Give a very short answer to the question given below:
 
Mendel crossed pure tall pea plants (TT) with pure short pea plants (tt) and obtained F1 progeny. When the plants of F1 progeny were self-pollinated, plants of F2 progeny were obtained.

a. What did the plants of F1 progeny look like? Give their gene combination.
 
b. Why could the gene for shortness not be expressed in plants of F1 progeny?
 
 
c. Write the ratio of the plants obtained in F2 progeny and state the conclusion that can be drawn from this experiment.