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.
Each cell in the offspring has half the chromosomes of the mother and half of the father.
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.
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.
In addition to animals, plants, and fungi, other eukaryotes (e.g.
the malaria parasite) also engage in sexual reproduction.This mode of reproduction is called asexual, and it is still used by many species, particularly unicellular, but it is also very common in multicellular organisms.In sexual reproduction, the genetic material of the offspring comes from two different individuals.the origin of chromosomal sex determination may have been fairly early in eukaryotes (see Evolution of anisogamy).The ZW sex-determination system is shared by birds, some fish and some crustaceans.This double-chromosome stage is called "diploid", while the single-chromosome stage is "haploid".