What the Shit Is "Alternation of Generations"

I created these bits as part of a crash course in a course I'm teaching and am posting them here in case anybody who's NOT ALREADY FAMILIAR WITH THEM (I assume most of you are) can get an idea on this process and why it's important. It will affect the way that you move through and interpret the world.

Remember........"Haploid" and "Diploid" are the main things to focus on when trying to understand "Alternation of Generations (AOG)", which ALL PLANTS partake in. It's weird and there's nothing really analogous to it in animals.

Two haploid halves (sperm cell and egg cell) come together to form a diploid whole. This is the same way that reproduction works in animals, but in plants there is an entire extra generation added into the process (the Gametophyte stage - remember, in ferns the Gametophyte is the small, inconspicuous little green leafy structure that looks like a piece of tiny lettuce).

Gametophytes are haploid. They only have one copy of the genome. In humans, only sperm and egg are haploid. In Humans, Haploid sperm or egg only have 23 chromosomes each.

Sporophytes are diploid. They have TWO copies of the entire genome, that is, two sets of the full list of chromosomes. 1 set from each parent. In Humans, every cell in the body that's not sperm or egg cells has 46 chromosomes - 23 from your father & 23 from your mother.

Meiosis is a kind of cell division that is synonymous with "haploid". Meiosis creates haploid cells.

Mitosis is just every-day run-of-the-mill cellular division that is happening when a plant grows (or in humans when your cells divide anywhere in your body that's not your gonads/sperm/egg etc.)

Two haploid halves come together to make a diploid whole (the zygote). That diploid whole (the zygote) will be a different phenotype than either of the parents, each of which donated a haploid cell to form the zygote. Parents and offspring are same genotype, but different phenotype. An Agave plant that produces offsets (pups) at the base is producing clones of itself. They are the same phenotype as the plant that they came from. No sexual reproduction or genetic recombination was involved. But when that same plant produces a flower which gets pollinated by another flower and the produces seed, those seeds then germinate and grow into new, different phenotypes. Phenotypic change is one of the key elements necessary for evolution. The environment selects for what phenotypes survive and which perish. "The environment" is all the circumstances and traits in an organism's direct vicinity, whether that's geology, climate, elevation, presence of a certain kind of herbivore or certain kind of pollinator, etc. In order for the environment to have something to select for, new phenotypes have to be present within a population. If the entire population were just composed of clones, they would generally all respond the same way to the same stresses.

Remember, in ferns (as in all plants), spores are haploid. The spore germinates and grows into a haploid Gametophyte. The Gametophyte produces haploid sperm and haploid egg (in antheridia and archegonia, respectively). The haploid sperm swims to haploid egg and they come together to form a DIPLOID SPOROPHYTE, which in every plant lineage except the mosses, because the dominant plant that you can readily see.

Any questions, consult the pdf presentation I uploaded today (all the material about haploid//diploid is in the bryophytes presentation but not in the other presentations) and look for the Alternation of Generations Diagram. If you're asking questions, that means you're actively thinking about this stuff. Focus on trying to understand it and understanding the processes, not just memorizing it. Memorization isn't fun. Understanding it will change the way you understand your world.