What is the most common cause of Down syndrome?

Down syndrome, also known as Trisomy 21, is most commonly caused by a nondisjunction error during cell division. Nondisjunction refers to the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leading to an abnormal distribution of chromosomes in the resulting gametes. This can result in a gamete with an extra copy of chromosome 21. If such a gamete participates in fertilization, the resulting embryo will have three copies of chromosome 21 instead of the usual two, leading to the characteristic features of Down syndrome.

This mechanism accounts for approximately 95% of all cases of Down syndrome. It is not associated specifically with paternal or maternal viral illnesses, nor is it directly linked to paternal translocation, which is a less common cause of the condition. Understanding nondisjunction is crucial for recognizing the primary genetic origin of Down syndrome and can help guide genetic counseling and risk assessment for affected families.