When Gregor Mendel crossed a tall plant with a short plant, the F1 plants inherited ...

One allele of tallness form one of the parent and one allele for dwarfness from another parent.

Explanation:

Mendel took two plants one was homozygous dominant (TT) for height trait and the other was homozygous recessive (tt) for height trait. The homozygous dominant individuals was tall and recessive was short.

So when they are crossed during fertilization the offsprings got one allele for tallness from the homozygous dominant parent and one allele for shortness from the homozygous recessive parent.

T T

t Tt Tt

t Tt Tt

So all the offsprings are heterozygous for the height trait that means all the individuals will be tall in appearance as they have one dominant trait for tallness.