Answer:
Both parents have a genotype of Tt
Step-by-step explanation:
Let's say the genotype for tall is T and the genotype of short is t
When the plants are crossed, it looks a little like this: 
  I T I t I
 I T I TT I Tt I
 I t I Tt I tt I
 The lower case tt represents the dwarf plants
Hope this helps :)