Final answer:
A Punnett square analysis shows that 100% of the F1 generation from a cross between true-breeding round, yellow seeded pea plants (RR YY) and wrinkled, green seeded pea plants (rr yy) will have round, yellow seeds.
Step-by-step explanation:
When Mendel mated a pea plant of true-breeding round, yellow seeds (genotype RR YY) with a pea plant of true-breeding wrinkled, green seeds (genotype rr yy), he performed what is known as a monohybrid cross.
Since the round seed shape (R) is dominant over the wrinkled seed shape (r) and yellow pea colour (Y) is dominant over green (y), all the offspring (F1 generation) from this cross would have the genotype Rr Yy. This genotype expresses the dominant phenotypes, hence, these offspring would all have round, yellow seeds. Therefore, a Punnett square analysis shows that 100% of the F1 generation has round yellow seeds, considering that the traits are independently inherited and the law of dominance applies.
In subsequent crosses, such as between F1 individuals, Mendel determined that the ratio of phenotypes in the F2 generation would show different probabilities, with 9/16 displaying the dominant phenotypes of round yellow seeds, but for the initial cross between the true-breeding parents, the result for the F1 generation is uniform with round and yellow peas.