Transcript Chances` Choices

Scene 1: Michelle is saved by the
starving bacteria
After this lesson, the student will be able to:
1) explain recessive inheritance using phenylketonuria (PKU) as
an example;
2) describe the Guthrie test for screening newborn babies for
PKU
3) explain how PKU can be treated by a special diet and how this
could postpone the effects of PKU until the next generation
4) discuss some of the economic issues associated with
screening for the treatment of genetic conditions
5) explain why females with PKU need to be followed very closely
in their reproductive years to avoid the problems associated
with maternal PKU.
Michelle’s Situation
• Stacy and Paul Chance
are referred to the
genetics center
because their newborn
daughter, Michelle, had
elevated phenylalanine
levels as determined by
newborn screening.
The diagnosis of
phenylketonuria (PKU)
is made.
Mandatory PKU Testing
• The couple wonders why their child was tested. They
have two other children, Alan (age 6) and Frank (12)
who are apparently healthy, active, intelligent boys.
The couple couldn't remember any special testing that
either boy had at birth. State mandated newborn
screening is explained to them. A description of PKU
and its inheritance is provided, and dietary
management is discussed.
PKU Testing: Guthrie Test
• Mandatory statewide
testing around day 2
($5/child)
• Bacteria grows on blood
sample if excess
phenylalanine is present
• Treatment begins
immediately
• Why is $5/child more
cost effective for the
state then no testing at
all?
What is PKU?
• PHENYLKETONURIA
(PKU)
– PAH gene on long arm of
chromosome 12 is
absent/reduced
– Excess phenylalanine
interferes with brain
development
• Autosomal recessive
disorder
1:10,000-15,000 births
• Effects: retardation,
seizures, destructive,
restlessness
Prognosis
• With treatment: Normal
development
• With minimal treatment:
Intellectual defects
• Without treatment:
Retardation
Treatments
• Low phenylalanine diet
• Baby fed special formula
(~$10,000/year)
• Vegetables and low
phenylalanine foods
added later in
development
• Regular blood samples
taken and phenylalanine
diet adjusted as needed
• Eventually, a normal diet
can be obtained
Chemistry of PKU
• Normally, phenylalanine
is converted into tyrosine
and then into a protein
• Enzyme phenylalanine
hydroxylase is
absent/reduced
• Phenylalanine builds up
in blood
Chances’ reaction
• Paul denies his contribution to his daughter's
condition. He is a successful businessman, proud of
his healthful lifestyle. He doesn't smoke, he jogs six
miles a day and he enjoys a healthy diet. He attributes
Michelle's condition to a "weakness on Stacy's side of
the family" and refuses to believe that he has any "bad
genes." The genetic counselor mentions that everyone
carries an estimated 6-10 "unusual" recessive genes
which cause no clinical problems in the carriers, but if
two carriers of the same unusual recessive gene were
to have children together, each child would have a 1 in
4 chance of inheriting the recessive condition for
which the genes were responsible.
Genetic
Counseling
• Paul is relieved to know that being a
carrier neither reflects negatively on his
manhood nor does it isolate him from his
peers. He quickly turns the situation to his
advantage and states, "Well, if we're all
carriers of some bad genes, I guess I'm
better off than most of you. At least I
know what one of my unusual genes is."
Stacy does not try to defend herself
against Paul's earlier accusations but is
relieved at his acceptance of the cause of
Michelle's condition.
• The further complication of maternal PKU
is mentioned. Paul and Stacy are
scheduled to meet with a nutritionist, and
follow-up appointments for monitoring
blood phenylalanine levels are made.
• Upon leaving, Stacy thanks the genetics
staff and says, "Well I guess we won't be
seeing you again for a long time."
• Little does she know...
Essay Topic
• Assume that you are
Stacy or Paul
Chance. Describe in
writing your feelings
after your visit to the
genetics center.