Of the various known organic (as opposed to social) causes of mental retardation, the genetic mutation known as Down syndrome is second only to fetal alcohol syndrome in prevalence and incidence. Ordinarily, the gametes (ovum and sperm) each possess 23 chromosomes, which results in the usual human complement of 23 pairs, or 46 chromosomes total, in every cell of the body. In Down syndrome, however, one of the gametes brings extra genetic material along, resulting in an extra chromosome, or at least a portion of one, in the twenty-ﬁrst pair. For this reason the syndrome is also known as trisomy 21. In 95 percent of cases, this results from an error in cell division called nondisjunction, in which prior to conception a pair of number 21 chromosomes, in either the sperm or the egg, fails to separate. The extra chromosome is then replicated in every cell of the developing embryo.
Two other types of chromosomal abnormalities, mosaicism and translocation, are also sometimes implicated in Down syndrome. Mosaicism occurs 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.
Down syndrome also invariably results in mental retardation, though it is usually of the mild to moderate variety rather than severe or profound. Because most cases are only mildly or moderately retarded, a majority of people with Down syndrome are able to live in community settings and lead productive lives, including earning a living, though even the highest functioning continue to need some help and support.
Much has been made of the role of maternal age in Down syndrome, and indeed the incidence of the disorder increases dramatically as maternal age increases. The probability of having a baby with Down syndrome rises, for example, from 1 in 885 at age thirty to 1 in 32 at age forty-ﬁve. What these widely reported numbers tend to obscure, however, is that more than 80 percent of babies with Down syndrome are born to women under thirty-ﬁve, as fertility rates are much higher for younger women. The more intriguing fact usually overlooked, however, is that most women over thirty-ﬁve who become pregnant do so in the company of men who are even older. The extra genetic material can be contributed by either parent and indeed can be shown to have come from the father in as many as 20 to 25 percent of cases. This of course still leaves the mother as the source in a majority of cases. There are several reasons for this, but the main one is the simple fact that the father produces new sperm cells throughout adulthood, whereas the woman’s ova are all present from birth and are thus subject to the effects of aging. It may be that the woman in her forties has simply been exposed for a longer time to environmental hazards that can affect the ova than a woman in her twenties. Research so far, however, has failed to ﬁnd a consistent relationship between nondisjunction and any particular environmental agent.
The signs and symptoms of Down syndrome have been observed and reported for many centuries, but they were not accurately described as making up a distinct syndrome until J. Langdon Down did so in 1866. In keeping with the casual racism of the time, however, the epicanthic fold around the eyes that is typical of the syndrome led him to call the pattern “Mongolism,” as it gave a vaguely Asian appearance. Individuals with the disorder were therefore known as Mongoloids, suggesting that their problems stemmed from somehow having been born in Asia. Perhaps there is some poetic justice, therefore, in the fact that the syndrome now bears Down’s own name instead.
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