Mendel's Law 



35 



In fig. 4 one zygote receives a double dose of tallness 

 and two others a single dose. These phrases are more 

 or less common in the Uterature of the subject, but the 

 more frequent terminology is as follows. When two 

 similar gametes unite to form a zygote it is called a homo- 

 zygote; when the two pairing gametes are different the 

 zygote is called a heterozygote. Using this terminology 



o o 



© ® 



Q O 



® ® 

 Q_0 



Fig. 4. — Diagram illustrating behavior of first hybrid generation 

 (F,) when inbred. Illustrates meaning of "segregation" and "purity of 

 gametes,'' and how chance matings of Fi gametes result in 3:1 ratio 

 in Fz generation; dwarf individual produced only by zygote in lower 

 right-hand comer. 



it is evident that the 3 : i ratio of the F2 generation is 

 really a 1:2:1 ratio, as follows: i homozygote for the 

 dominant character, 2 heterozygotes, and i homozygote 

 for the recessive character. The 1:2:1 ratio therefore 

 is the significant one and appears as a 3:1 ratio only 

 because of dominance. 



In the experiment represented in fig. 4 three tall 

 individuals appear in the F2 generation. Superficially 

 the individuals look alike, but it is realized that i differs 



