390 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



frequent terminolog>^ is as follows. When two similar gametes unite 

 to form a zygote it is called a homozygote; when the two pairing 

 gametes are different the zygote is called a heterozygote. Using this 

 terminology it is evident that the 3:1 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 charac- 

 ter. 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. 68 three tall individuals 

 appear in the F2 generation. Superficially the individuals look alike, 

 but it is realized that i differs from the other 2 in germinal constitu- 

 tion, for I will produce only one kind of gamete, while the other 2 

 will produce two kinds. To indicate this situation Johannsen has 

 introduced some appropriate terminology. Organisms which seem 

 to be alike, regardless of their germinal constitution, are said to be 

 phenotypically alike, or to belong to the same phenotype. On the 

 other hand, organisms having identical germinal constitution are said 

 to be genotypically alike, or to belong to the same genotype. From 

 the standpoint of phenotypes only, Mendel's F2 generation shows the 

 3:1 ratio; but if genotypes are considered, it shows the 1:2:1 ratio. 

 In other words, this group of forms contains two phenotypes but three 

 genotypes. 



Referring again to Fig. 68 several things may be inferred. It can 

 be seen what will happen in the F3 generation when the F2 individuals 

 are inbred. The dominant homozygote will produce only dominant 

 homozygotes in the F3 generation and will continue to produce them 

 as long as it is inbred. The two heterozygotes will split up in the 

 F3 generation in the same 1:2:1 ratio as did their hybrid parents of the 

 Fi generation. The recessive homozygote will produce only recessive 

 homozygotes as long as it is kept pure by being inbred. 



It is interesting to consider what will happen if a heterozygote 

 form is crossed with a homozygous recessive. It should be obvious 

 that one-half of the progeny would be pure recessives, while the other 

 aalf would be heterozygotes, that is, there would be a 1:1 ratio. A 

 similar result would be obtained by crossing a heterozygote with a 

 dominant homozygote, although all the immediate progeny would 

 show the dominant character. The real situation would be revealed, 

 however, when this progeny was inbred, for one-half would be homo- 

 zygous (pure breeders) and the other half would be heterozygous 

 (hybrid breeders). 



