Describe Down syndrome, and explain how it occurs. What are some of the consequences of Down syndrome?

What will be an ideal response?

Down syndrome is the most common chromosomal disorder, occurring in 1 out of every 770 live births. In 95 percent of cases, it results from a failure of the twenty-first pair of chromosomes to separate during meiosis, so the new individual receives three of these chromosomes rather than the normal two. In other, less frequent forms, an extra broken piece of a twenty-first chromosome is attached to another chromosome (called translocation pattern). Or an error occurs during the early stages of mitosis, causing some but not all body cells to have the defective chromosomal makeup (called mosaic pattern). Because the mosaic type involves less genetic material, symptoms may be less extreme.
The consequences of Down syndrome include mental retardation, memory and speech problems, limited vocabulary, and slow motor development. Affected individuals also have distinct physical features—a short, stocky build, a flattened face, a protruding tongue, almond-shaped eyes, and (in 50 percent of cases) an unusual crease running across the palm of the hand. In addition, infants with Down syndrome are often born with eye cataracts, hearing loss, and heart and intestinal defects. Because of medical advances, many individuals with Down syndrome survive into their fifties and a few into their sixties to eighties. However, more than half of affected individuals who live past age 40 show symptoms of Alzheimer's disease, the most common form of dementia. Genes on chromosome 21 are linked to this disorder.