Question

When Gregor Mendel crossed true breeding tall plants with true breeding short plants, all of the offspring were tall because of

Answer 1

Hello @Esmedee !
Welcome to openstudy. It was so because true breeding tall plants have genotype TT and true breeding short plant have a genotype tt. Now T is dominant over t. So F1 generation or offspring were all tall (phenotypically)

Answer 2

Yep, remember true breeding = homozygotes. This means more than just the same phenotype (appearance, essentially)—it means that the organisms in the breeding population are all homozygotes with the same allele.