Answer:
The tall plant is homozygous (TT) if all the progeny of the test cross is "tall". Presence of both tall and short progeny makes the "tall" plant heterozygous (Tt).
Explanation:
The "tall" plant with unknown genotype would be crossed with a homozygous short plant (tt) of the same species. This is called the testcross. If the progeny of test cross includes both tall and short plants, the "tall" plant with unknown genotype was heterozygous for the gene. On the other hand, the presence of only tall progeny in the testcross reflects the fact that the "tall" plant with unknown genotype was homozygous for the allele for "tallness".
