A mutagen is any agent that alters DNA
chromosomal structure, such as ionizing radiation and some chemicals. Pre-natal
exposure to mutagens may result in still births or increase the risk of
childhood cancer. Approximately 10% of abnormalities are genetic origins
(Sadler, 1990).
Some of the most genetic disorders
result not from mutagens but from failure of homologues chromosomes to separate
during meiosis resulting in conditions in which one pair of the chromosomes may
be duplicated or have more than two chromosomes. This disjunction of the
chromosomes produces daughter cells with 23 chromosomes each. Usually, the
normal human body cell is made up of 23 pairs of chromosomes, 22 pairs of which
are called autosomes and the remaining pair, the sex chromosomes. Chromosomal
abnormalities involving automomes are called automomal dependent abnormalities
while those related to sex chromosomes are said to be sex chromosome
abnormalities (Saladin, 1996).
In non-disjunction, both chromosomes
go to the same daughter cell which then receives 22. Aneuploid, the presence of
or absence of a chromosome accounts for about 50% of spontaneous abortions
(Saladin, 1996). Most notable for producing autosomal malformations are
chromosome pairs 23 to 15, 17, 18 and 21. The defects that occurs in chromosome
21 is the most common of chromosomal disorder referred to as Down’s syndrome,
it results from three copies of the chromosomes. Advancing maternal age
increases the risk of bearing a child with Down’s syndrome in the following
ratio: 1:3000 for women under 30, 1:265 by age 35 and 1:9 by age of 48
(Saladin, 1996, Carter, 1970). Symptoms include short stature, a relatively
flat face with a flat nasal bridge, mental retardation and congenital heart
malformations.
Trisomy of chromosome 13 is rare but
when it those occur, it’s often server. Affected babies do not make it to a
year and have server mental retardation, congenital heart defects, deafness,
and cleft/palate lip and eye defects. Another trisomy is that of chromosome 18
known as Edward’s syndrome. This incidence is also affected by increased
maternal age. Affected babies have low set of ears, flexion of fingers and
hand, mental retardation, heart defects as well as rocker bottom feet (Saladin,
1996).
Deletion is the loss of a portion of
a chromosome and when it occurs on chromosome 15, it causes prader willi
syndrome with features which include mental retardation and extreme obesity (Moore, 1999). A deletion
on chromosome 5 causes cri-du-chat syndrome in which a baby cry in infancy is
like that of a cat, high pitched; mental retardation as well as other physical
deformities are also observed (Smith, 1970).
Abnormalities associated with sex
chromosomes are notably Klinifelter’s, turners and triple x syndrome.
Klinifelter sufferers are sterile males, usually of average intelligence. The
affected boys have two or more x chromosomes along with their y chromosomes. They
tend to be tall with unusually long arms and legs, testicular atrophy,
gynaecomastia and hyalinization of testis (Young, 1992).
Turner’s syndrome found in females is a
condition where one sex chromosome is missing. Only 3% of those with turner
syndrome survive to birth (Saladin, 1996). No serious impairments are shown at
birth and as children; they tend to show webbed neck and widely space nipples.
At puberty, the secondary sex characteristics fail to develop; gonadal
dysgenesis and short stature are also observed (Tijo and Levan, 1956).
Difficulty in mathematics and spatial concept may be experienced and may have
health related problems (Young, 1996).
Triple x syndrome patients are
infertile and sometimes have mild intellectual impairment as well as scanty
menstruation (Saladin, 1996).
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