When looking at the inheritance of a single character, Mendel found that a cross between two true-breeding peas (between purple and white, for example) always yielded which genotype (s) in the F1 generation?

All progeny with heterozygous dominant genotype

Explanation:

Mendel crossed two pure breeding pea plants with respect to one genetic trait. He found that the F1 progeny obtained by crossing two pure breeding parent plants is always heterozygous dominant. The phenotype of the F1 progeny was always the phenotype of the dominant pure breeding parent plant.

For instance, purple flower color is a dominant trait over the white flower color in pea plants. When he crossed a pure breeding purple-flowered pea plant (PP) with a pure breeding white-flowered pea plant (pp), the F1 progeny was expressed the phenotype of the dominant parent and had purple flowers. The genotype of the F1 plants was Ww (heterozygous dominant).