Down syndrome, also called trisomy 21, is associated with the presence of an extra copy of chromosome 21. It is the most common genetic disorder and single cause of human birth defects.

FREQUENCY OFOCCURRENCESeen 1 in 750–800 live births
• The syndrome is statistically more common with older parents.
INHERITANCESporadic / not inherited
• Most cases of Down syndrome are not inherited, including mosacism.
• Translocation Down syndrome can be inherited. A carrier can have a rearrangement of genetic material between chr 21 and another chromosome. This is called a balanced translocation because there is no extra material from chr 21. Although they do not have signs of the syndrome, people who carry this type of balanced translocation are at an increased risk of having children with the condition.
LIFE EXPECTANCYLifespan close to normal average
• The median life expectancy is above 50 years.
CAUSEA third chromosome 21 appearing in all or some of the cells
• Primary trisomy: the presence of an entire third chr 21 in all cells (95% of cases).
• Unbalanced translocation: part of chr 21 becomes attached (translocated) to another chromosome (usually chr 14) during the formation of reproductive cells or very early in fetal development. Affected people have two copies of chr 21, plus extra material from chr 21 attached to another chromosome.
• Mosaicism: some normal cells with 46 chromosomes, others with extra chr 21.
• The only well known risk factor for conceiving a child with Down syndrome is advanced maternal age:
– 1:385 risk at 35 years
– 1:106 at 40 years
– 1:30 at 45 years

Trisomy 21 symptoms vary from person to person and can range from mild to severe. In mosaic trisomy 21 cases the percentage of cells with extra chr 21 varies, and they often do not have all the typical physical characteristics and may not be as severely intellectually impaired as people with full trisomy 21.

Key features:

  • Round, flat face
  • Upward slanting eyes with skin folds at the upper eyelid (epicanthal fold)
  • Brushfield spots (small white/gray spots on the periphery of the iris)
  • Abnormally shaped nose and ears
  • Small mouth with protruding tongue
  • Lateral teeth
  • Excess skin at the nape of the short neck
  • Short hands with short fingers
  • Single line in the palm (simian crease)
  • Hearing problems
  • Hip problems and risk of dislocation
  • Hypotonia and ligamental laxity
  • Congenital heart defects
  • Digestive malformations
  • Sleep apnea (because of narrowed mouth, throat and airway)
  • Autoimmune and endocrine problems
  • Infertility in males and severely decreased fertility in females
  • Earlier aging

Children with Down syndrome may also have delayed mental and social development.
Common problems may include:

  • Impulsive behavior and poor judgment
  • Short attention span
  • Slow learning with mild to moderate learning disabilities
  • Delayed language development and slow motor development
  • Intellectual deficiency

People with Down syndrome have an increased risk for developing epilepsy, leukemia, Alzheimer’s disease and immune deficiencies.

Early intervention can help people with trisomy 21 live productive lives well into adulthood. Corrective surgery for heart defects, gastrointestinal irregularities and other health issues is necessary for some individuals. Regular screening is needed for other medical conditions and disorders.

Patients can and should be fully included in family and community life. Educative programs are essential, aiming at the best possible personal fulfillment and integration in society, including physical, occupational and speech therapy.