chromosomal disorders

chromosomal disorders. Down syndrome was first described by John
Langdon Down in 1866, and although heredity was suspected in its etiology,
it was not until 1959 that it was discovered that Down syndrome patients had
one extra chromosome, for a total of forty-seven instead of the normal fortysix.
Down syndrome occurs at a frequency of about one in one thousand
births and is the single most prevalent cause of mental retardation. The
great majority of Down syndrome patients have three chromosomes number
21 instead of two (a condition called trisomy 21). The physical features
associated with Down syndrome are easily recognizable: short stature, a
short neck with excessive loose skin, thick lips, epicanthal folds of the eye,
malformed ears, poor muscle tone, and a flattened facial profile. Major
physical problems include heart and kidney defects, deafness, and gastrointestinal
blockages. Developmental milestones are delayed, and mental retardation
is common. Intelligence varies considerably, with an average IQ of 50
and only a small percentage of patients approaching the normal range. It is
essential that parents and educators assess the capabilities of each child and
provide an educational environment that maximizes achievement.
Although Down syndrome is genetic in the sense that it results from an
imbalance in the genetic material—an extra chromosome—it is not hereditary
in the sense that it does not run in families. The incidence of Down syndrome
shows a striking increase with maternal age, increasing dramatically
(to one in fifty births) in women giving birth beyond age thirty-five.
A normal human has twenty-two pairs of autosomes and one pair of sex
chromosomes—XX if a female, XY if a male. Cases involving an extra chromosome
or a missing chromosome, particularly if the missing chromosome
is one of the autosomes, usually lead to spontaneous abortion. The few that
survive have severe malformations, including those of the brain, and are
likely to have severe mental retardation. Malformations as a result of abnormalities
involving the sex chromosomes are usually less severe. Females with
an extra X chromosome (XXX) tend to have lower IQs than their siblings.
Males with an extra X chromosome (XXY), a condition called Klinefelter
syndrome, usually are not mentally retarded but may develop psychosocial
problems. Males with an extra Y chromosome (XXY) may have speech, language,
and reading problems.