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(1) A type of sexual process in which two gametes fusing during fertilization are distinguished by their external form. In heterogamy in the narrow sense the gametes of both sexes differ in size but do not differ in form and behavior (for example, the mobile, flagellate gametes of some algae). A large gamete is called a macrogamete (egg cell), a small one is called a microgamete (spermatozoon), and both are called heterogametes or anisogametes. In a broader sense, heterogamy also includes oogamy (a process in all animals, all higher plants, and many lower plants), in which the egg cell and spermatozoon (sperm) differ in size, form, and behavior.
(2) The transmission to offspring by the male individual of genes or their combination that differ from those donated by the female parent (for example, in Oenothera). If both sexes give the identical combination of genes, the process is called homogamy.
(3) The change in functions of male and female flowers or in their position on the plant (as an anomaly).