alternation of generations


alternation of generations
the alternation in an organism's life cycle of dissimilar reproductive forms, esp. the alternation of sexual with asexual reproduction.
[1855-60]

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In biology, alternation of a sexual phase (gametophyte) and a nonsexual phase (sporophyte) in the life cycle of an organism.

The two phases, or generations, are often distinct in structure and sometimes in chromosome makeup. Alternation of generations is common in algae, fungi (see fungus), mosses, ferns, and seed plants. The character and extent of the two phases vary greatly among different groups of plants and algae. During the course of evolution, the gametophyte stage has been progressively reduced. Thus in higher (vascular) plants, the sporophyte is the dominant phase; in more primitive, nonvascular plants the gametophyte is dominant. Among animals, many invertebrates (e.g., protozoans, jellyfish, flatworms) have an alternation of sexual and asexual generations.

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also called  Metagenesis, or Heterogenesis,  

      in biology, the alternation of a sexual phase and an asexual phase in the life cycle of an organism. The two phases, or generations, are often morphologically, and sometimes chromosomally, distinct.

      In algae, fungi, mosses, ferns, and seed plants, alternation of generations is common; it is not always easy to observe, however, since one or the other of the generations is often very small, even microscopic. The sexual phase, called the gametophyte, produces gametes, or sex cells; the asexual phase, or sporophyte, produces spores asexually. In terms of chromosomes, the gametophyte has a single (i.e., monoploid, or haploid) set, and the sporophyte has a double (diploid) set.

      Among animals, many invertebrates have an alternation of sexual and asexual generations (e.g., protozoans, jellyfish, flatworms), but the alternation of haploid and diploid generations is unknown.

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Universalium. 2010.

Look at other dictionaries:

  • Alternation of generations — (also known as alternation of phases or metagenesis) is a term primarily used in describing the life cycle of plants (taken here to mean the Archaeplastida). A multicellular sporophyte, which is diploid with 2N paired chromosomes (i.e. N pairs),… …   Wikipedia

  • alternation of generations — n. the regular alternation of two distinct body forms in the life cycle of a plant or animal, usually an alternation between a form that reproduces sexually and one that reproduces asexually …   English World dictionary

  • alternation of generations — noun the alternation between generations of asexual with sexual reproduction in certain plants and animals, as in mosses and tapeworms …   Australian English dictionary

  • alternation of generations — al·ter·na·tion of generations .ȯl tər nā shənalso .al n the occurrence of two or more forms differently produced in the life cycle of a plant or animal usu. involving the regular alternation of a sexual with an asexual generation but not… …   Medical dictionary

  • alternation of generations — The alternation of two or more generations reproducing in different ways; an alternation of sexual and asexual, or parasitic with a free living cycle; see alloiogenesis, digenesis, heterogamy, heterogenesis,heterogonymetagenesis …   Dictionary of invertebrate zoology

  • alternation of generations — alterna′tion of genera′tions n. dvl the alternation in an organism s life cycle of dissimilar reproductive forms, esp. the alternation of sexual with asexual generations • Etymology: 1855–60 …   From formal English to slang

  • alternation of generations — Date: 1858 the occurrence of two or more forms differently produced in the life cycle of a plant or animal usually involving the regular alternation of a sexual with an asexual generation …   New Collegiate Dictionary

  • alternation of generations — noun Biology a pattern of reproduction shown by some lower plants and invertebrates, involving a regular alternation between sexual and asexual forms (as in ferns) or dioecious and parthenogenetic forms (as in some jellyfishes) …   English new terms dictionary

  • alternation of generations — noun the alternation of two or more different forms in the life cycle of a plant or animal • Syn: ↑heterogenesis, ↑xenogenesis • Hypernyms: ↑organic phenomenon • Hyponyms: ↑metagenesis, ↑digenesis …   Useful english dictionary

  • alternation of life phases —   , less correctly alternation of generations. Having distinct sporophytic (diploid), e.g. stems, roots, leaves, cones, etc., and gametophytic (haploid), e.g. tissue within the seeds of cycads and all other seed plants, phases in the life cycle …   Expanded glossary of Cycad terms


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