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Down syndrome is the common name for Trisomy 21 or the triplication of the 21st chromosome.  It is a genetic condition that appears in one of every 691 live births. It is the most common chromosomal anomaly. Individuals with Down syndrome have 47 chromosomes instead of the typical 46. 

Every cell in the human body has a nucleus that stores genetic material in its genes. Genes carry the codes responsible for all of our inherited traits. 


Typically, each cell's nucleus contains 23 pairs of chromosomes  (46 in total), half inherited from each parent.  The genes store the genetic information along rod-like structures called chromosomes. Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21.  That is why you hear T21 or Trisomy 21 when people are referring to Down syndrome.


The cell division that causes Down syndrome happens immediately at conception, regardless of race, nationality, religion, or socioeconomic status. 




For centuries, people with Down syndrome have been alluded to in art, literature and science. It wasn’t until the late nineteenth century, however, that John Langdon Down, an English physician, published an accurate description of a person with Down syndrome. It was this scholarly work, published in 1866, that earned Down the recognition as the “father” of the syndrome. Although other people had previously recognized the characteristics of the syndrome, it was Down who described the condition as a distinct and separate entity, thus the common name for this medical condition is Down syndrome.  

In recent history, advances in medicine and science have enabled researchers to investigate people with Down syndrome's characteristics. In 1959, the French physician Jérôme Lejeune identified Down syndrome as a chromosomal condition. Instead of the usual 46 chromosomes present in each cell, Lejeune observed 47 in individuals with Down syndrome. 


It was later determined that an extra partial or whole copy of chromosome 21 results in the characteristics associated with Down syndrome. In the year 2000, an international team of scientists successfully identified and cataloged each of the approximately 329 genes on chromosome 21. This accomplishment opened the door to significant advances in Down syndrome research.


The image below is a karyotype of an individual with Down syndrome.  You can see that Chromosome 21 has three, instead of two, copies.  To obtain a karyotype, doctors draw a blood sample to examine the baby's cells. They photograph the chromosomes and then group them by size, number, and shape. By examining the karyotype, doctors can diagnose Down syndrome. Another genetic test called FISH can apply similar principles and confirm a diagnosis in a shorter amount of time.  





The most common type of Down syndrome is called Trisomy 21 which results in an embryo with three copies of chromosome 21 instead of the usual two. Before or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome replicates in every cell of the body. This type of Down syndrome, which accounts for 95% of cases, is called trisomy 21.




Mosaicism is the least common form of Down syndrome and accounts for only about 1% of all Down syndrome cases. Mosaicism (or mosaic Down syndrome) is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing 47. Those cells with 47 chromosomes have an extra chromosome 21. Research has indicated that individuals with mosaic Down syndrome may have fewer Down syndrome characteristics than those with other types of Down syndrome. However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess.




In translocation, which accounts for about 4% of Down syndrome cases, the total number of chromosomes in the cells remains 46; however, an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14. The presence of the extra full or partial chromosome 21 causes the characteristics of Down syndrome.



Down syndrome occurs in people of all races and economic levels, though older women have an increased chance of having a child with Down syndrome. A 35 year old woman has about a one in 350 chance of conceiving a child with Down syndrome, and this chance increases gradually to 1 in 100 by age 40. At age 45 the incidence becomes approximately 1 in 30. The age of the mother does not seem to be linked to the risk of translocation.


Since many couples are postponing parenting until later in life, the incidence of Down syndrome conceptions is expected to increase. Therefore, genetic counseling for parents is becoming increasingly important. Still, many physicians are not fully informed about advising their patients about the incidences of Down syndrome, advancements in diagnosis, and the protocols for care and treatment of babies born with Down syndrome.


Use this “people first’ language when referring to Down syndrome and people who have Down syndrome:



  • People with Down syndrome should always be referred to as people first.

  • Instead of “a Down syndrome child,” it should be “a child with Down syndrome.” Also avoid “Down’s child” and describing the condition as “Down’s,” as in, “He has Down’s.”

  • Down syndrome is a condition or a syndrome, not a disease.

  • People “have” Down syndrome, they do not “suffer from” it and are not “afflicted by” it.

  • “Typically developing” or “typical” is preferred over “normal.”

  • "Unique abilities", “Intellectual disability”, or “cognitive disability” has replaced “mental retardation” as the appropriate term.

  • We strongly condemn the use of the word “retarded” in any  context. Using this word is hurtful and suggests that people with unique abilities are not competent.



Down vs. Down’s



  • NDSS uses the preferred spelling, Down syndrome, rather than Down’s syndrome.

  • Down syndrome is named for the English physician John Langdon Down, who characterized the condition, but did not have it. An “apostrophe s” connotes ownership or possession.

  • While Down syndrome is listed in many dictionaries with both popular spellings (with or without an apostrophe s), the preferred usage in the United States is Down syndrome. The AP Stylebook recommends using “Down syndrome,” as well.


These downloadable versions of the Preferred Language Guide are available to print and distribute:





  • Do you want to explain Down syndrome to a child?  



This is a great resource to help you simplify and explain Down syndrome to a fellow student, sibling, friend or neighbor:

Who is doing research about Down syndrome?


  •  Down Syndrome Education
    Research explores how children with Down syndrome develop and learn and identifies more effective teaching approaches and therapies.




  • Down Syndrome Research Foundation
    Investigates best practices and initiates and participates in research studies to gain a better understanding of the learning styles of individuals with Down syndrome.







  • LuMind Foundation
    Supports research to improve cognitive functioning in people with Down syndrome.



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