Sexual Reproduction

Sexual reproduction is the process through which two parents produce offspring which are genetically different from themselves and have new combinations of their characteristics. This contrasts with asexual reproduction, where one parent produces offspring genetically identical to itself. During sexual reproduction, each parent contributes one haploid gamete (a sex cell with half the normal number of chromosomes). The two sex cells fuse during fertilization and form a diploid zygote (which has the normal number of chromosomes). Recombination, which is the production of variations in gene combinations, occurs at fertilization, so bringing together new combinations of alleles. Crossing-over, the exchange of pieces of chromosomes by two homologous chromosomes, also brings about genetic variation during sexual reproduction. Sexual reproduction is advantageous because it generates variations in characters that can adapt a species over time and improve its chances of survival.

Sexual reproduction occurs in practically all forms of life. Even bacteria, which are always haploid, exchange genetic material. Eukaryotes, organisms possessing a nuclear membrane, generally produce haploid gametes (or sex cells). A gamete, such as an egg or a sperm, possesses half the normal number of chromosomes, and is produced by meiosis, which is reduction cell division, which reduces the number of chromosomes from diploid in the parent cell to haploid in the gametes. When the gametes fuse at fertilization, they restore the normal number of chromosomes. Conjugation, alternation of generations, and animal reproduction illustrate various modes of sexual reproduction.