390 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
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 different the zygote is called a heterozygote. Using this 
terminology it is evident that the 3:1 ratio of the F, generation is 
really a 1:2:1 ratio, as follows: 1 homozygote for the dominant 
character, 2 heterozygotes, and 1 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 F, generation. Superficially the individuals look alike, 
but it is realized that 1 differs from the other 2 in germinal constitu- 
tion, for 1 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 F, 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 F; generation when the F, individuals 
are inbred. The dominant homozygote will produce only dominant 
homozygotes in the F, generation and will continue to produce them 
as long as it is inbred. The two heterozygotes will split up in the 
F, generation in the same 1: 2:1 ratio as did their hybrid parents of the 
F, 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). 
