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
Phenylalanine
hydroxylase
(enzyme)
Phenylalanine
(amino acid)
Tyrosine
(amino acid)
Melanin
(protein)
• Normally, phenylalanine is converted into tyrosine and
then into a protein
• The Problem: Enzyme phenylalanine hydroxylase is
absent/reduced
• The result: Phenylalanine builds up in blood
It’sMichelle’s
not my fault
problem
Michelle
is must
sick.
to a
I’mbea due
successful
weakness
of I
business
man.
Stacy’s
sideI of
don’t
smoke.
family.a Iday.
jogthe
6 miles
don’tahave
any
I have
healthy
“bad genes.”
diet.
Chances’ reaction
Everyone carries 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.
“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”
Stacy
h
HH
Hh
(healthy)
(healthy)
Paul
H
H
Key
H = Healthy
h
Hh
(healthy)
hh
hh
(PKU
disease)
h = PKU disease
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.
Genetic Counseling
• 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 #1
Assume that you are
Stacy or Paul Chance.
Describe in writing
your feelings after your
visit to the genetics
center.
•
Essay Format:
– Ink or typed (no pencil)
– Add title and date
– Minimal length = ¾ page
– If typed: 1” margins, 12-point times new roman
font, double spaced.
– Complete essays (Introduction, body,
conclusion)
– Spellcheck and proofread