Download Is He Blind? A Visual Case

Is He Blind?
A Visual Case
By Eric Ribbens
Western Illinois University
CQ1: Are there genetic diseases that
we cannot cure?
A.
B.
C.
D.
E.
No
Yes, but just a few
Yes, but we have cures for most
Yes, many of them
Yes, many of them, and none we can
cure
Scenario

Neil is a 48 year old male who is
nearsighted with a large blind spot in his
left eye.

When examined, a complete visual fields
evaluation was performed.
How to Read A Visual Field



The center of the
graph is the focal
point.
Each ring shows the
area where he can
see a light.
The innermost ring is
the dimmest light, the
outermost the
brightest.
Neil’s Visual Fields
Small Group Problem:

Describe Neil’s vision. Remember, you are
a scientist: be as accurate and precise as
possible! Don’t just say “He can’t see very
well.”
CQ2: Neil’s vision is…





A: He cannot see what he focuses on
B: He cannot see much outside his focal
area
C: He cannot see movement
D: He cannot see colors
E: He can see well, he just doesn’t realize it
Small Group Problem:

Describe Neil’s peripheral vision.
Describe Neil’s peripheral vision.

Neil has very limited peripheral vision. His
left eye is better than his right eye. In his
right eye he only has left some peripheral
vision above his focal spot; in his left eye
he has no peripheral vision to the left of his
focal spot.
Peripheral Vision

What do we use peripheral vision for? In
other words, what couldn’t Neil see that a
person with normal vision could see?
What do we use peripheral vision
for? In other words, what couldn’t
Neil see that a person with normal
vision could see?

Peripheral vision provides contextual cues,
shows movement of objects around the
viewer, and is especially used in nighttime
vision.
CQ3: So if Neil has very limited
peripheral vision, you can predict that
he cannot:





A: See stars
B: See movement
C: See things that are far away
D: See colors very well
E: See out of a periscope
What does the big cutout on the left
side of his left eye represent?
CQ4: The big cutout:




A: Is the area where he still can see
B: Is an area the doctor didn’t test
C: Is an area where bacteria have
damaged his retina
D: Is an area where he can’t see
anything
What does the big cutout on the left of
his left eye represent?

It is an area in which Neil can see nothing.
This area is especially important because it
covers an area outside the view of the right
eye. Because he has no peripheral vision
to the left, he is unable to sense anything
moving at him from his left.
Group work: Why is he much less
able to see a dim light than a bright

light?
Why is he much less able to see a
dim light than a bright light?

Because it requires greater sensitivity of
the pigment. One implication is that
eventually things like a computer screen
will appear brighter in the middle and
dimmer around the edges.
Variety of Defects

Retinitis pigmentosa is a syndrome caused
by various genetic defects in the genes that
code for the development of the pigments
in the retina. Today we know of about 70
different genetic defects that can cause
this. Why are there so many defects
possible in these pigments?
Why are there so many defects
possible in these pigments?

I suspect it is due to the very precise
configuration of these pigments that is
required; many defects are possible in the
way that these proteins are folded.
Sequencing

If all of the pigments are slowly breaking
down, why is his peripheral vision the first
to go?
If all of the pigments are slowly
breaking down, why is his
peripheral vision the first to go?

No one knows why peripheral vision
deteriorates first. The pigment layer is as
thick in the perimeter as in the focal area
before the disease takes effect.
What do you think will eventually
happen to his vision? Why?

Hint: think about the possibilities: it could
get better, it could stay the same, it could
get worse. Given his history and what you
know about the disease (genetic cause),
answer this question.
What do you think will eventually
happen to his vision? Why?

Pigment loss will continue to happen.
Eventually his vision will be restricted to his
focal spot, and he may become legally (10
degree or less peripheral vision) or
completely blind.
What do you think will eventually happen to his vision? Why?
Genetic Counseling

Build a Punnett Square showing the
possible offspring of Neil’s parents. Hint:
both parents are normal, and the disease is
recessive.
CQ5: What are the chances that a
sibling also has the disease?
A.
B.
C.
D.
E.
0%
25%
50%
75%
100%
What are the chances that a sibling
also has the disease?

If it’s recessive, than both parents must be
carriers (heterozygous). 50% chance of
getting it from each parent, 25% chance of
getting two copies.
More Genetic Counseling

Let’s assume that 1 out of 50 people is a
carrier for Usher’s Syndrome Type II. If
Neil’s brother Paul does not show RP, what
are the chances that Paul’s son has it?
Chances Paul’s son has it?

First, we need to calculate the probability
that Paul is a carrier. There are four
possible combinations of gametes Paul
could have received from their parents:




RP (Dad) and RP (Mom)
RP (Dad and normal (Mom)
normal (Dad) and RP (Mom)
Normal (Dad) and normal (Mom).
Chances Paul’s son has it?

However, we know Paul doesn’t have RP so the
first combination can be thrown out. Two of the
other three combinations have one copy of the
RP gene, so there is a 2/3 chance that Paul is a
carrier.
Chances Paul’s son has it?



1: Paul 2/3 likely to be a carrier.
Second, we need to know the probability
that Paul’s wife is a carrier. We’re told this
in the problem, 1/50.
Third, we need to know the probability that
two carriers have a child with both genes
RP. We calculated that in CQ5: 1/4
Chances Paul’s son has it?



Therefore, the chances that Paul’s son has
RP is:
2/3 * 1/50 * ¼ = 0.00333
Note that this is for one specific son! If we
rewrite the question as What are the
chances Paul has a son with RP, then you
need to multiply by the # of sons.
CQ6: What causes a genetic
disease?
A.
B.
C.
Coding in the DNA in a chromosome
A gene doesn’t get translated properly
into a protein
Something goes wrong inside your cells
as you get older
Quick Review




What’s RP?
How do you read a visual field chart?
How does it relate to the unit we are
studying in this course?
How did we calculate the probability of a
nephew with RP?
FollowUp

Neil was diagnosed with RP three years
ago. His eyes have gotten worse. He is
now legally blind, has stopped driving, and
cannot play basketball anymore. He hasn’t
seen stars for several years, but is grateful
he can still see well enough to teach.