chromosome 21

The gene on chromosome 21 was the first to be identified. (This finding is significant because an abnormality in chromosome 21—an extra copy—is found in patients with Down syndrome, virtually all of whom develop Alzheimer disease if they live to age 35.) The defective gene on chromosome 21…

…inherit an extra part of chromosome 21 or an entire extra copy of chromosome 21, a condition known as trisomy 21. British physician John Langdon Down first described the physical features of Down syndrome in 1866, and thus the disorder was later named for him.

…21, a condition known as trisomy 21. British physician John Langdon Down first described the physical features of Down syndrome in 1866, and thus the disorder was later named for him.