Alternation of generations
There are three different life cycles in which the organism alternates between a haploid genetic state and a diploid (or higher) genetic state.
- In animals, the cells of the multicellular adult body are usually diploid (or sometimes polyploid), and the sex gametes (sperm and eggs) are haploid.
- Animals spend most of their life in the diploid genetic state, and only undergo meiosis at the time of gamete production. Gametes are single cells that have no independent existence.
- Two haploid gametes fuse (fertilization) to produce a diploid zygote, which divides by mitosis to produce the large number of diploid somatic cells in the animal body.
- In some algae and some fungi the cells of the multicellular adult body are haploid. This is the longest part of the life cycle.
- Gametes are produced by mitosis (not meiosis) and after fertilization a diploid zygote is created.
- The single zygote cell never grows or divides my mitosis. It can only divide by meiosis to produce haploid cells once more, which then produce the main adult body.
- In plants and some algae, there is a multicellular diploid and a multicellular haploid period of the life cycle.
- Most common flowering plants are diploid for the majority of their existence (called the "sporophyte" phase), produce haploid "spores", which develop into small, but multinucleate haploid structures (the "gametophyte" phase), which in turn produce the haploid gametes (often just nuclei) used in fertilization and zygote formation.