396 Chapter 20
ABC D EFGH JIABDE FGHJIdeletion of segment C(Figure 20.13). the technology for rapid, low-cost genetic
screening is becoming widely available. With these tools, a
physician can order a genetic profile for a patient and use
it to select the best medicine to
deal with that person’s illness.
pharmacogenetics promises
to improve patient care by
allowing doctors to tailor
treatments to their patients’
genes. as medical treatment
moves in this direction, it will be
important to have safeguards in
place to protect the privacy of
patients’ genetic records.everybody’s different. and thanks to our genes, so is every
body. Because each of us has our own personal mix of
alleles—the varying chemical forms of genes—we also may
respond differently to therapeutic drugs. a field of study
called pharmacogenetics aims to pinpoint genetic variations
that influence how individuals respond to medications. the
idea is to allow physicians to custom-prescribe the drugs
that will be safest and most effective for each patient.
all medicines work for some people, but none is a perfect
fit for all. Blood pressure drugs are an example. there are
more than one hundred different ones, partly because
there are many individual differences in how well each one
controls high blood pressure. the medication and dose that
work well for one patient may be only modestly effective for
another, or may cause dangerous or unpleasant side effects
for someone else. Figuring out the best course often is a
matter of trial and error.
how can we take the guesswork out of prescribing drugs?
a first step is identifying the genes that control common
reactions to various drugs. that research is happening now
n the structure of a chromosome isn’t “written in stone.” It
can change in a variety of ways.n Link to eye disorders 14.10, Varying effects of genes 19.4
You may recall that DNA in chromosomes consists of vari-
ous types of nucleotides linked by chemical bonds (Section
2.13). A gene mutation is a change in one or more of the
nucleotides that make up a gene. Such mutations can arise
in several ways that we discuss in detail in Chapter 21. In
this section we are concerned with changes in the structure
of whole chromosomes. During meiosis, pieces of chromo-
somes can be deleted, duplicated, or moved around in other
ways. The result often is harmful.
Various changes in a chromosome’s
structure may cause a genetic disorder
A chromosome region may be deleted spontaneously, or by
a virus, irra diation, chemical assault, or some other envi-
ronmental factor:
Any part of a chromosome can be lost. Wherever such a
deletion happens, it per manently removes one or more
of the chromosome’s genes. The loss of a gene can lead to
serious problems. For example, one deletion from human
chromosome 5 leads to abnormal mental development and
an abnormally shaped larynx. When an affected infant
cries, the sounds produced resemble meowing—hence theF i g u r e 20.13 Dr. Stephen Liggett is a researcher in the field
of pharmacogenetics. He is studying genes that control how
patients respond to drug treatments for asthma and congestive
heart failure.AP Images/Al BehrmanChanges in a Chromosome or its Genes
20.7
20.8
personalized medicine
F i g u r e 20.14 Some genetic disorders are due to deletion of
part of a chromosome. A This child developed cri-du-chat
syndome due to a deletion on chromosome 5. Having ears low
on the head relative to the eyes is a typical sign of the disorder.
B A deletion on chromosome 11 causes aniridia, the lack of an
iris in the eye, as shown in this photograph.A B
Courtesy G. H. Valentine Courtesy of the Aniridia Foundation International, http://www.aniridia.net© Cengage LearningSCIENCE COMES TO LIFE
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