These are the long strands of tightly curled DNA that reside within the nucleus of all cells (except red blood cells). Each cell in your body has a full set of your chromosomes. The chromosomes contain the body’s genes, which are specific segments of DNA that contain the messages for the cell to create proteins, some of the building blocks of life. So the chromosomes have the genes, and the genes code for the proteins, and the proteins form things like blood, skin, and other organs. Most people have 46 chromosomes in each cell: that includes 22 pairs (which scientists number 1 to 22, from largest to smallest) of closely matching chromosomes (one of each pair from each parent) called autosomes. In addition to those 22 pairs, most people have two additional chromosomes that may or may not match, and these are called the “sex chromosomes.” Instead of being numbered like the autosomes are, the sex chromosomes are designated by the letters X and Y, because they kind of look like an X and a Y. Most females have two so-called X chromosomes, and so we say they have the karyotype 46,XX; the number 46 tells you they have 46 chromosomes total, and the “XX” tells you that two of those chromosomes are X chromosomes. Most males have one X chromosome and one Y chromosome, and so we say they have the karyotype 46,XY (46 chromosomes, including one X and one Y). However, there are many other patterns of chromosomes. Some people have an extra X or Y, some are missing an X, some females have a Y chromosome and some males have two X’s. The “sex chromosomes” are somewhat misnamed, because, although they are usually different between males and females, they are not by themselves responsible for whether a person develops as a male or female. There are many genes carried on many other chromosomes (autosomes) that are also responsible for development of the gonads and other sex aspects. There are also some genes on the X chromosome that don’t have anything to do with sex development. It’s critical to understand that “sex chromosomes” do not determine a person’s gender identity or sexual orientation.