In casual conversation, “having sex” refers to human sexual activity. Biologists, however, mean something far broader when speaking of sex. As applied to animals, the process of sex is a mechanism by which the genes of two individuals are combined to produce offspring. In the case of animal sexual reproduction, each parent produces specialized reproductive cells called gametes by meiosis. Because the gametes are products of meiosis, each gamete cell has only half as many chromosomes as the other cells in the body, and the gametes are diversified in their chromosomes and genes by independent assortment and crossing over (see Concept 7.4). The gametes of the female parent are relatively large, nonmotile cells called eggs or ova (singular, ovum). The gametes of the male, called spermatozoa or sperm, are small cells that swim, typically using flagella (FIGURE 37.1A). A sperm and egg fuse to produce a single-celled zygote that has a full set of chromosomes and develops into a sexually produced offspring (FIGURE 37.1B; see Chapter 38). In this way, chromosomes and genes from two parents are combined to produce each offspring, and the new individual is not genetically identical to either parent.:
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In casual conversation, “having sex” refers to human sexual activity. Biologists, however, mean something far broader when speaking of sex. As applied to animals, the process of sex is a mechanism by which the genes of two individuals are combined to produce offspring. In the case of animal sexual reproduction, each parent produces specialized reproductive cells called gametes by meiosis. Because the gametes are products of meiosis, each gamete cell has only half as many chromosomes as the other cells in the body, and the gametes are diversified in their chromosomes and genes by independent assortment and crossing over (see Concept 7.4). The gametes of the female parent are relatively large, nonmotile cells called eggs or ova (singular, ovum). The gametes of the male, called spermatozoa or sperm, are small cells that swim, typically using flagella (FIGURE 37.1A). A sperm and egg fuse to produce a single-celled zygote that has a full set of chromosomes and develops into a sexually produced offspring (FIGURE 37.1B; see Chapter 38). In this way, chromosomes and genes from two parents are combined to produce each offspring, and the new individual is not genetically identical to either parent.: