Download Cystic Fibrosis: A Case Study

Cystic Fibrosis:
A Case Study
Using a Pedigree to Solve a Genetic Problem
Cystic fibrosis (f) is an autosomal recessive disorder.
It is caused by nonfunctioning chloride
(Cl) carrier proteins (normal allele “F”) in the epithelial cells lining the respiratory tract and other ducts
throughout the body. The inability to move Cl- ions upsets the osmotic balance of the mucus in these
regions, causing the mucus to become thick and viscous (fewer ions in the mucus results in less movement of water by osmosis into the mucus). This leads to breathing problems and high risk of respiratory
infections because the bacteria and other contaminants which the mucus traps cannot be removed. Cystic
fibrosis also leads to problems in the pancreatic ducts, as the mucus accumulates there and interferes with
normal pancreatic function. Depending on the mutation and other physical conditions, persons born with
cystic fibrosis may have lives shorter than five years or spanning 20+ years. Due to interaction with other
genes & general health, carriers of the cystic fibrosis allele (f) occasionally experience mild symptoms of
the disease.
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Caroline & Ethan recently married and
are phenotypically normal. However,
they are concerned because family members have died from cystic fibrosis. Caroline & Ethan are trying to reconstruct a
pedigree for their family to figure out
what the risk is of them passing the gene
along to their offspring. You (being the
most-prominent geneticist in the Chicagoland area) have been asked to advise
them.
Their family history is as follows:
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Caroline’s mother and father were phenotypically normal, but Caroline’s paternal grandmother
had some symptoms of cystic fibrosis. Caroline’s paternal grandfather had no signs of cystic
fibrosis. Caroline’s uncle (her father’s brother) had cystic fibrosis.
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Caroline’s maternal grandparents were both phenotypically normal and none of her mother’s 2
brothers or sisters had the disease.
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Caroline’s only sibling (her sister) did not have the disease, though Caroline is uncertain if her
sister is actually a full sibling.
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Ethan’s three siblings by his father (two brothers and a sister) were all phenotypically normal.
His step-sister (the child his mother and step-father had when they married each other)
developed cystic fibrosis. Both his mother and step-father were phenotypically normal.
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Ethan’s paternal grandparents (and the uncle and aunt on his father’s side) were phenotypically normal.
1) Construct a pedigree showing the relationships and phenotypes of all individuals mentioned.
2) On the pedigree, label the genotypes of the individuals, as well as can be known.
3) Determine the highest probabilty that Caroline & Ethan will have a child who:
a) has cystic fibrosis
b) will be a carrier
4) Hypothesize as to why cystic fibrosis is a recessive disorder.
[Think carefully over the introductory paragraph above].