917
CHAPTER TWENTY-FOUR
Genetics and Genomics
DIRECT-TO-CONSUMER GENETIC TESTING
Here is an example of the information sent as part of the study:
The DNA from the baby girl was submitted as a thirty-three-year-old
woman, 185 pounds, 5
9
, who smokes, drinks a lot of coF
ee, doesn’t exer-
cise, and eats a lot of dairy, grains, and fats.
The DNA from the baby girl was also submitted as a ±
fty-nine-year-old
man, 140 pounds, 5
7
, who exercises, never smoked, takes vitamins, hates
coF
ee, and eats a lot of protein and fried foods.
The DNA from the man was submitted as being from a thirty-two-year-old
male, 150 pounds, 5
9
, who smokes, rarely exercises, drinks coF
ee, doesn’t
exercise, and eats a lot of dairy, grains, and fats.
The disorders with elevated risks found for the three fake people were exactly
the same: osteoporosis, hypertension, type 2 diabetes, and heart disease. One
company oF
ered the appropriate multivitamin supplements for $1,200, which
the investigation found to be worth about $35. Recommendations tended to
state the obvious, such as advising a smoker to quit. The advice tracked with
the ±
ctional lifestyle/diet information and not genetics. Concluded the study:
“Although these recommendations may be bene±
cial to consumers in that
they constitute common sense health and dietary guidance, DNA analysis is
not needed to generate this advice.” Some of the suggestions could even be
dangerous, such as recommending vitamin excesses in people with certain
medical conditions.
S
ome websites offer genetic tests. Submit a DNA sample on a
cheek swab or by spitting into a tube, and for a fee, receive infor-
mation about traits in²
uenced by genetics. The tests are either for
mutations of well-studied single genes or “associations” of pat-
terns of variation throughout the genome statistically linked to
certain illnesses or susceptibilities.
DNA tests may detect benign traits such as ear wax consistency or abil-
ity to taste bitter substances. ³or health matters, however, without genetic
counseling consumers can make decisions based on partial or inappropriate
information. Passage of the Genetic Information Nondiscrimination Act in
2008, which prevents employers and insurers from using results of genetic
tests to deny opportunities, may encourage people to have their DNA
probed.
A study from 2006, however, provided information about DNA testing
companies that oF
er nutritional supplements supposedly matched to per-
sonal genetic profiles. After the media spread the word of these services,
the U.S. government’s General Accounting Office researched the tests.
Investigators sent to four “nutrigenetics” companies two DNA samples—one
from a nine-month-old female and the other from a forty-eight-year-old man.
But the samples were sent along with diF
erent invented lifestyle dietary pro-
±
les, creating fourteen “±
ctitious consumers”—twelve for the female, two for
the male.
24.1
INTRODUCTION
Packaged into our cells are instruction manuals for our bodies.
The manual—the human genome—is written in the language
of DNA molecules. Recall from chapter 4 that DNA consists
of sequences of the nucleotide building blocks
A
(adenine),
G
(guanine),
C
(cytosine), and
T
(thymine). Sequences of DNA
that encode particular proteins are called genes. A gene has
different forms, because its sequence can vary from individ-
ual to individual. In somatic cells, two copies of the human
genome of 3 billion building blocks each are dispersed among
the forty-six chromosomes
(f gure 24.1)
.
Genetics
(je
˘-net
iks) is the study of inheritance of character-
istics. Genes provide our diversity, including eye, skin, and hair
color; many aspects of health; athletic ability and talents; and
hard-to-deF ne characteristics such as personality traits. We often
equate the study of genetics with disease, but it is more accu-
rately described as the study of inherited variation. Our genomes
are more than 99.9% alike in DNA sequence; within that less
than 0.1% of genetic variation lies our individuality.
The transfer of genetic information from one generation to
the next occurs through the processes of meiosis and fertiliza-
tion, when one copy of the genome from each parent join. The
cells that give rise to eggs and sperm, like all somatic cells, are
diploid, with two copies of each of the 23 chromosomes. Eggs
and sperm are haploid, with one set of chromosomes. Joining
of eggs and sperm reconstitutes diploidy.
RECONNECT
To Chapter 22, Sperm Cell Formation, pages 835–836, and
Oogenesis, page 849
The human genome is an economical information
store. It includes about 24,000 protein-encoding genes.
Different cell types access different subsets of the genome,
using the information to produce particular proteins and in
this way sculpt the hundreds of types of specialized cells in
the body. Yet at the same time, the genome encodes much
more than 24,000 bits of information. RNA molecules can
represent parts of different genes, so that the 24,000 genes
encode 100,000 to 200,000 different proteins. It is a little
like having a wardrobe of twenty items, but wearing them
in different combinations to create many dozens of differ-
ent outF
ts.
Genetic information functions at several levels. It is
encoded in DNA and expressed in RNA and protein; affects
cells and tissues; affects the individual; and is also passed to
the next generation.
Figure 24.2
depicts the inherited disease
cystic F
brosis (C±) viewed at several levels.
RECONNECT
To Chapter 4, Nucleic Acids and Protein Synthesis,
pages 131–133.
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