Dog Days of Science ■ 127A
disease is a condition that impairs health. It may
be caused by external factors, such as infection
by viruses, bacteria, or parasites, or injury from
harmful chemicals or high-energy radiation. Nutrient
deficiency can also lead to disease. Inadequate vitamin
C consumption, for example, produces scurvy, once
common among sailors and pirates. Disease may also
be caused by the malfunction of one or more genes.
Diseases caused exclusively by gene malfunction are
described as genetic disorders, distinguishing them
from infections and other types of diseases.
But many of the diseases that are most common in
industrialized countries—heart disease, cancer,
stroke, diabetes, asthma, and arthritis, for example—
are caused by multiple genes interacting in complex
ways with each other and with external factors. They
are complex traits: malfunctions in key genes make a
person susceptible to developing these diseases, but
environmental factors affect whether the disease
will actually appear and how severe the symptoms
will be. A large percentage of the estimated risk
of developing chronic diseases is preventable by
lifestyle choices such as maintaining good nutrition,
exercising regularly (see graph), and avoiding
tobacco. (The word “chronic” means “unceasing”—a
reference to the fact a person who develops one of
these diseases will have it for the rest of their lives.)
A major goal of modern genetics is to identify
genes that contribute to human disease.
Researchers have identified alleles associatedwith increased risk of a number of common ailments,
including high blood pressure, heart disease, diabetes,
Alzheimer disease, several types of cancer, and
schizophrenia. The hope is that one day soon, genetic
tests will tell us whether we are predisposed to a
disease before we become ill with it. Then, a person
carrying a risky allele might take preventive measures
to reduce the chance of developing the condition, and
treatment could be customized to fit the particular
allele involved. This tailored approach to treatment,
called “personalized medicine,” is already being used to
treat breast cancer and other chronic diseases.Most Chronic Diseases Are Complex Traits
400,000
300,000
200,000
100,000
0
Stroke20%–27%
Diabetes50%
Colon
cancerHeart
disease26%
30%–40%
Annual U.S. deathsChronic diseasesPreventable
by physical
activitypleiotropy (pleio, “many”; tropy, “change”). In
PWDs, Lark found that single genes can control
multiple related skeletal traits. The shape of a
dog’s head and the shape of its limb bones are
controlled by a single gene. That connection
makes sense, says Lark, since a small head
and long legs are advantageous for a fast dog,
while a strong dog, like a pit bull, uses both its
massive jaw and its short, thick legs for power.
Another good example of pleiotropy comes
from a long-term breeding experiment to tame
Russian silver foxes. Researchers found that as
foxes became tamer and tamer, they also devel-
oped floppy ears instead of straight ones and had
shorter legs and curlier tails than ordinary foxes
have (see “The New Family Pet?,” page 129).
Patterns of inheritance can get even more
complicated. Single traits governed by the action
of more than one gene a re c a lled polygenic traits.
In humans, polygenic traits include eye and skin
color, running speed, blood pressure, body size,
and more. Of the thousands of human genetic
traits, governed by an estimated 24,000 genes,
fewer than 4,000 are known or suspected to be
controlled by a single gene with a dominant and a
recessive allele. The rest are polygenic traits.
Another twist on inheritance is epistasis,
which occurs when the phenotypic effect of a
gene’s alleles depends on the presence of alleles
for another, independently inherited gene.
Labrador coat color, for example, is affected by
epistasis (Figure 7.9). Dog fur, as mentioned
earlier, has a dominant allele (B) that leads
to black fur and a recessive allele (b) that
produces brown fur. But the effects of these
alleles (B and b) can be eliminated completely,