Alteration of generations | definition of Alteration of generations by Medical dictionary
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interrupted occurrence, being interspersed with different or opposite events.
al·ter·na·tion of gen·er·a·tions
a succession of generations of individuals like and unlike the original parents, or an alternation of sexual and nonsexual generations.
alternation of generations
The regular alternation of forms or of mode of reproduction in the life cycle of an organism, such as the alternation between diploid and haploid phases, or between sexual and asexual reproductive cycles.
Fig. 26 Alternation of generations .(a) Life cycle of Obelia (Coelenterata). (b) Life cycle of fern (Pteridophyta). n = haploid number of chromosomes.
alternation of generations the occurrence within the life cycle of an organism of a sexually reproducing generation followed by an asexual generation. In some COELENTERATES, such as Obelia, the sexual organs are carried on MEDUSAS, and subsequent fertilization gives rise to an asexually producing HYDROID stage; here both generations are DIPLOID. Amongst the plants, ferns have a diploid SPOROPHYTE generation (the plant itself). This undergoes MEIOSIS to form spores that give rise to a HAPLOID GAMETOPHYTE generation known as a PROTHALLUS, which reproduces sexually. Male and female GAMETES fuse to form a ZYGOTE, which then develops into a new fern plant (sporophyte). Some definitions of the term require a haploid generation alternating with a diploid generation, so excluding all animal examples.