Alternation of generations is the plant life cycle pattern in which a multicellular diploid sporophyte generation alternates with a multicellular haploid gametophyte generation. It matters because it explains how plants make spores, gametes, seeds, and new offspring across many groups, from mosses to flowering plants. The cycle also shows how meiosis and fertilization balance chromosome number from one generation to the next.
Understanding this pattern helps connect genetics, reproduction, and plant evolution in one clear model.
In the sporophyte phase, the plant has two sets of chromosomes, written as 2n, and produces haploid spores by meiosis. Each spore can grow by mitosis into a gametophyte, which has one set of chromosomes, written as n, and produces gametes by mitosis. Fertilization joins two gametes to form a diploid zygote, which grows into the next sporophyte.
In mosses the gametophyte is the dominant visible plant, while in ferns and flowering plants the sporophyte is dominant and the gametophyte is reduced.
Key Facts
- Sporophyte chromosome number = 2n, meaning diploid with two sets of chromosomes.
- Gametophyte chromosome number = n, meaning haploid with one set of chromosomes.
- Meiosis in the sporophyte produces spores: 2n cell -> n spores.
- Mitosis in the gametophyte produces gametes: n cell -> n gametes.
- Fertilization restores diploidy: n egg + n sperm = 2n zygote.
- Plant life cycle sequence: sporophyte -> meiosis -> spores -> gametophyte -> gametes -> fertilization -> zygote -> sporophyte.
Vocabulary
- Sporophyte
- The diploid multicellular generation of a plant that produces haploid spores by meiosis.
- Gametophyte
- The haploid multicellular generation of a plant that produces gametes by mitosis.
- Spore
- A haploid reproductive cell that can grow into a gametophyte without fusing with another cell.
- Gamete
- A haploid sex cell, such as sperm or egg, that can fuse with another gamete during fertilization.
- Zygote
- The diploid cell formed when two haploid gametes fuse during fertilization.
Common Mistakes to Avoid
- Calling spores the same as gametes is wrong because spores grow into a new multicellular gametophyte, while gametes must fuse during fertilization.
- Saying meiosis makes gametes in plants is incomplete because in alternation of generations meiosis makes spores, and gametophytes make gametes by mitosis.
- Assuming the visible plant is always the gametophyte is wrong because mosses are gametophyte dominant, but ferns and flowering plants are sporophyte dominant.
- Forgetting that both generations are multicellular is wrong because alternation of generations specifically includes a multicellular sporophyte and a multicellular gametophyte.
Practice Questions
- 1 A plant species has a diploid chromosome number of 2n = 24. How many chromosomes are in each spore produced by meiosis?
- 2 A gametophyte of a fern has n = 18 chromosomes. How many chromosomes will be in the zygote after fertilization?
- 3 In a moss and a flowering plant, which generation is usually dominant in each, and how does that difference change what you see as the main plant body?