Germ cells are cells that create reproductive cells called gametes. Germ cells are located only in the gonads. They are called oogonia in females and spermatogonia in males. In females, germ cells are found in the ovaries, whereas male germ cells are found in the testes. During oogenesis, germ cells divide to produce ova, or eggs, in females.

Germ cells are the only cells that can undergo meiosis as well as mitosis.

When germ cells divide by mitosis, they produce genetically identical cells. The daughter cells created during this process have a full set of chromosomes (diploid). Diploid cells have 23 pairs of chromosomes.

When germ cells divide by meiosis, the daughter cells have half a set of chromosomes (haploid gametes). The cells formed in meiosis are all genetically different. This is because the chromosomes - and genes - separate and are reshuffled during the process.

In males, a single diploid cell yields four haploid sperm cells through meiosis. In females, diploid cells divide unevenly. Most of the cytoplasm is segregated into one daughter cell, which becomes the egg cell or ovum, while the smaller polar bodies only get a small amount of cytoplasm.

The figure on the right displays the formation of gametes.

2n is the chromosome number of the fertilized ovum and normal cells (46 in humans). The letter n describes one set of chromosomes, which in humans is 23 chromosomes.

The parent cell divides into two cells in the ovary or the testicle. At the same time, the number of chromosomes is halved (2n → n, a cell with 46 chromosomes is divided into two cells with 23 chromosomes).

The cells are further divided into two cells. The result is four gametes.

Ordinary human cells have 46 chromosomes, two of which are sex chromosomes. Chromosomal sex is determined at the time of fertilization; a chromosome from the sperm cell, either X or Y, fuses with the X chromosome from the egg cell.

There are two types of sperm. Some sperm have the sex chromosome X, whereas others have the sex chromosome Y. If a sperm cell with the sex chromosome X fertilizes the egg, the result will be a female. If a sperm cell with the sex chromosome Y fertilizes the egg, the result will be a male. Therefore, the chromosomes of a female are 44 + XX, whereas the chromosomes of a male are 44 + XY.