According to Mendel, gametes are always pure and contain one allele for each gene. Let's assume that the genotype of the tall plant is "TT" and the genotype of the short plant is "tt". The tall plant would produce all the gametes with one copy of the "T" allele while all the gametes from the short plant would carry only the "t" allele.
The random fusion of gametes from tall and short parent plant produces all the F1 progeny with genotype "Tt". Here, each F1 plant obtained one "T" allele from the tall parent plant and one "t" allele from the short parent plant.
