The normal human chromosome complement consists of 46 chromosomes comprising 22 
morphologically different pairs of autosomes and one pair of sex chromosomes. 
Variations in either chromosome number and/or structure frequently result in 
significant mental impairment and/or a variety of other clinical problems, among 
them, altered bone mass and strength. Chromosomal syndromes associated with 
specific chromosomal abnormalities are classified as either numerical or 
structural and may involve more than one chromosome. Aneuploidy refers to the 
presence of an extra copy of a specific chromosome, or trisomy, as seen in 
Down's syndrome (trisomy 21), or the absence of a single chromosome, or 
monosomy, as seen in Turner syndrome (a single X chromosome in females: 45, X). 
Aneuploidies have diverse phenotypic consequences, ranging from severe mental 
retardation and developmental abnormalities to increased susceptibility to 
various neoplasms and premature death. In fact, trisomy 21 is the prototypical 
aneuploidy in humans, is the most common genetic abnormality associated with 
longevity, and is one of the most widespread genetic causes of intellectual 
disability. In this review, the impact of trisomy 21 on the bone mass, 
architecture, skeletal health, and quality of life of people with Down syndrome 
will be discussed.