240 Experimental Transformation of Species 



nite proportions. That is to say, the white trait and the 

 black trait which the blues have derived from their mixed 

 ancestry are segregated in the offspring in one-half of the 

 generation, while the other half maintains the hybrid ap- 

 pearance of the immediate parents. Similarly the seed of 

 the pink-flowered four-o'clock will produce three types of 

 plants bearing respectively white flowers, red flowers, and 

 pink flowers. And again the proportions are the same — 25 



I s 



6 "v. 



Gametes X' 



6- 



Fi 



C- 



o 





Gametes 



-O 

 O 



I I I I I I I I 



6666 ^^0666 



Fig. 61. Mendel's Experiments in Back-crossing 



What happens if a hybrid (F ) showing the dominant character of one parent is mated 

 with the " pure " stocks of the two parental types (P) ? The dominant parent (repre- 

 sented say by the dark circles) produces only gametes that have the determiner for the 

 dominant character. The pure recessive parent also produces but one kind of gamete, that 

 bearing the determiner for the recessive character. The hybrid (F^), however, although 

 resembling the dominant parent, produces two kinds of gametes. If F^ is mated with the 

 dominant parent line, two kinds of individuals are produced, in equal numbers: pure 

 dominants, and hybrid dominants, as shown in the right half of the row F,. If Fj is 

 mated with the recessive parent line, two kinds of individuals are again produced, and 

 again in equal numbers: there are pure recessives and hybrids that appear dominant, as 

 shown in the left half of the F2 row. It does not matter which parent brings in the 

 dominant and which the recessive; nor does it matter which part of our diagram we use 

 to represent the dominant and which the recessive. The results of Mendel's experiments 

 lend themselves to broad mathematical treatment, which is presumptive, though not con- 

 clusive, evidence of the soundness of his g3neralization. 



per cent white, 25 per cent red, and 50 per cent pink or 

 hybrid type. 



The incompleteness of dominance may result in the ap- 

 pearance among the hybrids of a graded series between the 

 extreme types presented by the parents. Among the fruit 



