Some of the many plausible theories include: that sex creates variation among offspring, sex helps in the spread of advantageous traits, that sex helps in the removal of disadvantageous traits, and that sex facilitates repair of germ-line DNA.
Sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria.
Sex comprises the arrangements that enable sexual reproduction, and has evolved alongside the reproduction system, starting with similar gametes (isogamy) and progressing to systems that have different gamete types, such as those involving a large female gamete (ovum) and a small male gamete (sperm).
In complex organisms, the sex organs are the parts that are involved in the production and exchange of gametes in sexual reproduction.
This double-chromosome stage is called "diploid", while the single-chromosome stage is "haploid".
Genetic traits are contained within the deoxyribonucleic acid (DNA) of chromosomes—by combining one of each type of chromosomes from each parent, an organism is formed containing a doubled set of chromosomes.
Sexual reproduction involves the combining and mixing of genetic traits: specialized cells known as gametes combine to form offspring that inherit traits from each parent.
Gametes can be identical in form and function (known as isogamy), but in many cases an asymmetry has evolved such that two sex-specific types of gametes (heterogametes) exist (known as anisogamy).
In addition to animals, plants, and fungi, other eukaryotes (e.g.
the malaria parasite) also engage in sexual reproduction.