The resulting cells are called gametes, and contain only half the genetic material of the parent cells.
These gametes are the cells that are prepared for the sexual reproduction of the organism.
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).
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.
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.