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#1
05-28-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 88 Times in 61 Posts Reputation: 98
Autosomal Dominant Inheritance Question

Neurofibromatosis type 1 is one of the most common autosomal dominant disorders. A woman with neurofibromatosis type 1 has an unaffected partner. Which of the following is correct regarding their children?

a) The probability that each of their children will be affected is 1 in 4.
b) The probability that their second child will be affected if their first child is affected is 1 in 4.
c) The probability that their third child will be affected if their first two children are affected is 1 in 2.
d) If their first child is affected then their second child will not be affected.
 The above post was thanked by: bebix (05-28-2011), m82_ghasemi (06-07-2011)

#2
05-28-2011
 USMLE Forums Guru Steps History: Not yet Posts: 324 Threads: 70 Thanked 236 Times in 143 Posts Reputation: 246

its B i think
 The above post was thanked by: Claus_CU (05-28-2011)
#3
05-28-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,259 Times in 881 Posts Reputation: 3269

Quote:
 Originally Posted by Claus_CU Neurofibromatosis type 1 is one of the most common autosomal dominant disorders. A woman with neurofibromatosis type 1 has an unaffected partner. Which of the following is correct regarding their children? a) The probability that each of their children will be affected is 1 in 4. b) The probability that their second child will be affected if their first child is affected is 1 in 4. c) The probability that their third child will be affected if their first two children are affected is 1 in 2. d) If their first child is affected then their second child will not be affected.
I think is C)
 The above post was thanked by: Claus_CU (05-28-2011)
#4
05-28-2011
 USMLE Forums Master Steps History: 1 + CK Posts: 623 Threads: 62 Thanked 287 Times in 186 Posts Reputation: 297

i think its c
 The above post was thanked by: Claus_CU (05-28-2011)
#5
05-28-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,175 Threads: 38 Thanked 852 Times in 478 Posts Reputation: 862

Yes, I think its C) because:

Female is heterozygous affected & male is homozygous normal.

That gives a 50% chance of each child having a heterizygous affected genes.
 The above post was thanked by: Claus_CU (05-28-2011)
#6
05-28-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 88 Times in 61 Posts Reputation: 98

Quote:
 Originally Posted by Claus_CU Neurofibromatosis type 1 is one of the most common autosomal dominant disorders. A woman with neurofibromatosis type 1 has an unaffected partner. Which of the following is correct regarding their children? a) The probability that each of their children will be affected is 1 in 4. b) The probability that their second child will be affected if their first child is affected is 1 in 4. c) The probability that their third child will be affected if their first two children are affected is 1 in 2. d) If their first child is affected then their second child will not be affected.
The correct answer is C). Thanks.
#7
05-28-2011
 USMLE Forums Guru Steps History: Not yet Posts: 324 Threads: 70 Thanked 236 Times in 143 Posts Reputation: 246

But in autosomal dominant condition,if male is normal and female is heterozyogous then 2 child will be affected while the remaining two will not be affected..isn't it like that?
So i think if first two children are affected then the other two will be normal..
Can anyone please explain it?:sorry:
#8
05-28-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 88 Times in 61 Posts Reputation: 98

Quote:
 Originally Posted by struggle But in autosomal dominant condition,if male is normal and female is heterozyogous then 2 child will be affected while the remaining two will not be affected..isn't it like that? So i think if first two children are affected then the other two will be normal.. Can anyone please explain it?:sorry:
When an affected individual has children, and the other partner is not affected, the probability that each child will be affected is 1 in 2. Even if you have a child that has the disease, the next child will have the same 50%/50% chance of having the disease.
 The above post was thanked by: m82_ghasemi (06-07-2011), pass7 (05-28-2011), struggle (05-28-2011)
#9
06-07-2011
 USMLE Forums Addict Steps History: CK Only Posts: 178 Threads: 61 Thanked 250 Times in 105 Posts Reputation: 260

C is correct.
Aa (Mother) * aa (Father) =
Children: Aa - Aa - aa - aa --> 1/2 of children are affected.

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