Which are the causes of Down Syndrome?

Down syndrome is caused by the presence of all or part of a third copy of chromosome 21, a condition known as trisomy 21. This genetic anomaly disrupts the normal development of the body and brain, leading to the clinical features associated with the condition.

What exactly causes Down syndrome?

In a typical human cell, there are 46 chromosomes organized into 23 pairs, with one pair inherited from each parent. Down syndrome occurs due to a chromosomal error during cell division. In approximately 95% of cases, the condition is caused by "nondisjunction," where a pair of chromosome 21 fails to separate properly during the formation of the egg or sperm. As a result, the developing embryo ends up with three copies of chromosome 21 instead of two in every cell of the body. Because this is a fundamental genetic event occurring at conception, it is not caused by anything the parents did or did not do before or during pregnancy.

Are there different genetic types of Down syndrome?

While trisomy 21 is the most common form, researchers have identified three distinct genetic mechanisms that result in Down syndrome:

• Trisomy 21 (Nondisjunction): Accounts for about 95% of cases; every cell in the body contains an extra chromosome 21.


• Translocation: Accounts for about 3-4% of cases. In this form, a portion of chromosome 21 becomes attached (translocated) to another chromosome before or at conception.


• Mosaicism: Accounts for about 1-2% of cases. Only some of the body's cells contain the extra chromosome 21, while others have the typical pair, often resulting in milder clinical symptoms.

What are the known risk factors for Down syndrome?

It is crucial to distinguish between a cause and a risk factor. The cause of Down syndrome is the extra genetic material. A primary, well-documented risk factor is maternal age; the probability of conceiving a child with trisomy 21 increases as a woman gets older, particularly after age 35. However, because younger women have higher birth rates overall, many babies with Down syndrome are born to mothers under the age of 35. Environmental, lifestyle, or dietary factors have not been proven to cause the chromosomal nondisjunction that leads to the condition.

Is current research investigating the etiology of Down syndrome?

While the chromosomal basis of Down syndrome is well-understood, medical researchers are currently focusing on "genotype-phenotype correlation." This research aims to understand exactly how the extra genes on chromosome 21 interact with other genes to produce the specific health and developmental outcomes seen in individuals. Scientists are using advanced genomic mapping to identify which specific genes on the extra chromosome contribute to cognitive differences, heart defects, or immune system variations, with the hope of developing targeted therapies to improve quality of life for the 24 members of our DiseaseMaps community and others worldwide living with Down syndrome.

Next steps

• Consult with a clinical geneticist to discuss recurrence risks or to understand the specific type of Down syndrome diagnosed in your family.


• Connect with the 24 members of the Down syndrome community on DiseaseMaps.org to share experiences and supportive resources.


• Reach out to organizations like the National Down Syndrome Society (NDSS) for educational materials and local support networks.


• Schedule regular screenings with specialists, such as cardiologists or endocrinologists, as recommended by your primary care physician to manage potential health complications.

Medical disclaimer: This content is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment; always seek the advice of your physician or other qualified health provider with any questions regarding a medical condition.

References

• National Institutes of Health (NIH) Genetic and Rare Diseases Information Center (GARD): Down Syndrome


• Orphanet: Trisomy 21


• Online Mendelian Inheritance in Man (OMIM): Down Syndrome (#190685)


• National Down Syndrome Society (NDSS): About Down Syndrome