 |
 |
Chromosomal
Abnormalities
An error in chromosome number that arose during the development
of an egg or a sperm cell can be termed as a chromosomonal
abnormality. Unlike other cells, sperm and egg cells have
only 23 unpaired chromosomes. When they unite, pregnancy
begins and ultimately results in the formation of a fertilized
egg with 46 chromosomes. But if an error occurs during
cell division, it results in an embryo with a chromosomal
abnormality. A common type of chromosomal abnormality
is called a trisomy. This means that the individual has
3 copies, instead of two of a specific chromosome. Down
syndrome is the best cited example for trisomic condition
having 3 copies of chromosome 21.
Chromosomal aberrations also called structural alterations
may also occur when small pieces of chromosome gets deleted
, duplicated, inverted, misplaced or exchanged with part
of another chromosome. These structural rearrangements
may also result in pregnancy loss and birth defects.
One in 800 to 1000 infants are being affected by Down
syndrome. The risk of this and other trisomies increases
with mother age. The ratio of having a baby with Down
syndrome increases from 1 in 1250 if mother’s age
is 25 to 1 in 100 at age of 40. Mental retardation varies
from mild to moderate in such children. With early intervention
and special education, many learn to read and write and
participate in diverse childhood activities. Babies affected
with trisomies 13 or 18 show severe mental retardation
and birth defects. Most babies die before their first
birthday.
Sex Chromosome Abnormalities
One of the 23 pairs of human chromosomes is called Sex
chromosomes. Sex chromosome abnormalities are gender specific.
Normal males inherit an X and a Y chromosome while females
have two X's. A single Y chromosome is sufficient to produce
maleness while absence is necessary for femaleness. Female
abnormalities are the result of irregular numbers of either
the X or the Y chromosome or both.
Female Sex Chromosome Abnormalities
Turner syndrome is a rare chromosomal abnormality
(found 1 in 3000 to 5000 females) occurs when females
inherit only one X chromosome and their genotype is denoted
as X0. If they survive to birth, these girls have abnormal
growth patterns. They are short in stature, averaging
4 foot 7 inches as adults, and often characterised with
distinctive webbed necks (i.e., extra folds of skin),
small jaws, and high arched palates. They generally lack
prominent female secondary sexual characteristics. They
have exceptionally small, widely spaced breasts, broad
shield-shaped chests, and turned-out elbows. Their ovaries
do not develop normally and they do not ovulate. If diagnosed
in early childhood, regular injections of human growth
hormones can increase their stature by a few inches. Beginning
around the normal age of puberty, estrogen replacement
therapy can result in some breast development and menstruation.
These treatments allow Turner syndrome women to appear
relatively normal.
Metafemales or triple-X females, (found 1 in 10,000 females)
inherit three X chromosomes and their genotype is XXX
or more rarely XXXX or XXXXX. As adults, these "super-females"
are usually an inch or so taller than average with unusually
long legs and slender torsos, but otherwise appear normal.
They have normal development of sexual characteristics
and are fertile. They may have slight learning difficulties
and are usually in the low range of normal intelligence
(especially the XXXX and XXXXX individuals). They tend
to be emotionally immature for their size during childhood.
This sometimes results in teachers and other adults labeling
them as troublemakers. None of these traits prevent them
from being socially accepted as ordinary adult women.
Male Sex Chromosome Abnormalities
Klinefelter syndrome males inherit one or more extra X
chromosomes--their genotype is XXY or more rarely XXXY,
XXXXY, or XY/XXY mosaic. They characteristically have
relatively high-pitched voices, asexual to feminine body
contours as well as breast enlargement, and comparatively
little facial and body hair. They are sterile or nearly
so, and their testes and prostate glands are small. As
a result, they produce relatively small amounts of testosterone.
The feminizing effects of this hormonal imbalance can
be significantly diminished if Klinefelter syndrome boys
are regularly given testosterone from the age of puberty
on. Like Metafemales, many Klinefelter syndrome men are
an inch or so above average height. They also are likely
to be overweight. They usually face difficulties in learning
especially with language and short lived memory. They
are usually capable of normal sexual function, including
erection and ejaculation, but many, if not most, are unable
to produce sufficient amounts of sperm for conception.
Klinefelter syndrome males with more than two X chromosomes
usually have extreme symptoms and are often mentally retarded.
Men who are mosaic (XY/XXY) generally have the least problems.
There is no evidence that Klinefelter syndrome boys and
men are more inclined to be homosexual, but they are more
likely to be less interested in sex. They have a higher
than average risk of developing osteoporosis, diabetes
and other autoimmune disorders. This may be connected
to low testosterone production. The frequency of Klinefelter
syndrome has been reported to be between 1 in 500 and
1 in 1000 male births. This makes it one of the most common
chromosomal abnormalities.
XYY syndrome males inherit an extra Y chromosome and their
genotype is denoted as XYY. As adults, these "super-males"
are usually tall (above 6 feet) and generally appear and
act normal. However, they produce high levels of testosterone.
During adolescence, they often are slender, have severe
facial acne, and are poorly coordinated. They are usually
fertile and lead ordinary lives as adults. Many, if not
most, are unaware that they have a chromosomal abnormality.
The frequency of XYY syndrome is not certain due to statistical
differences between different studies. It may be as common
as 1 in 900 male births to as rare as 1 in 1500 or even
1 in 2,000.
However, some researchers suggest that the high testosterone
levels of XYY men can make them somewhat more prone to
violence and that this may cause higher rates of wife
beating.
Some rare disorders are caused by minor chromosomal changes.
For example, a tiny deletion on chromosome 15 can cause
Prader-Willi syndrome (characterized by mental retardation,
extreme obesity and other problems), while another on
chromosome 5 causes cri-du-chat (cat cry) syndrome (characterized
by a cat-like, high-pitched cry in infancy, mental retardation
and physical abnormalities). Small deletions from a specific
section of chromosome 22 also cause the heart defects,
cleft palate and other problems seen in DiGeorge and velocardiofacial
syndromes.
Parents who have had a baby with a chromosomal abnormality
should consult a genetic counselor. These health professionals
help families understand what is known about the causes
of a birth defect, and the chances that it will recur
in another pregnancy. Fortunately, in most cases, parents
of a baby with a chromosomal abnormality will learn that
the risk is low. For example, parents of a baby with Down
syndrome usually have a 1 percent risk of having another
affected baby, and also the additional risks a mother
has, based on her age. In certain cases, a genetic counselor
may recommend blood tests to determine whether parents
carry such a rearrangement, so he or she can give the
parents the most accurate picture of their risk in another
pregnancy.
Courtesy:
http://www.marchofdimes.com/professionals/681_1209.asp
http://anthro.palomar.edu/abnormal/abnormal_5.htm
|
|
|
|
