The Human Body


Sex is a physical and behavioral difference that distinguishes individual organisms according to their functions in the reproductive process. For information on issues of sexual health, sexual behavior, and sexual activity, see Human Sexuality.

Sex occurs at all levels of biological organization, with the exception of viruses. At the lowest level, bacteria conjugate and a length of the single chromosome is passed from the male, or donor cell, to the female, or recipient cell. At more advanced levels, multicellular individuals have specialized organs (gonads) that produce specialized sex cells (gametes). Upon fertilization, genetic information is transferred from the small, motile spermatozoa (male gametes) to the much larger ova (female gametes). Many organisms, including most plants, many protozoans and invertebrates, and some fishes, have both male and female gonads and are called hermaphroditic (see Hermaphroditism). Hermaphroditic organisms, however, are rarely self-fertilizing; the male and female reproductive organs ripen at different times—times that also coincide with those of other individuals, thereby ensuring cross-fertilization.