Down Syndrome
when the nondisjunction of the twenty-first chromosome occurs in one of the initial cell divisions after fertilization. This results in a mixture of two types of cells, some containing 46 chromosomes and some containing 47. Mosaicism is rare, being responsible for only 1 to 2 percent of all cases of Down syndrome. There is some evidence suggesting that individuals with mosaic Down syndrome are less affected than those with nondisjunction, but broad generalizations are not possible due to the wide range of abilities that people with Down syndrome possess. Translocation occurs when part of the number 21 chromosome breaks off during cell division and attaches to another chromosome. While the total number of chromosomes in the cells remains 46, the presence of an extra part of the number 21 chromosome causes the features of Down syndrome. As with nondisjunction trisomy 21, translocation usually occurs prior to conception. Regardless of the specific cause, all people with Down syndrome have an extra, critical portion of the number 21 chromosome present in all, or some, of their cells, and this additional genetic material alters the course of development and causes the characteristics associated with the syndrome. Many of the physical characteristics commonly found in people with Down syndrome are also found, to some extent, in the general population. It is the combination of traits that is characteristic of the syndrome, rather than any one feature. The most common visible traits include the following: • Flat, broad face with small ears and nose • Unusually short stature • Muscle hypotonia (poor muscle tone) • Upward slanting eyes with epicanthic folds (small skin folds on the inner corner of the eyes) • Short, broad hands with incurving fingers and a single deep crease across the center of the palm • Hyperflexibility (an excessive ability to extend the joints) • Excessive space between large and second toe • Small mouth with enlarged tongue, making articulation difficult • Incomplete or delayed sexual development
Most of these signs are obvious to the casual observer, but there are also some rather serious effects that are less evident. Prior to the last several decades, a large proportion of children with Down syndrome died before reaching adulthood, due primarily to congenital heart problems (most commonly in the form of holes) and bowel obstructions. Surgery in infancy to correct these problems is now fairly routine, and so people with Down syndrome regularly live well into adulthood, which has led to another interesting discovery: people with Down syndrome who live past their midthirties seem almost universally to develop early-onset Alzheimer’s disease. This lends a strong boost to the hypothesis that a genetic component is involved in that syndrome as well, and researchers are currently examining the role of the twenty-first chromosome pair in Alzheimer’s disease as a result. 66