Cystic Fibrosis Gene Found to Protect Against Typhoid
By William J. Cromie, Gazette Staff
Millions of people in the United States, Canada, and Europe carry a ticking time bomb in their cells -- a
mutated copy of a gene known as CFTR. If both mother and father possess the mutation, each of their
children has a one in four chance of dying before age 30.
A single copy of the mutated CFTR gene is present in one out of every 20 people of European origin. The
25 percent of those children who inherit two mutant copies get cystic fibrosis, a lethal disease that attacks
the lungs. Until the 1950s, almost all such newborns died in early childhood.
Cystic fibrosis sufferers produce unusually salty sweat, a trait used to detect the disease. In the past, if a
baby tasted salty when kissed, people knew the infant would die before its second birthday. Even today,
when lung infections can be controlled with antibiotics, most victims of cystic fibrosis, 30,000 people in
the United States, die before age 30.
Men with cystic fibrosis are usually sterile, and only recently have women with the disease been able to
become pregnant.
This lethality and sterility present medical scientists with a mystery. Why does the mutation persist when,
until quite recently, those who got the disease perished before passing it on? To survive the ruthless
culling of evolution, the mutation must provide some advantage. But what is it?
Researchers at Harvard University and their colleagues at the universities of Bristol and Cambridge in
England have found a likely answer.
"People with only one copy of the mutated gene apparently gain protection from infection by the
bacterium that causes typhoid," says Gerald Pier, professor of medicine at Harvard Medical School.
Typhoid comes from eating food or drinking water contaminated with Salmonella typhi, a bacterium
common in places with poor sanitation. Carried into the gut with corrupted water or food, the bug gets
into the intestinal wall, and then moves into the bloodstream. People with one copy of the mutated CFTR
gene gain protection against such infection.
Cystic Fibrosis Gene Found to Protect Against Typhoid
Multiple Choice
1. The relationship between humans and the typhoid bacteria can be described as
a. mutualistic
b. commensalistic
c. competitive
d. parasitic
2. Before 1950 most individuals born with cystic fibrosis died before their second birthday
because
a. malnutrition
b. sterility
c. lung infections
d. salty sweat
3. What is the evolutionary advantage of the mutated form of the CFTR gene?
a. Carriers of the gene have resistance to the typhoid disease.
b. Women with the gene are able to become pregnant.
c. Carriers of the gene are less likely to get lung infections.
d. Male carriers of the gene are usually sterile.
Open Response
A. Complete a Punnett square to show the phenotypic ratio that would result when two CF
carriers have children.
B. Considering that homozygous recessive CFTR individuals either die at a young age or are
sterile as adults, why does the mutated form of CFTR continue to persist? Use the points of
natural selection to justify your response.
Cystic Fibrosis Gene Found to Protect Against Typhoid
Open Response—Scoring Rubric
Score
4 A: The student correctly completes the Punnett square and identifies the phenotypic
ratios correctly.
B: The student describes the genotypes and/or phenotypes with respect to CF and
typhoid and correctly uses at least four of the five points of natural selection to justify
their response.
3 A: The student correctly completes the Punnett square and identifies the phenotypic
ratios correctly.
B: The student describes the genotypes and/or phenotypes with respect to CF and
typhoid and correctly uses three of the five points of natural selection to justify their
response.
2 A: The student correctly completes the Punnett square, but identifies genotypic ratios or
omits phenotypic ratios.
B: The student describes the genotypes and/or phenotypes with respect to CF and
typhoid and correctly uses two of the five points of natural selection to justify their
response.
1 A: The student completes the Punnett square with some errors.
B: The student describes the genotypes and/or phenotypes with respect to CF and
typhoid and correctly uses one of the five points of natural selection to justify their
response.
0 A: No attempt or no parts correct
B: No attempt or no parts correct