Syndrome caused by chromosome abnormality. Normally, humans have 23 pairs of chromosomes, including one pair of sex chromosomes. Any variation from this pattern causes abnormalities. A chromosome may be duplicated (trisomy) or absent (monosomy); one or more extra full sets of chromosomes can be present (see ploidy); or part of a chromosome may be missing (deletion) or transferred to another (translocation). Resulting disorders include Down syndrome, intellectual disability, heart malformation, abnormal sexual development, malignancies, and sex-chromosome disorders (e.g., Turner syndrome, Klinefelter syndrome). Chromosomal disorders occur in 0.5percnt of births; many can now be diagnosed before birth by amniocentesis.

Learn more about chromosomal disorder with a free trial on Britannica.com.