Final answer:
The correct option for the given question is 'A. F1, F2'. In this context, all of the F1 plants and 3/4 of the F2 plants show the tall stem phenotype, showing that tall stems are the dominant trait.
Step-by-step explanation:
In the context of genetics and Mendelian inheritance, when true-breeding tall stem pea plants are crossed with true-breeding short stem pea plants, the correct fill in for your question is: all of the F1 plants, and 3/4 of the F2 plants had tall stems. Therefore, tall stems are dominant.
This is the theory of dominance, according to which the dominant phenotype is expressed in the F1 generation of a cross between two true-breeding parents. The F1 generation is purely heterozygous, showing the dominant phenotype. The F2 generation, resulting from self-fertilizing the F1, follows a ratio of 3:1 dominant-recessive phenotypes.
For example, a tall pea plant (TT), when crossed with a short pea plant (tt), will result in offspring that are all heterozygous Tt (F1 generation) and hence tall. The F2 generation results from crossing the F1s and will display both phenotypes, with tall being more prevalent according to Mendelian ratio.
Learn more about Mendelian Inheritance