During asexual reproduction organisms undergo only mitotic divisions. The DNA of the cells involved are copied and then equally divided among the two daughter cells formed. Thus, chromosome number remains unchanged. In sexual reproduction, organisms produce gametes through meiosis, in which the original number of chromosomes becomes half. These two gametes combine to form the zygote and the original number of chromosome is restored.
For example, in humans, the parents each have 46 or 23 pairs of chromosomes. In the gametes, the sperm has half the number of chromosomes, i.e., 23 and the egg also has 23 chromosomes. When the sperm and the egg fuse, the zygote has 46 or 23 pairs of chromosomes.