Pedigree problem

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SheffieldWednesdayFC
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Pedigree problem

Post by SheffieldWednesdayFC » Fri Oct 16, 2009 9:49 pm

Hey everyone,

I am particularly stuck on this problem although I can normally do pedigrees well

Cystic fibrosis is an autosomal recessive disorder. A male whose brother has the disease has children with a female whose sister has the disease. It is not know if either the male or the female is a carrier. If the male and female have one child, what is the probability that the child will be a female with the disease?

If I knew the genotypes of the parents for the male and female, I would be able to solve this. However, not knowing their genotypes makes the situation a whole more complicated. Is there a systematic way to do this?

Thanks for reading

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mith
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Post by mith » Fri Oct 16, 2009 11:29 pm

they're saying they don't know whether he's a carrier, meaning he's either normal or carrier, assuming cystic fibrosis is easily noticeable. Thus you should do the punnets for if the man is a carrier, and then if he isn't. i.e. you'd be doing the squares for 2 men and 2 women.
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Accepting hardships as the pathway to peace;
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SheffieldWednesdayFC
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Re: Pedigree problem

Post by SheffieldWednesdayFC » Sat Oct 17, 2009 4:59 pm

okay,thanks, but if one of them wasn't a carrier, there would be no chance the daughter would have the disease. Thus, I could only do one punnett square with both parents being Aa. Since it is noticeable, it has at least the first A, and then 1/2 a chance of getting a small a? and same goes with the female. and then a 1/4 chance the child will be aa, and then 1/2 a chance she will be a girl.
Btw, the answer is 1/18, but I don't know how to get to this

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mith
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Post by mith » Sat Oct 17, 2009 7:32 pm

hmm, seems they're operating on the assumption

2/3 * 2/3 chance the parents are carriers (it's out of 3 because not diseased)
1/4 chance the offspring are affected
1/2 chance the offspring is female
Living one day at a time;
Enjoying one moment at a time;
Accepting hardships as the pathway to peace;
~Niebuhr

SheffieldWednesdayFC
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Post by SheffieldWednesdayFC » Mon Oct 19, 2009 1:19 am

thanks, i think that was it! :)

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