A gamete is a cell produced by an organism for the purpose of sexual reproduction. In humans, the egg and the sperm are the two sexual cells, which differ in their size and in other qualities such as the quantity of each that the body produces. Each of these cells has 23 chromosomes, exactly half of the number found in the other cells of the body. They are formed by special cellular division called meiosis, which occurs only in the primary sex organs — the testes and the ovaries. Fertilization combines the gametes of both parents into a zygote.
Sexual organisms make a special type of cell, the gamete, that combines with another cell for reproduction. In humans, each of these contains one pair of 23 chromosomes, and thus is haploid, while other cells are diploid, with two pairs of chromosomes. The 23rd pair differs in males and females, and is what distinguishes the two genders biologically, with males having the pair XY and females XX. Chromosomes consist of long strings of genes bound together in a sequence. Since the egg or ovum, the female cell, can only carry X chromosomes, sex is determined by the male gamete.
Normal division of cells to produce a new copy of the original occurs through a process called mitosis. A slightly different division, meiosis, generates a new gamete. Both processes involve the copying of DNA from the nucleus of the parent cell and its transfer into the new one, but meiosis involves a special combination of DNA from both of the parents' original gametes. This recombination of hereditary information allows for diversity of inherited traits in the newly produced gamete; it is the reason why children have a mixture of genes from both parents.
Size and relative quantity distinguish the male from the female gamete. Male gametes, sperm, are motile, small, and produced in large numbers, of which few ever achieve fertilization. Eggs, the female cell, are large, with a cytoplasm that will nourish an embryo if fertilization occurs. Some species of algae and plant do not have separate male and female gametes, but reproduce by combining genetically identical cells. In asexual species, no gametes are produced, and cells only divide by a form of mitosis.
The male gamete, sperm, is produced in the testicle, in a process called spermatogenesis. Each testicular cell that undergoes meiosis produces four new gametes. In the female's ovaries, the ovarian folicles generate the egg cells during a process calling oogenesis that largely is accomplished at birth, but which is completed after puberty during the monthly ovarian cycles, when the eggs mature and become ready to be fertilized. At the moment of fertilization, the gametes unite, and a zygote is formed. This cell has 46 chromosomes, with an equal number contributed by each parent.
