19151 Outline on the Theory of Descent 13 



Mendel's method differed from that of earlier students of 

 heredity in that he focused his attention upon the behavior 

 of a single pair of alternative characters. He selected the 

 ordinary garden pea to work with, and for eight years car- 

 ried on his experiments in hybridization. The parental 

 characters reappear in their hybrid offspring in a perfectly 

 definite fashion, which has come to be known as '^Mendel's 

 Law." 



An example or two may make this clear. A tall pea, 

 crossed with a dwarf, produces offspring which are invariably 

 tall. So it is when round seeded pears are crossed with 

 wrinkled ones, yellow with green and with a host of other 

 character-pairs which have been tested by Mendel and his 

 followers. The offspring are not intermediate in character 

 between the two parents as had been universally supposed to 

 be the case, but wholly like one parent which Mendel called 

 the dominant. The character which does not appear in the 

 offspring — dwarfness in this case — is called recessive. 



IsTow when these hybrids are interbred, three kinds of 

 offspring are produced and, when the numbers are large, in 

 fairly definite proportions. One fourth are dwarfs and 

 breed true. The remaining three-fourths are tall, and look 

 alike, but behave differently in reproduction. One of the 

 fourths is tall and breeds true, the other two-fourths are tall, 

 but not true breeding — they split up again in the same ratio 

 — 25 per cent true breeding tall, 25 per cent true breeding 

 dwarfs, and 50 per cent tall but not breeding true, and re- 

 peating this behavior in the next generation. 



Another case — the blue Andalusian fowl does not breed 

 true. Its offspring is 25 per cent white, 25 per cent black 

 and 50 per cent blue. The whites and the blacks breed true 

 to color, but the blues repeat the story. The Andalusian is 

 thus seen to be not a true race of fowls but merely a hybrid 

 between a white and a black race, in which black is not en- 

 tirely dominant as it is in guinea pigs for example, and as it 

 usually is, and as tallness is to dwarfness in the case of 

 garden peas. 



