What structural change occurs in cells affected by Down's Syndrome?

Down's Syndrome, also known as Trisomy 21, is characterized by an extra copy of chromosome 21 in an individual's genetic makeup. This means that instead of the typical two copies of chromosome 21, individuals with Down's Syndrome have three copies. This additional genetic material alters the normal development of the body and brain, leading to the distinctive physical characteristics and developmental challenges associated with the syndrome.

The presence of this extra chromosome affects various biological processes and can result in a range of health and developmental issues. Understanding that the defining feature of Down's Syndrome is this trisomy—specifically, the extra chromosome 21—is essential for grasping how it influences a person's health and development.