Final answer:
Crossing two pure-breeding plants with antagonistic traits yields progeny exhibiting only the dominant trait in the F1 generation; F2 offspring exhibit both traits in a 3:1 ratio.
Step-by-step explanation:
The outcome of crossing two pure-breeding plants with antagonistic traits results in progeny that all exhibit the dominant trait, with the recessive trait not appearing in this first generation (F1). However, when these F1 offspring are self-crossed, the F2 generation will exhibit both the dominant and recessive traits in a 3:1 ratio, as predicted by Gregor Mendel's laws of inheritance.
To illustrate, if true-breeding pea plants with yellow seeds (YY) are crossed with true-breeding pea plants with green seeds (yy), the F1 offspring will all have yellow seeds because yellow is the dominant trait. If we use a Punnett square, we can show that the F1 generation will all have genotype Yy. When the F1 plants are self-crossed, the F2 generation will have the genotypic ratio of 1 YY (yellow seeds), 2 Yy (yellow seeds), and 1 yy (green seeds), which translates to the aforementioned phenotypic ratio of 3:1 for dominant (yellow) vs. recessive (green) traits.