This double-chromosome stage is called "diploid", while the single-chromosome stage is "haploid".
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.
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).
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.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.For instance, mate choice and sexual selection can accelerate the evolution of physical differences between the sexes.One of the basic properties of life is reproduction, the capacity to generate new individuals, and sex is an aspect of this process.the origin of chromosomal sex determination may have been fairly early in eukaryotes (see Evolution of anisogamy).