Alternation of generations

The term alternation of generations is used to describe an alternation of forms in the life cycle of plants (and some protists).

One form is diploid, with 2n chromosomes: the sporophyte. The other form is haploid with only one set of chromosomes: the gametophyte. Both forms are multicellular.^[1]

In sexual reproduction, organisms have a haploid phase with one set of chromosomes and a diploid phase with two sets of chromosomes. In animals the body (soma) is usually diploid, while the haploid stage is only the gametes.

In eukaryotes an alternation of generations may occur. This means that both the diploid and the haploid phases are multi-cellular organisms. The classic example is the mosses, where the green plant is a haploid gametophyte, and the reproductive phase is the diploid sporophyte.^[1] The two forms often occur together, as shown in the illustration on the right

The term alternation of generations refers only to the sexual cycle: organisms may have asexual reproduction as well. The term should not be confused with life cycle stages in animals, which may look very different, but where all cells have two sets of chromosomes.

1. ↑ ^{1.0 1.1} "Alternation of generations: reproductive cycles in which a haploid phase alternates with a diploid phase". King R.C. Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics. 7th ed, Oxford University Press, p18. ISBN 0-19-530761-5