Biology Now, 2e

A Deadly Inheritance ■ 147

in a gene involved in bone growth. People with

achondroplasia have a decreased life span, so

few of them live long enough to pass the muta-

tion on to their offspring. Instead, infants with

achondroplasia are born to unaffected parents

at a rate of between one in 10,000 and one in

100,000. Almost all of these births are to fathers

older than 35 years, who produce this mutation

during sperm production.

Huntington disease, a dominant genetic

disorder, is an exception to that rule because

symptoms of the disease—uncontrolled move-

ments and loss of intellectual faculties caused

by dying brain cells—arise later in life, in one’s

forties, after the person carrying that allele has

had the opportunity to reproduce. In this way

the allele is readily passed from one generation

to the next. This is why some couples whose

families have a history of Huntington’s may

choose to screen their developing fetus for the

gene that causes the disorder.

Replacing Deadly Genes: A Work in Progress

Most inherited genetic disorders, including

cystic fibrosis and Huntington disease, have no

cure. Patients and their families do everything

they can simply to manage the disease. At the

age of 7, Zoe was taking 25 pills each day just

to digest food. At the age of 10, she underwent

four extended hospitalizations, and almost

her entire summer was spent on drip IVs. Zoe

continues to take antibiotics and mucus thin-

ners, uses nasal sprays, and spends 2 hours a

day doing breathing therapy. “We don’t talk

about the future,” says Jada, Zoe’s mother. Her

voice cracks. “But she is aware that she’s not like

everybody else.”

Scientists have not given up the race to find

effective treatments, even cures, for genetic

disorders. Thanks to Christoph Klein, WAS is

one of the few such disorders for which an effec-

tive treatment has been identified.

It was in 2003 that Klein, then at Boston

Children’s Hospital, saw the first glimmer of

hope that there might be a therapy for boys

afflicted with WAS. He and colleagues believed

aa Aa

Eggs

Sperm

Gamete

production

Affected

child

Noncarrier

child

Noncarrier

child

Affected

child

a

a

Aa aa

Aa aa

a

a

A a

× A a

Noncarrier mother Affected father

Figure 8.13

Inheritance of an autosomal dominant

disorder

The pattern of inheritance for a human

autosomal dominant genetic disorder is the same

as for any other dominant trait. This Punnett

square shows the possible children of a normal

female (genotype aa) and an affected male

(genotype Aa).

Q1: What is the probability that a child

with one parent who has an autosomal

dominant disorder will inherit the disease?

Q2: Why are there no carriers with a

dominant genetic disorder?

Q3: Because dominant genetic disorders

are rare, it is extremely rare for both

parents to have the condition (genotype

Aa). Draw a Punnett square with two Aa

parents. What proportion of the offspring

would have the disorder? What proportion

would be normal?