Chapter *6 



INDEPENDENT SEGREGATION 



I: 



N THE preceding Chapters we 

 have discussed the transmission 

 -genetics of akernative genes for 

 a single trait and have found that a single 

 pair of genes could explain the data in each 

 case. The question asked now is, what will 

 be the genetic unit of transmission when two 

 or more different traits are followed simul- 

 taneously in breeding experiments? 



The answer to this may lie in the results 

 of some additional experiments performed 

 with the garden pea.^ Previous studies had 

 already shown that, like the flower color trait 

 described in Chapter 2, seed shape and seed 

 color were each due to a single pair of 

 genes. That is, a Pi of pure line round X pure 

 line wrinkled seeds gave round Fi, round 

 being dominant. Self-fertilizing the Fi round 

 gave F2 which were in the proportion of 3 

 round : 1 wrinkled. Similarly, a Pi of pure 

 line yellow X pure line green seeds gave yel- 

 low F], yellow being dominant, and self-ferti- 

 lization of the yellow Fi gave 3 yellow : 1 

 green in F2. 



The question presented above may be 

 asked relative to the seed traits of shape and 

 color. What will happen when individuals 

 are crossed that simultaneously difi'er with 

 regard to both of these traits? Suppose in 

 Pi a round yellow strain is crossed with a 

 wrinkled green strain, these strains being 

 available as pure lines. In Fi only round yel- 

 low seeds are obtained (Figure 6-1). This 

 result is what would be expected had we been 

 studying shape and color of seeds separately. 

 We find in this case, therefore, that there is 



^ Based upon experiments of G. Mendel. 

 39 



■^ 



I 



y 



Round Yellow x Wrinkled Green 

 ALL Round Yellow 

 F, Round Yellow x F, Round Yellow 

 PHENOTYPE NUMBER RATIO 



Round Yellow 

 Round Green 

 Wrinkled Yellow 

 Wrinkled Green 



315 



101 



108 



32 



9.06 



2.9 



3.1 



T^ 



^^^m 



^ 



FIGURE 6-1. Phenotypic results from studying 

 two traits simultaneously. 



no phenotypic efi'ect of the dominance of one 

 trait upon the phenotypic expression of the 

 other trait. 



Self-fertilization of the round yellow Fi 

 gives offspring which, when counted in suffi- 

 ciently large numbers, occur in the relative 

 frequencies of 9 round yellow : 3 round 

 green : 3 wrinkled yellow : 1 wrinkled green. 

 Notice that segregation and recombination 

 have occurred for each trait, as revealed in 

 F2 by 12 round : 4 wrinkled and by 12 yel- 

 low : 4 green. So, in this case there is also 

 no effect of one trait upon the segregation- 

 recombination behavior of the genetic ma- 

 terial for a different trait. 



