So, genetically the cross from the pea plants is as follows:

P     AA   X    aa

F1    All of the plants are Aa.  So the cross for the F2 generation is Aa   X    Aa


F2   Punnet Square   

     A          a

A  AA     Aa

a   Aa     aa


Which yields a 3:1 Ratio   Tall:Dwarf.



TERMS

GENE- A location on a DNA molecule that codes for a General Trait (Plant Height Gene)
ALLELES - Different forms of the same gene.   (A and a)
PHENOTYPE - the "physical appearance" of an organism. (Tall and Dwarf)
GENOTYPE - the genetic makeup of an organism.  3 basic ones:
1. HOMOZYGOUS DOMINANT - when both alleles are dominant (AA)
2. HOMOZYGOUS RECESSIVE - when both alleles are recessive (aa)
3. HETEROZYGOUS - the alleles in a pair are different.  (Aa)