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toyin2
Joined: 18 Jan 2008
Posts: 29
Location: London
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 Posted: Thu Aug 07, 2008 10:49 am Post subject: cystic fibrosis calculations EMQ |
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Hi guys...this is an abridged version of an EMQ that always gives me a headache to work out these calculations  Any tips or am i just being dumb  ???
Options:
A amniocentesis+karyotype
B CVS+karyotype
C 1 in 160
D Fetal MRI
E 1 in 120
F Low risk reassurance
g CVS+DNA diagnosis
Q1: 25 year old woman at 6 weeks gestation in her 1st pregnancy has a brother with cystic fibrosis.Her partner is unrelated and has no family history of CF. what is the chance of them having a child with CF? The UK carrier rate frequency is 1 in 20.
Q2: The young woman in Q1 is found to carry the DF508 CF mutation. Her partners 30 mutation test is negative.what can u offer them?
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Xerxes I
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Joined: 01 Mar 2007
Posts: 228
Location: Winchester
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| Posted: Tue Aug 12, 2008 6:40 pm Post subject: |
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Q1: This is a tricky one actually:
Chance of partner being a carrier: 1/20
Chance of partner being a carrier and passing the dodgy chromosome: 1/20*1/2= 1/40
Now thisbit is tricky:
Chances of her being a carrier: 2/3 (this is the key to answering this q, imagine the four children her parents have, one is her brother who has the disease, three remain, two carriers and one non carrier, therefore there is a 2/3 risk of her being a carrier)
chances of her passing the dodgy chromoome: 1/3
1/3*1/40= 1/120
fo the second question, even though I don't know if the 30 mutation included that particular mutation, I would reassure. mainly because her mutation is known, so even if 30 mutation does not include that one, I would test for that mutation instead of CVS. |
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Nick Raine-Fenning
Course Director
Joined: 27 May 2006
Posts: 1854
Location: Nottingham
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| Posted: Wed Aug 13, 2008 8:46 am Post subject: |
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| toyin2 wrote: | | Q1: 25 year old woman at 6 weeks gestation in her 1st pregnancy has a brother with cystic fibrosis.Her partner is unrelated and has no family history of CF. what is the chance of them having a child with CF? The UK carrier rate frequency is 1 in 20. |
Very difficult - not sure you are right Xerxes.
If her brother has CF her parents (let's assume they are the same  ) must have been carriers (Unless one also had CF although we would expect to be told this in all fairness). This means there are four outcomes as you say for any sibling:
1. CF
2. carrier
3. carrier
4. normal
which you quite rightly modifed to 2/3 carrier and a 1/3 normal as she does not have the disease
You then made a mistake as you used a 1/3 in your calculation but should have used 2/3!
Her partner has the background risk of 1 in 20.
The answer should be 2/3 * 1/20 therefore which = 2/60 or 1/30.
Am I right??
Toyin - where was this question from and were there other risks given??
Last edited by Nick Raine-Fenning on Wed Aug 13, 2008 8:50 am; edited 1 time in total |
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Nick Raine-Fenning
Course Director
Joined: 27 May 2006
Posts: 1854
Location: Nottingham
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| Posted: Wed Aug 13, 2008 8:50 am Post subject: Re: cystic fibrosis calculations EMQ |
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| toyin2 wrote: | | Q2: The young woman in Q1 is found to carry the DF508 CF mutation. Her partners 30 mutation test is negative.what can u offer them? |
Reassurance.
DF508 is the most common I believe (? up to 80%) and should be detectable therefore.
A normal screen offers a >95% chance of normality as the undetectable mutations account for 5% or less.
We get these reuslts for guys with azoospermia all the time but I can't remember the exact figures. I'll check next clinic |
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Xerxes I
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Joined: 01 Mar 2007
Posts: 228
Location: Winchester
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| Posted: Wed Aug 13, 2008 12:01 pm Post subject: |
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Hi Toyin,
The reason I used 1/3 in my calculations is that I multiplied 2/3 by 1/2 to get the risk of her actually giving the bad chromosome to the baby. Same with the partner. He has a 1/20 risk of being the carrier (having one of two chromosome pair with the mutaation) but the risk of him being a carrier AND passing this chromosome to the baby is 1/2*1/20=1/40
Therefore 1/3*1/40=1/120
What you have calculated (2/3*1/20=1/30) is the risk of them both being carriers. Now the risk of them both being carriers AND having a baby with CF (1/4) is again 1/30*1/4=1/120. two different way of reaching the same result. |
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Maud
Joined: 11 Oct 2007
Posts: 73
Location: Bristol
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| Posted: Wed Aug 13, 2008 7:55 pm Post subject: |
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I agree with Xerxes. I think Nick gave us the risk of them both being carriers and Xerxes gave us the risk of the baby having CF.
I had it wrong, I always forget using the 2/3 risk instead of the 2/4 risk of the healthy person being a carrier.  |
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Nick Raine-Fenning
Course Director
Joined: 27 May 2006
Posts: 1854
Location: Nottingham
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| Posted: Thu Aug 14, 2008 6:58 pm Post subject: |
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| Xerxes I wrote: | | The reason I used 1/3 in my calculations is that I multiplied 2/3 by 1/2 to get the risk of her actually giving the bad chromosome to the baby. |
Why? I am sure you are right but I have just had a TIA and am totally lost |
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Xerxes I
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Joined: 01 Mar 2007
Posts: 228
Location: Winchester
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| Posted: Thu Aug 14, 2008 7:12 pm Post subject: |
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Hi Nick, I think I have overcomplicated it (as I do with everything in life! and I promise I won't do it in the exam).
We all agree that the risk of him being a carrier is 1/20 and the risk of her being carrier is 2/3. Therefore the risk of them both being a carrier is 1/30. We also know that if two persons are both carriers, the risk of them having an affected child is 1/4. Now the risk of them being both carriers AND having an affected child will be 1/4x1/30=1/120.
the 1/4 we used comes from: 1/2(the risk of a carrier contributing his bad chromosome to the ovum) multiplied by 1/2 (the risk of the other carrier ....) and I used the two 1/2 seperately. It's basically the same thing in slightly different ways. |
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Nick Raine-Fenning
Course Director
Joined: 27 May 2006
Posts: 1854
Location: Nottingham
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| Posted: Thu Aug 14, 2008 7:24 pm Post subject: |
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I am sure you are right and what you say makes absolute sense but if we get 1 in 4 for the risk of a baby being affected when we have two carriers presumably by multiplying 1/2 * 1/2 why can we not do the same and simply multiply 2/3 * 1/20?
Is the answer given in the book this comes from? |
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toyin2
Joined: 18 Jan 2008
Posts: 29
Location: London
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| Posted: Fri Aug 15, 2008 4:59 pm Post subject: |
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well the answers as given are:
1 in 160 for the first q
low risk reassurance for the 2nd q
if anyone knows how they got to that calculation...fill me in
questions from a well known revision course(not TF!) |
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Nick Raine-Fenning
Course Director
Joined: 27 May 2006
Posts: 1854
Location: Nottingham
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| Posted: Sat Aug 16, 2008 8:25 am Post subject: |
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| toyin2 wrote: | well the answers as given are:
1 in 160 for the first q |
Now the cat is well and truly amongst the pigeons
Maud, Xerses??? |
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