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#1
08-26-2012
 USMLE Forums Newbie Steps History: Step 1 Only Posts: 9 Threads: 3 Thanked 1 Time in 1 Post Reputation: 11
genetics ques!!

A woman who is a known heterozygous carrier of a PKU mutation marries her half second cousin . What is the probability that this couple will produce a child with PKU??

A.1/16

B.2/9

C.1/64

D.1/256

E.1/128

#2
08-26-2012
 USMLE Forums Scout Steps History: Not yet Posts: 72 Threads: 8 Thanked 11 Times in 11 Posts Reputation: 21

Wow half second cousin

Could you please tell where u picked up this Q from?

Thanks!
#3
08-26-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

I think it's D 1/256 but it's based on many assumptions so really not sure...

my calculations:
1/2 x 1/4 x (1/8 x 1/2 x 1/2) = 1/256

1/2 - probability that the husband is a carrier (we have to include that, right??? I mean I assume that he's not affected since the stem doesn't say anything and if he's healthy then they will not have a child with PKU but not sure...)
1/4 - probability that, provided both parents are carriers, they will have a child with PKU

1/8 - pool of genes their grandaparents shared (half first cousins)
x 1/2 x 1/2 - portion of genes passed on through 2 generations (from grandparents to the present couple)

#4
08-26-2012
 USMLE Forums Newbie Steps History: Step 1 Only Posts: 9 Threads: 3 Thanked 1 Time in 1 Post Reputation: 11

Viral i pick this question from another forum.

Casandra yes i got the same answer when i calculated it
1/32*1/2*1/4
1/32 for second cousins
1/2 for half cousins
1/4 probability of PKU child if both parents are carriers
Yes i think we should include the probability of father being a carrier
Thanks!
#5
08-26-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

Quote:
 Originally Posted by electricblue Viral i pick this question from another forum. Casandra yes i got the same answer when i calculated it 1/32*1/2*1/4 1/32 for second cousins 1/2 for half cousins 1/4 probability of PKU child if both parents are carriers Yes i think we should include the probability of father being a carrier Thanks!
why 1/32 for second cousins? and then separately 1/2 for half cousins?
can you explain how and why did you calculate 1/32? thanks
#6
08-26-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

is D correct?
#7
08-26-2012
 USMLE Forums Newbie Steps History: Step 1 Only Posts: 9 Threads: 3 Thanked 1 Time in 1 Post Reputation: 11

D is correct

for siblings 1/2
for first cousins 1/8 (1/2*1/2*1/2)
for second cousins 1/32 (1/8*1/2*1/2)
ans so on...
please Correct me if i am wrong
regarding half cousin i am not sure i was thinking that we should multiply the whole thing with 1/2....
#8
08-26-2012
 USMLE Forums Scout Steps History: Not yet Posts: 72 Threads: 8 Thanked 11 Times in 11 Posts Reputation: 21

Thanks for the Q or else I would not have known the concept of half cousins!!

#9
08-26-2012
 USMLE Forums Scout Steps History: Step 1 Only Posts: 15 Threads: 2 Thanked 3 Times in 3 Posts Reputation: 13

What would have been the answer if they were
1. half cousins
2. Second cousins.

Thanks
#10
08-26-2012
 USMLE Forums Addict Steps History: Not yet Posts: 109 Threads: 24 Thanked 19 Times in 11 Posts Reputation: 29

i dnt get wat u mean by HALF cousins!! cud sm1 explain d relashionship!!
#11
08-26-2012
 USMLE Forums Master Steps History: Step 1 Only Posts: 864 Threads: 62 Thanked 374 Times in 225 Posts Reputation: 384
cousin

second cousin-1/8x 1/2x 1/2
Husband being carrier- 1/2
Affected child of carrier parents- 1/4

=1/256
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#12
08-26-2012
 USMLE Forums Veteran Steps History: 1+CK+CS+3 Posts: 205 Threads: 6 Thanked 151 Times in 80 Posts Reputation: 161

Quote:
 Originally Posted by Casandra I think it's D 1/256 but it's based on many assumptions so really not sure... my calculations: 1/2 x 1/4 x (1/8 x 1/2 x 1/2) = 1/256 1/2 - probability that the husband is a carrier (we have to include that, right??? I mean I assume that he's not affected since the stem doesn't say anything and if he's healthy then they will not have a child with PKU but not sure...) 1/4 - probability that, provided both parents are carriers, they will have a child with PKU 1/8 - pool of genes their grandaparents shared (half first cousins) x 1/2 x 1/2 - portion of genes passed on through 2 generations (from grandparents to the present couple)
chance that 2 related half cousins have same disease is 1/2 x1/2x1/2X1/2=1/16
chance that two heterozygotes will produce a homozygous affected child is in1/4
total probability of these events happening together is 1/16x1/4=1/64

please casandra /anyone else correct me as i did a similar question in KLN in population genetics but now i am not getting how the answer turns out to be 1/256
#13
08-26-2012
 USMLE Forums Guru Steps History: 1 + CK Posts: 454 Threads: 20 Thanked 340 Times in 161 Posts Reputation: 360

