CHROMOSOMAL
DIFFERENCES AND GENETIC CONDITIONS
Differences in the number or structure of chromosomes are a major cause
of birth defects, mental retardation, and malignancies. Chromosomal
differences are present in approximately 1 in every 160 live-born infants and
are the cause of greater than 50% of all spontaneous first-trimester pregnancy
losses (Lashley, 1998; Thompson et al., 2001). Chromosomal differences most
com-monly involve an extra or missing chromosome. This is called ane-uploidy.
Whenever there is an extra or missing chromosome, there is always associated
mental or physical disability to some degree.
Down syndrome, or trisomy 21, is a common chromosomal condition that
occurs with greater frequency in pregnancies of women who are 35 years or
older. A person who has trisomy 21 Down syndrome has a complete extra number 21
chromosome, which causes a particular facial appearance and increased risk for
congenital heart defects, thyroid and vision problems, and mental retardation.
Other examples of chromosomal differences include trisomy 13 and 18, both more
severe than Down syn-drome, and conditions involving extra or missing sex
chromo-somes (eg, Turner syndrome, in which females have only one X chromosome
instead of the usual two) (Lashley, 1998).
Chromosomal differences may also involve a structural re-arrangement
within or between chromosomes. These are less common than chromosomal
conditions in which there is an extra or missing chromosome, but occur in 1 in
every 500 newborns (Thompson et al., 2001). People who carry “balanced”
chromo-some rearrangements have all of their chromosomal material, but it is
rearranged. A person who carries a balanced chromosomal re-arrangement has an
increased risk for spontaneous pregnancy loss and for having children with an
unbalanced chromosomal arrange-ment that may result in physical or mental
disabilities. Known carriers are therefore offered prenatal counseling and
testing.
Chromosome studies may be needed at any age, depending upon the
indication. Two common indications are for confirma-tion of a suspected
diagnosis such as Down syndrome, and for a his-tory of two or more unexplained
pregnancy losses. Chromosome studies are accomplished by obtaining a tissue
sample (eg, blood, skin, amniotic fluid), preparing and staining the
chromosomes, and analyzing them under a microscope. The microscopic studyof
chromosomes is called cytogenetics and is an area that is rapidly evolving.
Today, cytogenetics is used with new molecu-lar techniques such as fluorescent
in situ hybridization (FISH), which permits more detailed examination of
chromosomes. FISH is useful to detect small abnormalities, including
characterizing chromosomal rearrangements (Thompson et al., 2001).
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