446 



THE MAINTENANCE OF SPECIES 



other hand, being constituted like their hybrid parents, when interbred 

 furnished again the typical 3 : 1 ratio. In Mendel's original experi- 

 ments there were actually obtained from the Ss peas 5474 smooth 

 and 1850 wrinkled peas, which is very near the expected 3 : 1 ratio. 

 Such pure SS peas and hybrid Ss peas are said to be phenotypically 

 alike and genotypically different. That is, they look alike, but have 

 different possibilities when it comes to producing gametes. The 

 way to distinguish the one from the other is to breed them hack with 

 the recessive ss peas, which can conceal nothing, and observe the kind 

 and proportion of the offspring produced. SS X ss gives 100 per cent 

 Ss (phenotypically smooth), while Ss X ss gives 50 per cent Ss 

 (phenotypically smooth) and 50 per cent ss (phenotypically wrinkled), 

 as shown in the checkerboard below, in which the gametes of the two 

 sexes are placed outside the double lines, and the resulting kinds of 

 individuals are represented by double letters within the squares. 



Sometimes dominance may be incomplete, in which case it is not 

 necessary to back-cross with the corresponding recessive in order to 

 determine which are pure and which are hybrid dominants. The 

 four-o'clock {Mirahilis jalapa), as pointed out by Correns, furnishes 

 a well-known demonstration of this point, for the hybrid produced by 

 red X white flowers is not dominant red, as might be expected, but 

 pink. The pink hybrids give in turn the proportion of three colored 

 flowers (one red and two pink) to one white. 



Mendel carried through the same hybridization procedure and sub- 

 sequent follow-up, with each of the seven pairs of contrasting char- 

 acters, and found that the approximate 3 : 1 result always obtained, 

 regardless of whatever other characters were present in the individual 

 plants. Each pair of characters, in other words, behaved inde- 

 pendently of every other pair. This is called the principle of inde- 

 pendent assortment. 



It is apparent, moreover, that the determiner for each character 

 retains its integrity, reappearing in the next generation true to 

 itself, regardless of the company it has been keeping within the germ 

 cell. This integrity of the hereditary determiners, together with the 



