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's syndrome , mental retardation, heart malformation, abnormal sexual development, malignancies, and sex-chromosome disorders (e.g., Turner's syndrome , Klinefelter's syndrome ). Chromosomal disorders occur in 0.5% of births; many can now be diagnosed before birth by amniocentesis .