Any chromosome that is additional to the regular karyotype of a species. Such chromosomes vary in size and composition and may or may not affect the phenotype. In humans, about one person in every 1500 carries an accessory chromosome, and some of these are associated with mental or physical abnormalities. For example, cat-eye syndrome occurs in people who carry an extra chromosome that partly duplicates chromosome 22. It is characterized by enlarged pupils, skin malformations, cardiac and urinary defects, and, sometimes, mental handicap.