Down syndrome is caused by the presence of an extra chromosome. What is the number of this chromosome?

Down syndrome is specifically caused by the presence of an extra copy of chromosome 21, which results in a total of three copies of this chromosome instead of the usual two. This genetic condition, also known as trisomy 21, leads to the characteristic physical features and developmental challenges associated with Down syndrome. The presence of this additional chromosome disrupts normal development, impacting cognitive abilities and contributing to the array of physical traits commonly seen in individuals with Down syndrome. Understanding that the condition is linked to chromosome 21 is crucial for recognizing its genetic basis and implications for individuals and families affected by it.