36 Plant Genetics 



from the other 2 in germinal constitution, for i will 

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

 produce two kinds. To indicate this situation Johann- 

 SEN (i) 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 : i 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. 4 several things may be 

 inferred. It can be seen what will happen in the Fj 

 generation when the F2 individuals are inbred. The 

 dominant homozygote will produce only dominant 

 homozygotes in the Fj generation and will continue to 

 produce them as long as it is inbred. The two hetero- 

 zygotes will split up in the Fj generation in the same 

 1:2:1 ratio as did their hybrid parents of the Fi genera- 

 tion. 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 reces- 

 sive. It should be obvious that one-half of the progeny 

 would be pure recessives, while the other half would be 

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

 similar result would be obtained by crossing a hetero- 

 zygote with a dominant homozygote, although all the 



