Question

what will happen if both the characters present on F1 generation pass together in F2 generation ?

Answer 1

then self fertilization does not occur and 3:1 ratio will not seen

Answer 2

This condition is called incomplete dominance.
It was found dat sometimes the F1 had a phenotype that did not resemble either of the two parents and was in between the two.
Eg: flower of Snapdragon.