A chromosome is a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying the genes that determine heredity. The word chromosome comes from the Greek χρῶμα (chroma, meaning “color”) and σῶμα (soma, meaning “body”), referring to their property of being stained by particular dyes.
All eukaryotic cells have chromosomes with linear DNA molecules arranged in two sets: one set inherited from each parent. Eukaryotes also have non-chromosomal DNA, which is usually organized into mitochondria and chloroplasts. Prokaryotes lack chromosomes altogether, having instead a single circular chromosome located in the cytoplasm. Each gene on a chromosome is functionally related to other genes on the same or different chromosomes through interactions between proteins encoded by those genes.
The study of chromosomes and gene expression is called genetics. Variations in the number or structure of chromosomes can lead to genetic disorders such as Down syndrome or Klinefelter syndrome. Aneuploidy, a condition in which an individual has an abnormal number of chromosomes, is often caused by errors during cell division known as meiotic nondisjunction; it may also result from mitotic nondisjunction during early embryonic development. Gene duplications are another source of variation; they occur when segments of DNA are copied more than once within a chromosome or when whole chromosomes are duplicated (as happens in polyploid organisms). Defects resulting from chromosomal abnormalities can be corrected surgically in some cases (e.g., removal of an extra copy of chromosome 21 corrects Down syndrome), but often they cannot be cured and lead to birth defects or death soon after birth.