40 



CHAPTER 6 



From these results what else can we decide 

 regarding the gene? Until now, we have been 

 able to explain all the experimental data pre- 

 sented by supposing that each sexually re- 

 producing individual contains only a single 

 pair of genes. Accordingly, we shall still 

 consider that each Pi individual carries but 

 a single pair of genes, but require each gene 

 to have two simultaneous effects, one on seed 

 shape and the other on seed color. The re- 

 sults obtained are consistent with this expec- 

 tation in the following respect: the Fi are 

 round yellow, and the F2 give a 3 : 1 ratio for 

 yellow vs. green and also for round vs. 

 wrinkled. But, on this hypothesis, the F2 

 would be of only two types — 3 round yel- 

 low : 1 wrinkled green! The facts are that in 

 F2 not only these grandparental (Pi) combina- 

 tions are found but two new, recombinational 

 classes of offspring appear, namely, round 

 green and wrinkled yellow! Apparently, then, 

 what is genetically transmitted is not composed 

 of a single pair of indivisible units, but is com- 

 posed of separable pairs of units, each pair 

 capable of undergoing segregation and recom- 

 bination. 



Let us assume, therefore, that each sexually 

 reproducing organism contains more than 

 one pair of genes. In the present case, let R 

 (round) and r (wrinkled) be the alleles of one 

 pair of genes while Y (yellow) and y (green) 

 are the alleles of the second pair. The Pi, 

 then, would be RR YY (round yellow) and 

 rr yy (wrinkled green). Each pair of genes 

 would undergo segregation so that a gamete 

 would contain only one member of each pair. 

 In this manner the former parent would 

 produce only RY gametes and the latter one 

 only ry, and all Fi would be Rr Yy (round 

 yellow) as observed. 



On the current hypothesis, the gametes 

 formed by the Fi would contain either R 

 or r, and, moreover, would contain either Y 

 or y. Since R and Y do not always go to- 

 gether 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 F2 

 would contain the diploid genotypes and 

 their corresponding phenotypes indicated in 

 Figure 6 2. Note that nine different geno- 

 types are possible in F2, four giving the round 

 yellow phenotype, two giving round green, 

 two wrinkled yellow, and one wrinkled green. 

 This is consistent with the fact that four 

 phenotypes were actually found in F2, sub- 

 stantiating our hypothesis that the genetic 

 material transmitted in a gamete is composed 

 of subunits, each of which has the properties 

 of a gene. 



How can we account for the fact that the 

 observed F2 phenotypes occurred in the rela- 

 tive proportions of 9 : 3 : 3 : 1 , respectively? 

 This can be done by making the simple 

 assumption that the segregation of the mem- 

 bers of one pair of genes occurs independently 

 of the segregation of the members of another 

 pair of genes. As a result of this (see Fig- 

 ure 6-3) half of all gametes receive R, of 

 which half will carry 7 and half>'; the other 

 half of all gametes receive r, of which half 

 will carry Y and half y. Thus the male 

 gamete population in the P2 will be 25% RY, 

 25% Ry, 25% rY, and 25%o ry. The Po also 

 produce female gametes of the same four 

 genotypes in the same relative frequencies. 

 Since fertilization has already been assumed 

 to occur at random the F2 expected are shown 

 in Figure 6-4. 



The branching track in Figure 6-4 can be 

 read beginning at the top: % of female 

 gametes are RY and are fertilized % of the 

 time by RY male gametes (producing Ks of 

 all offspring as RR YY), % of the time fertiliza- 

 tion is by Ry male gametes (so that Ke of all 

 offspring are RR Yy from this origin), etc. 

 By summing up like classes, the kinds and 

 relative numbers of genotypes and of pheno- 

 types are obtained as shown in the chart. 



The branching track may be used to obtain 

 the 9 : 3 : 3 : 1 phenotypic ratio more simply. 



