Chapter 29
Heredity
1103
29
alert the new parents that treatment is necessary to ensure the
well-being of their infant. Anatomical defects are usually treated
surgically, and PKU is managed by strict dietary measures that
exclude most phenylalanine-containing foods.
Adult children of parents with Huntington’s disease are obvi-
ous candidates for these services, but many other genetic condi-
tions also place babies at risk. For example, a woman pregnant at
age 35 may wish to know if her baby has trisomy-21, or Down
syndrome (see Related Clinical Terms, p. 1107), a chromosome
abnormality with a high incidence in children of older mothers.
Depending on the condition being investigated, screening
can occur before conception by carrier recognition, or during
fetal testing.
Carrier Recognition
±ere are two major avenues for identifying carriers of detri-
mental genes: pedigrees and blood tests. A
pedigree
traces a
genetic trait through several generations and helps predict the
future. For prospective parents, a genetic counselor collects phe-
notype information on as many family members as possible and
uses it to construct the pedigree (o²en called the family tree). By
working backward from current individuals and applying the
rules of dominant-recessive inheritance, a counselor can deduce
the genotypes of the parents and figure out the genotypes of the
other individuals in their parents’ generation.
Simple blood tests are used to screen for the sickling gene in
heterozygotes, and sophisticated
blood chemistry tests
and
DNA
probes
can detect the presence of other unexpressed recessive
genes. At present, carriers of the Tay-Sachs and cystic fibrosis
genes can be identified with such tests.
Fetal Testing
Fetal testing is used when there is a known risk of a genetic dis-
order. ±e most common type of fetal testing is
amniocentesis
(am
0
ne-o-sen-te
9
sis), in which a wide-bore needle is inserted
into the amniotic sac through the mother’s abdominal wall, and
about 10 ml of fluid is withdrawn
(Figure 29.7a)
. Because there
is a chance of injuring the fetus before ample amniotic fluid is
present, this procedure is not normally done before the 14th
week of pregnancy. Using ultrasound to visualize the position of
the fetus and the amniotic sac has dramatically reduced the risk
of this procedure.
±e fluid is checked for enzymes and other chemicals that
serve as markers for specific diseases, but most tests are done
on the sloughed-off fetal cells in the fluid. ±ese cells are cul-
tured in laboratory dishes over a period of several weeks. ±en
the cells are examined for DNA markers of genetic disease
and karyotyped to check for chromosomal abnormalities (see
Figure 29.1).
Chorionic villus sampling (CVS)
suctions off bits of the
chorionic villi from the placenta for examination (Figure 29.7b).
A small tube is inserted through the vagina and cervical canal
and guided by ultrasound to an area where a piece of placental
tissue can be removed. CVS allows testing at 8 weeks but waiting
until a²er the 10th week is usually recommended. Karyotyping
Mutations of imprinted genes may lead to pathology. For
example, victims of Prader-Willi syndrome are mildly to mod-
erately retarded, short, and grossly obese. Children with Angel-
man syndrome are severely retarded, unable to speak coherently,
laugh uncontrollably, and exhibit jerky, lurching movements as
if tied to a puppeteer’s strings. ±e symptoms of these two dis-
orders are very different, but the genetic cause is the same—
deletion of a particular region of chromosome 15. If the defec-
tive chromosome comes from the father, the result is Prader-
Willi syndrome; the mother’s defective chromosome confers
Angelman syndrome. ±us, it seems that the same allele can
have different effects depending on which parent it comes from.
In short, protein-coding genes are not the only instructions
to which cells refer. RNA matters, and so do the tiny chemical
tags that attach to the chromatin.
Extranuclear (Mitochondrial) Inheritance
Describe the basis of extranuclear (mitochondria-based)
genetic disorders.
Although we have focused on the chromosomal basis of inheri-
tance, remember that not all genes are in the cell’s nucleus.
Some 37 genes (referred to as mtDNA) are in mitochondria.
Mitochondrial genes are transmitted to the offspring almost ex-
clusively by the mother because the ovum donates essentially all
the cytoplasm in the fertilized egg. Additionally, sperm mito-
chondrial DNA is selectively destroyed by elimination factors in
both the sperm and egg.
A growing list of disorders, all rare, is now being linked to
errors (mutations) in mitochondrial genes. Most involve prob-
lems with oxidative phosphorylation within the mitochondria,
but a few lead to unusual degenerative muscle disorders or neu-
rological problems. Some researchers suggest that Alzheimer’s
and Parkinson’s disease may be among them.
Check Your Understanding
12.
What process labels genes as paternal or maternal?
13.
What is the source of the genes that confer extranuclear
inheritance?
For answers, see Appendix H.
Genetic Screening, Counseling,
and Therapy
List and explain several techniques used to determine or
predict genetic diseases.
Describe briefly some approaches of gene therapy.
Genetic screening
and
genetic counseling
provide information and
options for prospective parents not even dreamed of 100 years
ago. Newborn infants are routinely screened for a number of an-
atomical disorders (congenital hip dysplasia, imperforate anus,
and others), and testing for phenylketonuria (PKU) and other
metabolic diseases is mandated by law in many states. ±ese tests
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