In cells, long strands of DNA are compacted to form units called chromosomes. Most human chromosomes contain hundreds or thousands of genes. A given chromosome, such as “chromosome 7” or “chromosome 19”, contains the same set of genes in all humans.
Humans inherit two sets of chromosomes, with one version of each chromosome inherited from each parent. Humans have 23 pairs of chromosomes, and most cells in our bodies contain a pair of every chromosome. Important exceptions are sperm and eggs, which contain one copy of each chromosome that will be passed on to any future offspring (see How are genes inherited?).
Chromosomes can be seen with specialized microscopy techniques. Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” Each chromosome arm is divided into regions called cytobands that can be seen using special stains. In rare cases where a chromosome has large regions of DNA deleted or rearranged, these changes can sometimes be detected by changes in the cytoband patterns.