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  #1  
Old 07-21-2011
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Genetics Heterozygous carrier marries a half-cousin

A man who is a known heterozygous carrier of oculocutaneous albinism marries his half-cousin (they share one common grandparent). This trait is transmitted as a fully penetrant autosomal recessive. What is the probability that this couple will produce a child with this disorder?

A) 1/2
B) 1/4
C) 1/8
D) 1/16
E) 1/32
F) 1/64
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  #2  
Old 07-21-2011
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Is it 1/64?
I dont know when i am gona master these types of questions..
(i solved it by this method..1/2 chance of father giving the defective gene.
mother shares 1/8 genes as they are cousins..
and if both mother and father are affected ,1/4 will be homogyzous affected..

so ,1/2 x 1/8 x 1/4 = 1/64..
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  #3  
Old 07-21-2011
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I think its b ....
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Old 07-21-2011
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That's a good question.

I suck at these kinda questions but I was trying to work it out -

The question stem says the man is heterozygous carrier (Aa) & shares a common grandparent with the cousin.

Do we assume the grandparent Heterozygous (Aa) or Homozygous (aa)? If the grandparent was aa then the chances of offsprings to become carriers are 100%. If the grandparent was Aa then the chances of offsprings to become carriers is 50% (1/2).

Now, I will assume that the grandparent was Heterozygous carrier (Aa), & 50% of his offsprings were carriers.

Since we know that the man who married the half-cousin is a carrier that means his parent received has a carrier status. The chance that the cousin is a carrier = 1/2 (probability his parent is a carrier) * 1/2 (probability that the cousin received the heterozygous mutation) = 1/4.

Probability that the cousin (1/4) & the man (carrier) will produce an affected child = 1/4 (cousin's probability of being a carrier) * 1/4 (probability of two carriers of AR dx producing an affected child)

---A----a
A-AA--Aa
a-Aa--aa

So I would go with 1/4 * 1/4 = 1/16.

Is the answer D ?

lol I was quite confused but tried to work it out!
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  #5  
Old 07-21-2011
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d 1/16 Aa
aa
baby 1/4 affected 1/2*1/2*1/4=1/16
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  #6  
Old 07-21-2011
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1/64 ...?

Because half cousin implies they share 1/2 * 1/2 * 1/2 * 1/2 = 1/16 taking into account the genes shared by their parents and one grand parent

Autosomal Recessive ==> 1/4 or 25 % affected

therefore 1/16 * 1/4 = 1/64
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  #7  
Old 07-21-2011
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Default F

1/64.
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  #8  
Old 07-21-2011
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I think 1/16 ... These question i found it... sooo hard...
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Old 07-21-2011
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Correct Answer correct answer

grandparent - > parent = they share 1/2
parent -> son = they share 1/2

Then, grandparent --> son = 1/2*1/2 = 1/4

In this case, both have one common grandparent, so we'll have 1/4*1/4

If both parents are heterozygous carrier for this autosomal recessive:
Aa x Aa => AA Aa aA aa => 1/4 affected

Finally, the probability that this couple will produce a child with this disorder will be equal to 1/4*1/4*1/4 = 1/64

This qs. is from Kaplan.
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  #10  
Old 07-21-2011
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Yep also got 1/64:

My method:

Cousins share 1/8 of your genes

1/8 x 1/2 (1/2 grandma relation) x 1/4 (AR chances) = 1/64
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  #11  
Old 07-21-2011
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They already said One Parent Is Heterozygous ,then why we should calculate probability of being heterozygous carrier for that parent ???

According to me answer should be 1/16 .

Male parent - already Heterozygous carrier - Probability - 1
Female parent - Probability of being heterozygous 1/2 * 1/2 = 1/4


Now , Probability of child being affected with disorder =
1/4 * 1/4(Probabilty of both parents being Heterozygous 1 * 1/4)

=1/16

Quote:
Originally Posted by bebix View Post
grandparent - > parent = they share 1/2
parent -> son = they share 1/2

Then, grandparent --> son = 1/2*1/2 = 1/4

In this case, both have one common grandparent, so we'll have 1/4*1/4

If both parents are heterozygous carrier for this autosomal recessive:
Aa x Aa => AA Aa aA aa => 1/4 affected

Finally, the probability that this couple will produce a child with this disorder will be equal to 1/4*1/4*1/4 = 1/64

This qs. is from Kaplan.
I have checked kaplan and they gave answer 1/64, but i really doubt it.

Please correct if i am wrong.

Thanks
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Old 07-22-2011
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siblings share 1/2 genes
first cousins share 1/8 genes
second cousins share 1/32
half cousins share 1/16
half brothers share 1/4
uncle/niece share 1/4

I never understood how to solve such questions.. so have just mugged up the relation.. and it works!
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  #13  
Old 07-22-2011
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ans is 1/16
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Old 07-22-2011
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Star E

It is Option E i.e 1/32
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Old 07-22-2011
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Quote:
Originally Posted by jaimin View Post
They already said One Parent Is Heterozygous ,then why we should calculate probability of being heterozygous carrier for that parent ???

According to me answer should be 1/16 .

Male parent - already Heterozygous carrier - Probability - 1
Female parent - Probability of being heterozygous 1/2 * 1/2 = 1/4


Now , Probability of child being affected with disorder =
1/4 * 1/4(Probabilty of both parents being Heterozygous 1 * 1/4)

=1/16

I have checked kaplan and they gave answer 1/64, but i really doubt it.

Please correct if i am wrong.

Thanks
"Why we should calculate probability of being heterozygous carrier for that parent ???"...we are calculating the probability of sharing the same genes...they are cousins
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