Final answer:
The false statement about round-seeded pea plants in the F2 generation is that two-thirds of these plants are expected to be heterozygous for the seed shape allele; in reality, only 50% should be heterozygous according to Mendelian genetics.
Step-by-step explanation:
The question pertains to Mendelian genetics, specifically the inheritance of seed shape in pea plants. When considering the statements about round-seeded pea plants obtained in the F2 generation, each can be assessed for its truthfulness based on Mendelian principles. For example, the statement that these plants are phenotypically identical for seed shape is true, as they all exhibit round seeds. However, stating that two-thirds of these plants are expected to be heterozygous for the seed shape allele is incorrect because in the F2 generation, we expect a 3:1 phenotypic ratio, which means 1/4 would be homozygous dominant, and 1/2 would be heterozygous. Furthermore, expecting 25% of these plants to be homozygous for the seed-shape allele aligns with Mendel's laws since 1/4 of the F2 should be homozygous recessive.
Now, if these round-seeded plants were crossed with wrinkled-seeded plants (rr), the heterozygous plants (Rr) would produce a mix of round and wrinkled seeds, while the homozygous dominant plants (RR) would produce only round seeds. Therefore, the statement that some round-seeded plants would produce only round-seeded plants when crossed with wrinkle-seeded plants is true for the homozygous dominant plants. The false statement in this scenario is that two-thirds of them are expected to be heterozygous, as mentioned earlier.