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.Multiplicity of gamete types within a species would still be considered a form of sexual reproduction.
Among humans and other mammals, males typically carry XY chromosomes, whereas females typically carry XX chromosomes, which are a part of the XY sex-determination system.
Other animals have a sex-determination system as well, such as the ZW sex-determination system in birds, and the X0 sex-determination system in insects.
Some bacteria use conjugation to transfer genetic material between cells; while not the same as sexual reproduction, this also results in the mixture of genetic traits.
The defining characteristic of sexual reproduction in eukaryotes is the difference between the gametes and the binary nature of fertilization.
In addition to animals, plants, and fungi, other eukaryotes (e.g.
the malaria parasite) also engage in sexual reproduction.
XY sex determination is used by most mammals, and from a comparison between chicken and human, the Z chromosome appeared similar to the autosomal chromosome 9 in human, rather than X or Y, suggesting that the ZW and XY sex-determination systems do not share an origin, but that the sex chromosomes are derived from autosomal chromosomes of the common ancestor of birds and mammals.
A paper from 2004 compared the chicken Z chromosome with platypus X chromosomes and suggested that the two systems are related.
Disregarding intermediates, the basic distinction between asexual and sexual reproduction is the way in which the genetic material is processed.
Typically, prior to an asexual division, a cell duplicates its genetic information content, and then divides. In sexual reproduction, there are special kinds of cells that divide without prior duplication of its genetic material, in a process named meiosis.
As sexual reproduction developed by way of a long process of evolution, intermediates exist.