218 BOTANY: PRINCIPLES AND PROBLEMS 



heterozygous at all, but half of them are P and half W. Thus the 

 hybrid character of a plant cannot be carried by its gametes, 

 which must be entirely one thing or entirely the other. The 

 factors P and W, brought in from the original purple and white 

 parents, have coexisted in the hybrid without influencing each 

 other in the least and have now sharply parted company, or 

 become segregated. 



Mendelian Ratios. — In a cross between two of these hybrid 

 Fi plants (or in the case of a self-fertilization of one of them) 

 the occurrence of the three-to-one ratio in the F^ generation is 

 thus easy to explain. Of the gametes of each parent, approxi- 

 mately half carry the factor P and half the factor W, so that in the 

 perfectly free and random union which takes place between these 

 gametes there are four possible combinations which may occur in 

 the offspring produced. P male cells may fertilize P eggs, pro- 

 ducing PP plants; P male cells may fertihze W eggs, producing 

 PW plants; W male cells may fertilize P eggs, also producing PW 

 plants, or W male cells may fertilize W eggs, producing WW 

 plants. Each of these combinations, on the basis of pure chance, 

 is apt to occur just as often as any other. Approximately one- 

 quarter of the new generation, the PP plants, will not only look 

 purple but will breed just as truly for this color as did their 

 purple-flowered grandparent; approximately one-half, the PW 

 plants, will also look purple (perhaps somewhat paler) but are of 

 course heterozygous, and when selfed or when crossed among 

 themselves will behave just as did their parent, the Fi hybrid, 

 and yield three colored-flowered plants to every white; and the 

 final quarter, comprising the WW plants, will appear white and 

 will breed as truly to this color as did their white grandparent 

 (Fig. 124). The characteristic Mendelian ratio is therefore not 

 three-to-one at all, but rather one-to-two-to-one. Of course it 

 should be remembered that the results of actual breeding do 

 not always display these ratios exactly, any more than in the 

 tossing of coins or the throwing of dice there are always exact 

 and predictable results. The ratios merely indicate what may 

 be expected on the basis of probability. 



Obviously when dominance is absent the F^ generation will 

 not include simply two sorts of plants, one three times as 

 numerous as the other, but a third, as well. A crimson snap- 

 dragon, for example, when crossed with a white one gives apinkFi 

 hybrid. When selfed, this produces an Fo in which one-fourth of 



