

 to a single pair of genes. That is, a P] of 

 pure-line round X pure-line wrinkled seeds 

 gives round F,, round being dominant. Self- 

 fertilizing the F, round produces Fj in the 

 proportion of 3 round: 1 wrinkled. Simi- 

 larly, a P, of pure-line yellow x pure-line 

 green seeds gives yellow F,, yellow being 

 dominant, and self-fertilization of the yellow 

 Fi gives 3 yellow: 1 green in F 2 . 



What actually happens in a crossing of 

 individuals that differ simultaneously with 

 regard to both seed shape and seed color? 

 A round yellow strain is crossed with a 

 wrinkled green strain, these strains being 

 available as pure lines. In F, only round 

 yellow seeds are obtained (Figure 4-1). 

 This result is what we would expect had we 

 been studying shape and color of seeds sep- 

 arately. In this case, there is no phenotypic 



Based upon experiments of G. Mendel. 

 42 



etleet ol the dominance of one trait upon the 

 phenotypic expression o\ the other trait. 



Self-fertilization of the round yellow F, 

 gives offspring which, when counted in sulli- 

 ciently large numbers, occur approximately 

 in the relative frequencies of 9 round \ el- 

 low: 3 round green : 3 wrinkled yellow:! 

 wrinkled green. Notice that segregation and 

 recombination are involved for each trait, 

 as revealed in F 2 by approximately 12 

 round: 4 wrinkled and by about 12 yel- 

 low^ green. In this generation also there 

 is no effect of one trait upon the recombina- 

 tional behavior of the genetic material for 

 a different trait. 



From these results, what else can we de- 

 cide regarding the gene? Until now, we 

 have been able to explain all the experi- 

 mental data on the basis of only a single 

 pair of genes, as if the total genetic material 

 of a diploid cell is divisible into only two 

 genes, any one gene having numerous dif- 

 ferent alleles, each one having effects on 

 many different traits. To continue to con- 

 sider that each Pi individual in the present 



Round Yellow x Wrinkled Green 



ALL Round Yellow 



F, Round Yellow x F, Round Yellow 



PHENOTYPE NUMBER RATIO 

 Round Yellow 315 9.06 



Round Green 101 2.9 



Wrinkled Yellow 108 3.1 



Wrinkled Green 32 0.9 



FIGURE 4-1. Phenotypic results from studying 

 two traits simultaneously. 



