X and Y chromosomes. Computer artwork of an X (left) and a Y (right) chromosome. Chromosomes are made of very condensed deoxyribonucleic acid (DNA). DNA contains sections, called genes, which encode the body's genetic information. An individual's sex is determined by the presence or absence of the Y chromosome. Women have two X chromosomes, while men have an X and a Y. The Y chromosome is much smaller than the X. This is because it only contains genes for 'maleness', for example, a gene that triggers the development of the testes.