This animation shows how meiosis, the form of cell division unique to egg and sperm production, can give rise to sperm that carry either an X or a Y chromosome.

Meiosis starts with a diploid cell (a cell with two sets of chromosomes) and ends up with four haploid cells (cells with only one set of chromosomes), which are called gametes (eggs and sperm). When an egg and sperm combine at fertilization, the embryo regains a diploid number of chromosomes. Before a cell begins meiosis, its two sets of chromosomes come together and swap segments in a process known as recombination (or crossing over).

The first part of this animation shows recombination between the X and Y chromosomes. Most women have two X chromosomes, whereas most men have one X chromosome and one Y chromosome. After a cell with both X and Y chromosomes divides twice, it produces four gametes. Half the gametes get an X chromosome, and half get a Y chromosome. Because of recombination and random assortment, the exact composition of chromosomes in each gamete varies.

The second part of the animation shows how recombination may result in the transfer of a gene called SRY from the Y chromosome to the X chromosome. SRY plays an important role in male sexual development. An embryo with two X chromosomes, one of which has SRY, may develop more typical male sexual characteristics.