What is Down syndrome?

Down syndrome (DS), also called trisomy 21, is the most common genetic cause of developmental disability currently known. Down syndrome is caused by an extra copy of chromosome 21 (trisomy 21), and is manifested by microcephaly and varying degrees of mental retardation. Due to a process called nondisjunction, genetic materials fail to separate during a crucial part of the formation of gametes, resulting in an extra chromosome. Down syndrome varies in severity and causes delays in the way a child develops, both mentally and physically, particularly with language, short term memory, and with changing tasks. People with Down syndrome may have a variety of birth defects. 

There are three types of Down syndrome. Physical features and behaviors are similar between each type without looking at the chromosomes. 
  • Trisomy 21. Each cell in the body has 3 separate copies of chromosome 21 instead of the usual 2 copies.
  • Translocation Down syndrome. Occurs when an extra part or a whole extra chromosome 21 is present, but it is attached or “trans-located” to a different chromosome rather than being a separate chromosome 21.
  • Mosaic Down syndrome. Mosaic means mixture or combination. For children with mosaic Down syndrome, some of their cells have 3 copies of chromosome 21, but other cells have the typical two copies of chromosome 21. May have fewer features of the condition due to the presence of some (or many) cells with a typical number of chromosomes.
Risk factors
  • Advancing maternal age. A woman's chances of giving birth to a child with Down syndrome increase with age because older eggs have a greater risk of improper chromosome division. At age 35, a woman's risk of conceiving a child with Down syndrome is about 1 in 350 and by age 45, the risk is about 1 in 30.
  • Having had one child with Down syndrome. Parents who have a child with Down syndrome or another chromosomal disorder, or who have a chromosomal disorder themselves.
  • Being carriers of the genetic translocation for Down syndrome. Both men and women can pass the genetic translocation for Down syndrome on to their children.
  • Screening tests can indicate the likelihood a mother is carrying a baby with Down syndrome
  • Diagnostic tests can identify whether your baby has Down syndrome
  • Chromosomal karyotype testing uses a sample of blood which analyzes your child's chromosomes
Down syndrome can’t be prevented but it can be detected before a child is born. Health problems that tag along with Down syndrome can be treated, and may resources are available to help kids and their families who are living with the condition. 
  • 6,000 babies are born in the United States each year with Down syndrome
  • 400,000 people are living with Down syndrome in the United States
  • The most frequent chromosomal cause of mild to moderate intellectual disability 
  • Occurs in all ethnic and economic groups
  • Older women are much more likely to give birth to an infant with DS
  • 60-80% of children with Down syndrome have hearing deficits 
  • Typically, a woman who has one child with Down syndrome has about a 1 in 100 chance of having another child with Down syndrome
about Down syndrome

  • "Down Syndrome - Genetics Home Reference." U.S. National Library of Medicine. National Institutes of Health, 18 Apr. 2017. Web. 19 Apr. 2017.
  • "Facts about Down Syndrome." Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 03 Mar. 2016. Web. 19 Apr. 2017.
  • "Down Syndrome." Mayo Clinic. Mayo Foundation for Medical Education and Research, 19 Apr. 2014. Web. 19 Apr. 2017.
  • Gavin, MD, Mary L., and Charles I.Scott, MD. "Down Syndrome." Down Syndrome - Johns Hopkins All Children's Hospital. John Hopkins Medicine, Feb. 2012. Web. 19 Apr. 2017.
  • "Down Syndrome (Trisomy 21)." Down Syndrome (Trisomy 21) | Center for Interdisciplinary Brain Sciences Research | Stanford Medicine. Stanford Medicine, n.d. Web. 19 Apr. 2017.
  • "Genes and Human Disease." WHO | Genes and Human Disease. World Health Organization, n.d. Web. 19 Apr. 2017.
  • "How Many People Are Affected by or at Risk for Down Syndrome?" National Institutes of Health. U.S. Department of Health and Human Services, n.d. Web. 19 Apr. 2017.



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