Sexual reproduction involves two processes: meiotic cell division and fertilization. Meiotic cell division, as we just saw, produces cells with half the number of chromosomes present in the parent cell. In multicellular animals, the products of meiotic cell division are gametes: An egg cell is a gamete and a sperm cell is a gamete. Each gamete is haploid, containing a single set of chromosomes. In humans, meiosis takes place in the ovaries of the female and the testes of the male, and each resulting gamete contains 23 chromosomes, including one each of the 22 numbered chromosomes plus either an X or a Y chromosome.

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During fertilization, these gametes fuse to form a single cell called a zygote. The zygote is diploid, having two complete sets of chromosomes, one from each parent. Therefore, fertilization restores the original chromosome number.

As we discuss further in Chapters 16 and 42, sexual reproduction plays a key role in increasing genetic diversity. Genetic diversity results from meiotic cell division (the cells that are produced are each genetically different from one another as a result of crossing over and the random segregation of homologous chromosomes), and from fertilization (different gametes are combined to produce a new, unique individual). The increase in genetic diversity made possible by sexual reproduction allows organisms to evolve and adapt more quickly to their environment than is possible with asexual reproduction.

In the life cycle of multicellular animals like humans, then, the diploid organism produces single-celled haploid gametes that fuse to make a diploid zygote. In this case, the only haploid cells in the life cycle are the gametes, and the products of meiotic cell division do not undergo mitotic cell division but instead fuse to become a diploid zygote. However, there are a number of life cycles in other organisms that differ in the timing of meiotic cell division and fertilization, discussed more fully in Chapter 27. Some organisms, like most fungi, are haploid. These haploid cells can fuse to produce a diploid zygote, but this cell immediately undergoes meiotic cell division to produce haploid cells, so that the only diploid cell in the life cycle is the zygote (Chapter 34). Other organisms, like plants, have both multicellular haploid and diploid phases (Chapter 30). In this case, meiotic cell division produces haploid spores that divide by mitotic cell division to produce a multicellular haploid phase; subsequently, haploid cells fuse to form a diploid zygote that also divides by mitotic cell division to produce a multicellular diploid phase.

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