A Deadly Inheritance ■ 145copy of that gene. In other words, because males
cannot be heterozygous for any X-linked genes,
the effects of an a allele cannot be masked. In
general, males are more likely than females to get
recessive X-linked disorders, because they need
to inherit only a single copy of the disorder allele
to exhibit the disorder. Females, on the other
hand, must inherit two copies to be affected.
X-linked recessive inheritance explains why boys
are more likely to get WAS than girls are.
Other X-linked genetic disorders in humans
include red-green color blindness, hemophilia,
and Duchenne muscular dystrophy—a lethal
disorder that causes muscles to waste away,
often leading to death at a young age. All of
these X-linked disorders are caused by recessive
alleles.
More Common, but No Less Deadly: Zoe’s Story
X-linked disorders like WAS are rare compared
to autosomal disorders. Both sexes are equally
likely to be affected by recessive genetic
disorders on autosomes, since both males and
females have two copies of autosomal chromo-
somes and therefore identical odds of being
homozygous or heterozygous for a disorder allele.
Several thousand human genetic disorders are
inherited as recessive traits on autosomes. These
include sickle-cell disease, Tay-Sachs disease,
and the most common fatal genetic disease in
the United States, cystic fibrosis (Figure 8.10).
Scott and Jada first began to suspect some-
thing was wrong when their newborn daugh-
ter, Zoe, didn’t put on any weight. Every time
Zoe ate, her belly became hard and bloated. She
screamed in pain. “I was beside myself,” recalls
Jada. “I knew something wasn’t right.”
Like Felix, Zoe spent the first year of her life
in and out of the pediatrician’s office, until the
eve of her first birthday, April 6, 2005. On that
day, Scott and Jada sat with Zoe in a children’s
hospital in Florida, waiting for a second opinion.
The doctor came in and asked them to sit down.
The diagnosis was cystic fibrosis (Figure 8.11).
Cystic fibrosis (CF) is a lethal recessive genetic
disorder caused by one or more mutations in the
cystic fibrosis transmembrane regulator gene
(CFTR). A CFTR mutation causes the body to
produce abnormally thick, sticky mucus, which
clogs the airways and leads to lung infections.
The thick mucus also obstructs the pancreas,
preventing enzymes from reaching the intes-
tines, where they are needed to break down
and digest food. The average life span of people
with CF who live to adulthood is approximately
35 years. There is no cure.
Recessive genetic disorders vary in severity;
some, like cystic fibrosis, are lethal, whereas
others have relatively mild effects. Adult-onset
lactose intolerance, for example, is caused by a
single recessive allele that leads to a shutdown
in the production of lactase, the enzyme that
digests milk sugar.
The only individuals who get a disorder
caused by an autosomal recessive allele (a)
are those who have two copies of that allele
(aa). Usually, when a child inherits a reces-
sive genetic disorder, both parents are hetero-
zygous; that is, they both have the genotype
Aa (Figure 8.12). It is also possible for one or
both parents to have the genotype aa and thus
the disease. Because the A allele is dominant
and does not cause the disorder, heterozygous
individuals (Aa) like Scott and Jada, Zoe’sFigure 8.11
Cystic fibrosis damages lung tissue
The panel on the left shows a chest x-ray from a cystic fibrosis patient and
the panel on the right shows a normal chest x-ray. Notice the damaged
fibrotic lung tissue in the cystic fibrosis x-ray.