Why is it necessary to reduce the chromosome number of gametes, but not other cells of an organism?
Human cells have 23 pairs of chromosomes (for a total of 46 chromosomes in all). Most cells undergo cell division via mitosis, thus producing 46 *2 = 92 chromosomes that are then split between the two new cells to give each one 46 chromosomes (as 23 pairs). So mitosis takes one diploid cell and produces two diploid cells with the proper number of chromosomes for a human cell. Gametes, or sex cells, are unique in that two of them (an egg and a sperm) will ultimately join together during sexual reproduction to produce a new organism (zygote). In order to accomplish this, gametes undergo cell division via meiosis. In this process, a diploid cell (46 chromosomes) replicates its DNA and undergoes two cell divisions (meiosis I and II) to produce 4 haploid cells (23 chromosomes each). So each daughter cell produced has half the number of chromosomes (23 versus 46) of a regular human cell. This is because when two gametes join during sexual reproduction, they will add their chromosomes together to make a total of 46 chromosomes (23 pairs). In addition, the new zygote will be an even genetic blend of the mother and father with half of the chromosomes originating with the mother and the other half originating with the father.