DNA X Chromosome CGI.  DNA is a molecule composed of two polynucleotide chains that coil around each other to form a double helix. It carries genetic instructions for the growth and reproduction of all known organisms.  The X chromosome is one of the two sex chromosomes found in mammals, including humans. It is present in both males and females and is responsible for determining the sex of an individual. Females have two copies of the X chromosome (XX), while males have one X and one Y chromosome (XY). The X chromosome contains many genes that are important for normal development and function of the body. Some of these genes are involved in sex-specific traits, while others are involved in non-sex specific traits such as immune function and blood clotting.