Question

why were all of the offspring tall when Gregor Mendel crossed true breeding tall plants with true breeding short plants

Answer 1

All of offspring had the "tall" phenotype because one breed of the plant was "true breeding", which means that, for that gene, it only has one allele (one version of the gene), and so it passed it on to all it's offspring. Look at the punnett square below.



T= tall allele; true breeding tall genotype= TT
t=short allele; true breeding short genotype= tt

All of the progeny have the genotype Tt, and because the "tall" phenotype is observed in all the progeny, we say that the "tall" phenotype is \(\sf dominant\) over the "short" phenotype.