Summary
A condition caused by an extra copy of chromosome 21, also called trisomy 21.
In depth
Trisomy 21 occurs in about 1 in 700 births. It causes intellectual disability of variable degree, characteristic facial features, heart defects and an increased risk of leukaemia and early Alzheimer's. Most cases arise from nondisjunction; a few from translocation.
Prenatal screening includes ultrasound, blood tests and cell-free DNA.
About 95 percent of cases are standard trisomy 21 from nondisjunction, 3 to 4 percent are due to Robertsonian translocation (which can be inherited) and 1 to 2 percent are mosaic. Genes on chromosome 21 include APP, linked to Alzheimer disease, which is common in adults with Down syndrome.
Life expectancy has risen from about 10 years in 1960 to around 60 today with improved cardiac surgery and care. Screening uses ultrasound (nuchal translucency), blood markers and cell-free fetal DNA tests, with diagnosis by amniocentesis.
Frequently asked questions
What is Down Syndrome?
A condition caused by an extra copy of chromosome 21, also called trisomy 21.
Which area of biology covers Down Syndrome, and how advanced is it?
Down Syndrome is one of the 54 topics in Genetics. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Inherited Disorders, Genetic Counselling, Penetrance and Expressivity.
Why does Down Syndrome matter?
Early intervention, education and support help people with Down syndrome reach their potential, and many lead fulfilling lives.
Can you give an example of Down Syndrome?
Life expectancy has risen to around 60 years with better care.
What should I learn before Down Syndrome?
Work through the Beginner topics in Genetics first, such as Heredity, Gene, Allele, then return to Down Syndrome. Intermediate topics assume the Beginner vocabulary.
What topics are related to Down Syndrome?
The closest topics to Down Syndrome are Inherited Disorders, Genetic Counselling, Penetrance and Expressivity, Turner and Klinefelter Syndromes. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Down Syndrome?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Down Syndrome. To practise the whole of Genetics, use the Genetics quiz.
How is Down Syndrome different from Inherited Disorders?
Down Syndrome: A condition caused by an extra copy of chromosome 21, also called trisomy 21. Inherited Disorders: Conditions caused by faulty genes passed through families, such as cystic fibrosis and sickle cell disease. Read both side by side and note what each one explains.
How should I study Down Syndrome step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Inherited Disorders and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.