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Genetic Causes of Down Syndrome


  • Down syndrome is caused by the presence of extra genetic material from chromosome 21.
  • Although an experienced doctor or nurse might recognize several clinical features suggestive of Down syndrome, the actual diagnosis is confirmed by chromosome analysis.
  • Most people have 23 pairs of chromosomes with one member of each pair inherited from the mother and the other member of each pair passed on from the father. They have 46 chromosomes.
  • The majority of individuals with Down syndrome have three copies of chromosome 21 in every cell of their body, known as trisomy 21. They have 47 chromosomes. 
  • Trisomy 21 occurs when either the egg or the sperm contains an extra copy of chromosome 21. This is the result of an inaccuracy in the division process, called nondisjunction. When nondisjunction occurs, one parent contributes two copies of chromosome 21 and the other parent gives one copy of chromosome 21 for a total of three copies of chromosome 21.
  • Trisomy 21 is not caused by anything a parent did or did not do in the past or during the pregnancy.
  • Although trisomy 21 occurs more frequently as women age, most babies with Down syndrome are born to women under the age of 35 years, because younger women have higher fertility rates.
  • About 5% of individuals with Down syndrome have mosaicism or a translocation.
  • Individuals that have mosaic Down syndrome have two different types of cells in their bodies – some with trisomy 21 and 47 chromosomes, and others with a pair of chromosomes 21 and 46 chromosomes.
  • Some individuals with Down syndrome have 46 chromosomes. A translocation is caused by a chromosomal rearrangement.
  • To learn more about Trisomy 21, mosaicism and translocations, please contact a genetic counselor. Genetic counselors can be found at the National Society of Genetic Counselors website.

For more information, please see:

  • Morris JK, Mutton DE and E Alberman. Recurrences of free trisomy 21: analysis of data from the National Down Syndrome Cytogenetic Registry. Prenatal Diagnosis 25: 1120-1128, 2005.
  • Sherman SL, Allen EG, Bean LH, and Freeman SB. Epidemiology of Down Syndrome. Mental Retardation and Developmental Disabilities Research Reviews 13: 221-227, 2007.
  • Steinberg C, Zackai EH, Eunpu DL, Mennuti MT, Emanuel BS. Recurrence rate for de novo 21q21q translocation Down syndrome: a study of 112 families. American Journal of Medical Genetics. 17: 523-530, 1984.