220 



BOTANY: PRINCIPLES AND PROBLEMS 



dominant, the sixteen types will not all be visibly distinguishable, 

 for the hybrids or heterozygous plants will resemble the pure 

 dominants. The resulting F2 generation is shown in Fig. 125. 

 Our expectation in such a population is evidently that nine- 



Parents 



PP 5S 



(Purple, Smooth) 



WW RR 



(White^Roush) 



Gametes 



PW SR 

 (Purple, Smooth) 



F; Game 



F2 



(PWSRx 

 PWSR) 



PS 



PR 



tWR 



PS 



iPR 



iws 



ZWR 



Total Fz 



15 Purple Smooth) j^ Purple, Rough; 7^ White, Smooth^, 



4 White, Rough 



Fig. 125. — Diagram showing genotype (in letters) and appearance of parents, 

 of Fi, and of F2 in a cross between a purple-flowered, smooth-seeded pea and a 

 white-flowered, rough-seeded one. 



sixteenths will show both dominant characters, three-sixteenths 

 one dominant and one recessive, three-sixteenths the other com- 

 bination of dominant and recessive, and one-sixteenth, both 

 recessive characters. The results of another such dihybrid 

 cross are shown in Fig. 126. The method by which new combi- 

 nations of characters are secured through hybridization is thus 

 clear; but we must remember that many of the F2 plants are 



