Genome

Ploidy reflects the reproductive mechanisms of an organism. Animals commonly have both a maternal and a paternal parent. Through meiosis, the former forms a haploid gamete called an ovum (or egg); the latter forms a haploid gamete called a sperm. During fertilization, the egg and sperm unite to form a diploid zygote that matures to an adult organism. Thus, the genome of adult animals is diploid, while the genome of their gametes is haploid.

Plants exhibit an alternation of generations; sporophytes (the mature, visible plant) are diploid; through meiosis, they produce spores that germinate into gametophytes; the gametophytes are haploid and produce gametes that fuse to reestablish the diploid state. Fungi also exhibit an alternation of generations. They commonly exist as multinucleate tubes of cytoplasm called hyphae. The individual nuclei are most often haploid (though may be diploid in the lower fungi).

Hyphae of different members of a fungal species sometimes fuse; in this circumstance (called heterokaryosis) the genome becomes the sum of the two (dikaryotic) haploid complements. Unicellular protistan organisms, a group that includes protozoans and most algae, exhibit many variations. For example, the ciliates (such as paramecia) have diploid micronuclei and polyploid macronuclei; the former are the basis of inheritance; the latter establish the genetic character of an existing organism.