What is one of the main characteristics of down syndrome?

One of the main characteristics of Down syndrome is the presence of an extra copy of chromosome 21. This genetic condition, also known as Trisomy 21, results in three copies of this chromosome instead of the usual two, which leads to the variety of developmental and physical traits often associated with the syndrome. It is crucial in understanding Down syndrome because this chromosomal abnormality is a fundamental aspect of the condition and is the basis for many of the associated features, including distinctive facial characteristics, potential for intellectual disability, and other health-related challenges.

The other options do not accurately describe Down syndrome. Severe vision impairment can occur in some individuals with Down syndrome, but it is not a defining characteristic. Likewise, excellent physical coordination and increased cognitive function are generally not associated with Down syndrome, as individuals typically experience delays in cognitive development.