In a monohybrid cross by Mendel, when a pure tall plant was crossed with a dwarf plant, all the $F_1$ progeny were tall because of:

According to Mendel's Law of Dominance, when two contrasting alleles for a specific trait are present together in a heterozygous condition ($F_1$ generation), one allele completely expresses itself (the dominant allele, e.g., Tallness '$T$ ') and masks the expression of the other alternative allele (the recessive allele, e.g., Dwarfness '$t$ ').