931
CHAPTER TWENTY-FOUR
Genetics and Genomics
pleting meiosis. During this time, the oocytes
may have been exposed to chromosome-dam-
aging chemicals or radiation. Other trisomies are
more likely to occur among the of
spring oF older
women, too. In the nineteenth century, when
physicians noted that people with Down syn-
drome were oFten the youngest in their Families,
they attributed the condition to “maternal repro-
ductive exhaustion.”
Many oF the medical problems that people
with Down syndrome suf
er are treatable, so liFe
expectancy is now FiFty-Five years. In 1910, liFe
expectancy was only to age nine.
However, 80% oF children with trisomy 21 are
born to women under age thirty-Five, because
younger women are more likely to become preg-
nant and have been less likely to have prenatal
testing. About 5% oF cases oF trisomy 21 can be
traced to nondisjunction in the sperm.
The age Factor in Down syndrome may be
because meiosis in the Female is completed aFter
conception. The older a woman is, the longer her
oocytes have been arrested on the brink oF com-
T
he most common autosomal aneuploid is
trisomy 21,
an extra chromosome 21. The
characteristic slanted eyes and ±
at Face oF
af
ected individuals prompted Sir John Langdon
Haydon Down to coin the inaccurate term “mon-
golism” when he described the syndrome in 1886.
As the medical superintendent oF a Facility For the
proFoundly mentally retarded, Down noted that
about 10% oF his patients resembled people oF
the Mongolian race. The resemblance is coinci-
dental. Males and Females oF all races can have
the syndrome.
A person with Down syndrome (either tri-
somy or translocation) is short and has straight,
sparse hair and a tongue protruding through thick
lips. The eyes slant and have upward “epicanthal”
skin Folds in the inner corners. Ears are abnor-
mally shaped. The hands have an unusual pat-
tern oF creases, the joints are loose, and re±
exes
and muscle tone are poor. Developmental mile-
stones (such as sitting, standing, and walking) are
slow, and toilet training may take several years.
Intelligence varies greatly, From proFound mental
retardation to being able to Follow simple direc-
tions, read, and use a computer. At least two col-
leges specialize in educating people with Down
syndrome (²
g. 24B).
Down syndrome (either type) is associated
with many physical problems, including heart or
kidney deFects, susceptibility to inFections, and
blockages in the digestive system. An af
ected child
is ²
Fteen times more likely to develop leukemia than
a healthy child, but this is still a low ²
gure. Prenatal
testing cannot reveal how severely af
ected an indi-
vidual with Down syndrome will be.
The likelihood oF giving birth to a child
with trisomy 21 Down syndrome increases dra-
matically with the age oF the mother (table 24A).
24.2
CLINICAL APPLICATION
Down Syndrome
chemicals in serum. Levels of these maternal serum markers,
which include alpha fetoprotein, an estrogen, pregnancy-
associated plasma protein A and human chorionic gonadotro-
pin, can indicate an underdeveloped liver, a sign of trisomies
13, 18, and 21. Screening maternal serum markers is routine
in the management of pregnancy, and it identiF es fetuses at
elevated risk.
After a maternal serum marker pattern indicates
increased risk, the patient is offered
amniocentesis,
in
which a needle is inserted into the amniotic sac and with-
draws about 5 milliliters of fl
uid. ±etal F
broblasts in the
Prenatal Tests Detect Chromosome
Abnormalities
Several types of tests performed on pregnant women can
identify anatomical or physiological features of fetuses that
can indicate a chromosomal problem or detect the abnormal
chromosomes
(f g. 24.13)
. An ultrasound scan, for example,
can reveal the fusion of the eyes, cleft lip and/or palate, mal-
formed nose, and extra F ngers and toes that indicate trisomy
13
(f g. 24.14)
. A blood test performed on the woman during
the F fteenth week of pregnancy detects levels of certain bio-
FIGURE 24B
Many people with Down
syndrome can learn, go to school, and hold
jobs. This young lady is learning From a cheF.
TABLE
24A
|
± ±Risk±of±Trisomy±
21 Increases with
Maternal Age
Maternal
Age
Trisomy 21
Risk
Risk for Any
Aneuploid
20
1/1,667
1/526
24
1/1,250
1/476
28
1/1,053
1/435
30
1/952
1/385
32
1/769
1/322
35
1/378
1/192
36
1/289
1/156
37
1/224
1/127
38
1/173
1/102
40
1/106
1/66
45
1/30
1/21
48
1/14
1/10
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