HERITAGE OF THE INDIVIDUAL 283 



and more apparent that exceptions which prove the rule, 

 make it necessary to revise somewhat our ideas regarding the 

 unity of unit characters and the dominance of dominants, and 

 to accentuate the principle of segregation as the prime Men- 

 delian contribution. A few examples will serve to bring the 

 main facts before us. 



The seven pairs of contrasting characters in Peas which 

 Mendel studied showed essentially complete dominance of 

 one character in each pair, and therefore, quite naturally, he 

 laid stress on this principle. As a matter of fact we may 

 say that dominance is hardly the rule because there are in- 

 numerable cases in which the hybrid (Fi) shows a different 

 condition from either of the parents. For instance, on cross- 

 ing homozygous red and white races of the Four-o'clock, all 

 the progeny in the heterozygous (Fi) generation bear pink 

 flowers, or, we may say, flowers intermediate in color between 

 the two parents. Neither red nor white is dominant. But in- 

 breeding these give an F 2 of 1 red, 2 pink, and 1 white. Thus 

 the typical Mendelian 3 : 1 ratio is, so to speak, automati- 

 cally resolved into the 1:2:1 ratio which, when one character 

 is dominant, is only patent on further breeding. (Fig. 144.) 



In the case of the Four-o'clock, only the hybrids are inter- 

 mediate; segregation occurs as usual and the homozygous 

 progeny show the original parental characters unmodified. 

 But sometimes, with the apparent lack of dominance, segre- 

 gation seems not to take place. The cross between white and 

 black races of Man is a typical example. 



The mulatto (Fi) is intermediate in skin color between the 

 parental types and even in the F 2 and later generations rarely 

 gives pure white or black offspring. But an adequate Men- 

 delian explanation is not far to seek. It has been found that 

 both white and black are really composite characters, each 

 made up of varying amounts of black, yellow, and red pig- 



