Which of the following images depicts alternation of generations?

Question

BlackTom AI · Accepted Answer

To determine which image depicts alternation of generations, we first recall the concept: alternation of generations involves two distinct multicellular phases, a sporophyte that is diploid (2n) producing spores by meiosis, and a gametophyte that is haploid (n) producing gametes by mitosis. These gametes fuse (fertilization) to form a diploid zygote, which grows into the next sporophyte generation, continuing the cycle.

Option A: In this diagram, the cycle emphasizes a line that moves from a zygote through meiosis back to a 2n state, and then to a mitotic process. This presentation tends to emphasize one part of the cycle (the diploid phase and meiosis) without clearly showing the separate haploid (n) gametophyte stage producing gametes by mitosis, which is essential to alternation of generations. The lack of a clearly distinct n phase and the explicit production of gametes by mitosis makes this depiction incomplete for the alternation concept.

Option B: This image shows both the haploid (n) and diploid (2n) stages in a connected loop. The cycle includes meiosis producing spores (n), those spores growing into a haploid stage, mitosis producing gametes, fertilization forming a diploid zygote (2n), and then meiosis again to restart the cycle. This presentation captures the two alternating generations (sporophyte via meiosis to spores, and gametophyte via mitosis to gametes), along with fertilization returning to the diploid state. The presence of both n and 2n phases and their transitions aligns with the classic alternation of generations model.

Option C: Here, the diagram appears to depict a cycle that moves from haploid or diploid stages but emphasizes a continuous loop without clearly separating the sporophyte and gametophyte generations, or it pairs meiosis and fertilization in a way that bypasses the distinct mitotic production of gametes and the production of spores as the alternate generation. This can misrepresent the cyclical alternation between the two generations, making it less accurate for the concept.

In summary, option B uniquely demonstrates the core components of alternation of generations: both haploid (n) and diploid (2n) stages, meiosis producing spores, mitosis producing gametes, and fertilization completing the cycle. The other options either underemphasize one generation or fail to explicitly show the two-generation alternation that defines the concept.

Which of the following images depicts alternation of generations?

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