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.
As sexual reproduction developed by way of a long process of evolution, intermediates exist.
Bacteria, for instance, reproduce asexually, but undergo a process by which a part of the genetic material of an individual (donor) is transferred to an other (recipient).
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.
For instance, mate choice and sexual selection can accelerate the evolution of physical differences between the sexes.
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.
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".Diploid organisms can, in turn, form haploid cells (gametes) that randomly contain one of each of the chromosome pairs, via meiosis.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.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.Life has evolved from simple stages to more complex ones, and so have the reproduction mechanisms.Initially the reproduction was a replicating process that consists in producing new individuals that contain the same genetic information as the original or parent individual.