Final answer:
Mendel's cross of homozygous purple-flowered plants with homozygous white-flowered plants resulted in all purple offspring due to the dominance of the purple flower trait over the white flower trait.
Step-by-step explanation:
When Gregor Mendel crossed homozygous purple-flowered plants with homozygous white-flowered plants, all the offspring were purple because the purple flower trait is dominant over the white flower trait. The first generation of offspring, known as the F1 generation, all displayed the purple flowers, indicating that the white flower trait did not express itself when paired with the purple flower trait. This observation led Mendel to propose the concept of dominant and recessive traits, which laid the foundation for modern genetics.