The second way that organisms reproduce is by combining complete sets of genetic information from two individuals to make a new genetically unique individual. This form of reproduction is called sexual reproduction.

At its core are two basic biological processes—meiotic cell division and fertilization (Fig. 42.2). Meiotic cell division halves the number of chromosomes; fertilization involves fusion of the products of meiotic cell division to restore the original chromosomal content. Meiotic cell division evolved in the ancestors of modern eukaryotes, and so sexual reproduction occurs only in eukaryotes.

As we saw in Chapter 11, meiotic cell division consists of one round of chromosome replication followed by two rounds of cell division, resulting in four cells each with half the number of chromosomes of the parent cell. So, in a diploid species with 2n chromosomes, the result of meiosis is four cells with n chromosomes. The resulting cells are haploid and are called gametes or spores. Human individuals, for example, have 23 pairs of chromosomes (2n = 46 chromosomes) and human gametes have half this number (n = 23 chromosomes).

Many species produce two types of gamete that differ in shape and size. The smaller, male gametes are called spermatozoa or sperm, and the larger, female gametes are called ova or eggs. For other organisms, including green algae, diatoms, slime molds, and most fungi, the two types of gamete are the same size. In spite of their similar size and appearance, however, they come in two (or more) distinct mating types and cannot reproduce sexually with individuals of the same type.

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Fertilization involves the fusion of two gametes, one from each parent (Fig. 42.2). The result is a zygote. If each gamete has n chromosomes, the resulting zygote has 2n chromosomes. For diploid multicellular organisms, fertilization is the direct basis for reproduction. The zygote divides by mitosis and develops into an embryo, an early stage of multicellular development.

Meiotic cell division and fertilization have important genetic consequences. During meiotic cell division, pairs of homologous chromosomes undergo recombination, shuffling the combination of alleles between the chromosomes. In addition, meiotic cell division randomly sorts homologous chromosomes into different gametes. As a result, the products of meiotic cell division, the haploid gametes, are each genetically unique and represent a mixture of the parental genetic makeup. Then, during fertilization, two of these unique haploid gametes fuse, creating a new genetic combination and a unique diploid individual.

Quick Check 1 Sexual reproduction results in offspring that are genetically different from one another and from their parents. Describe four mechanisms that produce this genetic variation. Do any of these mechanisms occur in asexual reproduction?