Haploid and diploid stages in an organism's life cycle

This is an in-progress expansion of a blog post.

After watching this nice Youtube video about plant evolution, I have kind of an interesting question about how chromosome pairing works. The presenter1 talks about "the alternation of generations" between haploid versus diploid — that is, from having one set of chromosomes to having two sets of chromosomes.

Humans and other mammals spend most of our life cycle in the "diploid" condition: our body cells have two sets of chromosomes. Only when making sperm and egg cells do the chromosomes get unpaired. But apparently plants can be different: algae are mostly haploid, and mosses have a weird life cycle where the haploid and diploid phases of life look like different organisms. (Also, mosses and algae apparently have motile, swimming sperm cells.)

  1. It's strange to me that folks who are doing Science Communication Youtube make it hard to find out their actual names. His channel is Mugsy Explains, but his name is ... a mystery. When the hosts of different channels get together, they mostly call each other by their channel names. It's a little wild. ↩