I agree with offpiste .. That's how I worked it out but half second cousin I dont get

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#14
08-26-2012
 USMLE Forums Scout Steps History: Not yet Posts: 89 Threads: 3 Thanked 49 Times in 35 Posts Reputation: 59

1/ 32 for second cousin
1/2 for half cousin
1/4 for child being diseased...

1/256

Half first cousins are , cousins who share a single grand parent ..
similarly 1/2 second cousins share single great grand parent....
#15
08-26-2012
 USMLE Forums Scout Steps History: Not yet Posts: 89 Threads: 3 Thanked 49 Times in 35 Posts Reputation: 59

Quote:
 Originally Posted by electricblue Viral i pick this question from another forum. Casandra yes i got the same answer when i calculated it 1/32*1/2*1/4 1/32 for second cousins 1/2 for half cousins 1/4 probability of PKU child if both parents are carriers Yes i think we should include the probability of father being a carrier Thanks!
From your calculation we also include probability of father being carrier answer would be 1/256 x 1/2 ...
I didnt understand why you said "Yes i think we should include the probability of father being a carrier" ??

#16
08-26-2012
 USMLE Forums Scout Steps History: Not yet Posts: 89 Threads: 3 Thanked 49 Times in 35 Posts Reputation: 59

Quote:
 Originally Posted by Casandra I think it's D 1/256 but it's based on many assumptions so really not sure... my calculations: 1/2 x 1/4 x (1/8 x 1/2 x 1/2) = 1/256 1/2 - probability that the husband is a carrier (we have to include that, right??? I mean I assume that he's not affected since the stem doesn't say anything and if he's healthy then they will not have a child with PKU but not sure...) 1/4 - probability that, provided both parents are carriers, they will have a child with PKU 1/8 - pool of genes their grandaparents shared (half first cousins) x 1/2 x 1/2 - portion of genes passed on through 2 generations (from grandparents to the present couple)

Do you think we should take probability of husband being carrier as 1/2 ?? The ( 1/8x 1/2x1/2) factor that you have used itself is the probability of husband sharing the same gene as his carrier wife..... Am I wrong ?
#17
08-26-2012
 USMLE Forums Scout Steps History: Not yet Posts: 89 Threads: 3 Thanked 49 Times in 35 Posts Reputation: 59

Quote:
 Originally Posted by offpiste chance that 2 related half cousins have same disease is 1/2 x1/2x1/2X1/2=1/16 chance that two heterozygotes will produce a homozygous affected child is in1/4 total probability of these events happening together is 1/16x1/4=1/64 please casandra /anyone else correct me as i did a similar question in KLN in population genetics but now i am not getting how the answer turns out to be 1/256
your calculation is right but you missed the point that question asked for half second cousins , You have calculated half first cousins ,

.. so the factor 1/16 which u got should be 1/ 64 Hope that helps ,, thanks
 The above post was thanked by: offpiste (08-26-2012)
#18
08-26-2012
 USMLE Forums Master Steps History: 1+CK+CS Posts: 1,039 Threads: 189 Thanked 570 Times in 323 Posts Reputation: 580

Quote:
 Originally Posted by dr.nitowl Do you think we should take probability of husband being carrier as 1/2 ?? The ( 1/8x 1/2x1/2) factor that you have used itself is the probability of husband sharing the same gene as his carrier wife..... Am I wrong ?
1/8x 1/2x1/2 - is a calculation that they share common genes (it doesn't mean that he's a carrier, he can share those genes and be perfectly healthy = noncarrier)
1/2 - is a chance of him being a heterozygous carrier of autosomal recessive disease
 The above post was thanked by: dr.nitowl (08-27-2012)
#19
08-27-2012
 USMLE Forums Scout Steps History: Not yet Posts: 89 Threads: 3 Thanked 49 Times in 35 Posts Reputation: 59

Quote:
 Originally Posted by Casandra 1/8x 1/2x1/2 - is a calculation that they share common genes (it doesn't mean that he's a carrier, he can share those genes and be perfectly healthy = noncarrier) 1/2 - is a chance of him being a heterozygous carrier of autosomal recessive disease
oh ! thanks.. then how can we account for the half cousin thing??

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