I've been reading on Wikipedia about how plants alternate generations between a diploid sporophyte (usually the dominant part) and haploid gametophyte (in flowering plants, the pollen and ovule sacs).
Mammals also produce haploid gametes. But in mammals we don't describe the gametes as a separate generation that alternates with the dominant "sporoid." The gametes are just the organism's means of reproduction.
What is it about plant reproduction that warrants describing their gametes as a separate "generation" or "phase"?
Answer
The big difference is that in humans, there is no mitosis in the haploid phase.
There are three terms that are important here:
- Haplontic: Most of the life is spent in the haploid phase
- Diplontic: Most of the life is spent in the diploid phase
- Haplodiplontic (aka. diplohaplontic): About as much time is spent in the haploid phase than in the diploid phase
At the extreme, when there is absolutely no mitosis in one of the phase, then the organism is clearly not haplodiplontic. Haplodiplontic organisms have an alternation of generations.
Note that the term alternation of generations is also used for systems that are much more complex than haplodiplontie. Examples of such complex alternation of generation include many parasites such as the giant liver fluke
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