X chromosome red video clip.  The X chromosome is one of the two sex chromosomes in mammals and some other species. The other sex chromosome is the Y chromosome. In humans, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). The X chromosome is larger and carries many more genes compared to the Y chromosome. Many of the genes on the X chromosome are involved in a variety of functions, including the regulation of gene expression, the control of cell division, and the development of the brain and other organs.