44 Outline of Genetics 



brids is indicated in the accompanying diagram (fig. 4), 

 which represents the chance matings of the two kinds of 

 gametes. The obvious results are three tall individuals 

 and one dwarf. This is the so-called monohybrid ratio, 

 which means the ratio when a single pair of allelomorphs 

 is considered. 



Before discussing the further development of Mendel's 

 law, it will be necessary^ to explain some of the terminol- 

 ogy of genetics. When each gamete carries the same 

 kind of determiner the resulting zygote is said to receive 

 a double dose; when a zygote receives only a single such 

 determiner it is said to receive a single dose. 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 literature of the subject, but the more 

 frequent terminology is as follows. When two similar 

 gametes unite to form a zygote it is called a homozygote; 

 when the two pairing gametes are dift'erent the zygote is 

 called a heterozygote. Using this terminology, it is e\'ident 

 that the 3 : i ratio of the F2 generation is really a 1:2:1 

 ratio, as follows: i homozygote for the dominant char- 

 acter, 2 heterozygotes, and i homozygote for the recessive 

 character. The 1:2:1 ratio, therefore, is the significant 

 one and appears as a 3 : i ratio only because of dominance. 



In the experiment represented in fig. 4, three tall 

 indi\dduals appear in the F2 generation. Superficially the 

 individuals look alike, but it is realized that one differs 

 from the other two in germinal constitution, for one will 

 produce only one kind of gamete, while the other two 

 will produce two other kinds. To indicate this situation 

 JOHANNSEN (2) has introduced some appropriate termin- 

 ology. Organisms which appear to be alike, regardless 



