Male sex chromosomes. Coloured scanning electron micrograph (SEM) of human X (centre) and Y (upper left) sex chromosomes. Each chromosome has replic- ated to form two identical strands (chromatids). The area linking the chromatids is the centromere. The sex chromosomes inherited during fertilisation determine a person's gender. Males have an X and a Y chromosome (as seen here), whilst females have two copies of the X chromosome. The Y chromosome carries instructions for the development of male characteristics. The sex chromosomes are one pair of 23 pairs present in most cells in the body, which contain the DNA necessary for growth and development. Magnification: x7150 at 6x7cm size.