Sexual reproduction involves the fusion of gametes from similar or different parents. The size and shape of these gametes may vary to a greater extent, as shown in this figure gametes of some organisms. According to the size of male and female gametes types of gametes are- 1. Isogametes: When the fusing gametes are morphologically similar, they are known as 'isogametes' or 'homogametic'. They are produced in some algae and fungi.
E.g., Algae: Cladophora, Chlamydomonas, Ulothrix; Fungi: Synchytrium, Rhizopus. 2. Heterogametes: When the fusing gametes are morphologically distinct types, they are known as 'heterogametes'. It is the feature of the majority of sexually reproducing organisms. E.g., i) Algae: Fucus, Volvox, Chara ii) All Bryophytes, Pteridophytes, Gymnosperms and Angiosperms. In such organisms, male gamete is called 'antherozoid' or 'sperm' and the female gamete is called 'egg' or 'ovum'. 3. Oogamy: When the motile male gamete fertilizes the large non-motile female gamete. E.g., Humans.
