GENETIC SYSTEM : RELATION TO CHARACTERISTICS 1 1 1 



unite to form a zygote are unlike, as A and a, then the 

 zygote so produced is called a heterozygote, as Aa. 



The two corresponding characters, one dominant, the 

 other recessive, that are produced by the two differing mem- 

 bers of a pair of chromosomes, are spoken of as alternative 

 characters, or alleles, or allelomorphs. Any given individual 

 manifests one or the other of the two alternatives, not 

 both. For the alternative characters we shall employ hence- 

 forth the term alleles. 



The characteristic manifested by a zygote is its phaeno- 

 type, while its actual constitution is its genotype. Thus, 

 two zygotes AA and Aa both manifest the dominant char- 

 acter; the dominant character is the phaenotype, or the 

 individuals are said to be phaenotypically dominant. But 

 they are diverse in genotype; one is AA, the other Aa. In 

 the case of a homozygote AA or aa, the phaenotype and 

 genotype may be held to coincide. 



Mendel crossed yellow peas with green peas, and found, 

 as before stated, that in the Fi generation all the peas 

 were yellow, so that yellow is the dominant allelomorph. 

 In the F2 there were three times as many yellows as greens. 

 The actual number of seeds in the F2 generation was in 

 his experiments 8023, of which 6022 were yellow while 

 2001 were green, so that the ratio of dominants to reces- 

 sives was, accurately expressed, 3.01 to i. 



Mendel found that if he bred further the individuals of 

 the F2 generation, some of these gave different results from 

 others. In the pea plant it is possible to allow each plant to 

 fertilize itself, so that both kinds of germ cells come from 

 the same parent. When this was done with the individuals 

 of the F2 generation, the following results were observed: 



The green plants (recessive) gave only recessive green 

 offspring (they "bred true"). 



Of the yellow plants (dominant), one-third gave only 

 yellow offspring (they "bred true"), while two-thirds of 



