Independent Recombination by Nonalleles 



43 



case carries but a single pair of genes, each 

 gene must have two simultaneous effects, 

 one on seed shape and the other on seed 

 color. The results obtained are consistent 

 with this requirement in the following re- 

 spect: the F, are round yellow, and the F L . 

 give a 3 : 1 ratio for yellow vs. green and 

 also for round vs. wrinkled. According to 

 this hypothesis, the F L . should be of only two 

 types — 3 round yellow: 1 wrinkled green. 

 But in F L . not only are these grandparental 

 (Pi) combinations found, but two new, re- 

 combinational classes of offspring appear. 

 namely, round green and wrinkled yellow! 

 Apparently, then, what is genetically trans- 

 mitted is not composed of a single pair of 

 indivisible units, but is composed of pairs 

 of units, or genes, with each gene pair ca- 

 pable of undergoing segregation separately . 

 Let us assume, therefore, that each sex- 

 ually reproducing organism contains more 

 than one pair of genes. In the present case, 

 let/? (round) and r (wrinkled) be the alleles 

 of one pair of genes and Y (yellow) and y 

 (green) be the alleles of the second pair. 

 The Pj, then, would be RR YY (round yel- 



RR YY 



RR Yy A n 



^ Round 



Rr YY W Yellow 

 Rr Yy 



/2 Y 



RR yy 



r yy 



Round 

 Green 



rr YY 



r r Y y 



Wrinkled 

 Yellow 



r yy » 



Wrinkled 



figure 4-2. Expected genotypes and pheno- 

 types in F 2 following segregation. 



'/a R 



— %RY 



— Va R y 



V 2 r 



y 2 y 



Va r Y 



y 2 y 



Va r y 



i IGl rh 4-3. Genotypes of gametes formed by 

 a dihybrid, Rr Yy, undergoing independent 

 segregation. 



low) and rr yy (wrinkled green). Each 

 pair of genes would undergo segregation so 

 that a gamete would contain only one mem- 

 ber of each pair. In this manner the former 

 parent would produce only RY gametes and 

 the latter only ry, and all F, would be Rr Yy 

 (round yellow), as observed. 



Based on the current hypothesis, the gam- 

 etes formed by the Fj would contain either 

 R or r. and. moreover, would contain either 

 Y or y. Since the F 2 show that R and Y do 

 not always go together into a gamete, nor 

 do r and y, there must be four genotypes 

 possible in gametes. RY. Ry. rY. ry. Since 

 these possible haploid genotypes will be 

 found both in male and in female gametes, 

 it is expected that the ¥_. would contain the 

 diploid genotypes and their corresponding 

 phenotypes indicated in Figure 4—2. Note 

 that nine different genotypes are possible in 

 F 2 , four giving the round yellow phenotype, 

 two giving round green, two wrinkled yellow, 



