Part 1 of Essay
The chromosomal abnormality involved in most cases of Down syndrome is trisomy-21, or the presence of three copies of the 21st chromosome. As a result, the affected person has 47 chromosomes in all body cells instead of the normal 46, although how this causes the condition's symptoms is not yet known. Scientists assume th ....

Part 2 of Essay
.... the extra chromosome 21, children with Down syndrome often have some characteristic physical features, such as a small head, a flat face, slightly upward slanted eyelids, skin folds at the inner corners of the eyes, small nose and mouth, and small hands and feet. Most of these characteristic do not interfere with the child's functioning, a doctor primarily uses the characteristics for diagnostic purposes. These physical features are variable, and children with Down syndrome are usually more like other children ....