Animals and plants, however, have generation times that are measured in days, month and usually years. The environment can change drastically over this time period and random mutations cannot provide for adequate changes to adapt. Thus, sexual reproduction of these organisms makes use of enhanced mixing up of genetic variation from generation to generation by using multiple copies of each gene (e.g. diploid if two copies) that can be randomly assorted before and mixed during fertilization of a male and female gamete (egg and sperm cells). For this purpose, sexually reproducing organisms depend on two parents, rather then one, allowing mixing of genetic material causing random distributing of the of genetic variability through out a population. The resulting gene pool is characteristic of a species. As a result, each individual is genetically unique unlike descendant of a single bacterial cell (a clonal population).

The process of sexual reproduction much like asexual reproduction strives to maintain the number of genes (or chromosomes which contain the genes) of the organism from generation to generation. Since sexual reproduction involves fusion of gametes from two parents, the number of genes or chromosome sets first has to be reduced. Otherwise, the number of the copies of genes would increase exponentially from generation to generation. For this mechanism to work, each gene (and chromosome) must be present in two copies. Sexually reproducing organisms contain two full genetic complements (genomes). They are said to be diploid. During reproduction the reproductive organs of males and females produce single copy genomes (haploid cells) from diploid cells in a process called meiosis. Unlike mitosis which consists of one round of duplication and separation of sister chromatides followed by a single cell division (cytokinesis), meiosis adds two additional features: first, after duplication, the sister chromatids of homologous pairs of chromosomes are joined in a tetrad (synapsis formation) where homologous chromatids exchange portions of their structures (recombination or crossing-over); second, after recombination the cell undergoes two rounds of cell divisions (cytokinesis) without further duplication of chromosomes, separating first homologous chromosome pairs forming haploid cells where recombined sister chromatides are still joined, followed by separation of these sister chromatides into single copy cells (sperm or egg). These gametes contain one set of chromosomes composed of randomly assorted chromosomes inherited from both parents. Random fusion of one egg and one sperm from two different individuals during fertilization produces a diploid zygote, the first step in the life cycle of a new individual.

Because of this random mixing of two parental chromosome sets into a new single set of chromosomes during mitosis, each gamete is genetically different from any other gamete decreasing the probability of subsequent fertilization events producing genetically identical offspring. The only exception in sexual reproduction, where genetically identical offspring (not identical with parents) are formed are twins (and triplets etc.… ) that are the result of early separation of single cells after a few rounds of cell division by mitosis of the zygote into an embryo (see embryonic stem cells).

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