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#1
05-31-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 96 Times in 64 Posts Reputation: 106
If you don't die after this...you're done with genetics!

Subject I 1 and II 2 have Becker muscular dystrophy (X-linked recessive inheritance)
Subject III 2 has oculocutaneous albinism (autosomal recessive inheritance, incidence in the general population of 1/10 000.
The disease status for both Becker muscular dystrophy and oculocutaneous albinism is unknown for IV 1 and IV 2.

click image to enlarge

1) What is the probability that III 3 is a carrier of Becker muscular dystrophy?
a) 1/4
b) 2/3
c) 1/2
d) 1

2) What is the probability that III 3 is a carrier of oculocutaneous albinism?
a) 1/4
b) 2/3
c) 1/2
d) 1

3) What is the probability that IV 1 is a carrier of oculocutaneous albinism?

a) 1/4
b) 1/2
c) 2/3
d) 99/100

4) What is the probability that IV 2 will be affected with Becker muscular dystrophy?

a) 1/4
b) 1/2
c) 0
d) 1

5) What is the probability that IV 2 will be affected with oculocutaneous albinism?

a) 1/100
b) 1/50
c) 1/24
d) 1/12

Have fun
 The above post was thanked by: bebix (05-31-2011), DrSick (05-31-2011), Hope2Pass (05-31-2011), INCOGNITO (06-04-2011), Med.Student (05-31-2011), rahulpatel21295 (07-07-2018), vaibhavmanu (05-31-2011)

#2
05-31-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 8 Threads: 2 Thanked 3 Times in 2 Posts Reputation: 13

those questions were some how confusing
these are my answers by the way:
1- a
2- b
3- a
4- a
5- d

how many of them are correct & how many wrong??
 The above post was thanked by: Claus_CU (05-31-2011)
#3
05-31-2011
 USMLE Forums Scout Steps History: Step 1 Only Posts: 46 Threads: 8 Thanked 37 Times in 18 Posts Reputation: 47

1 - d
2 - c
3 - d
4 - b
5 - a

Last edited by maitribhandari; 05-31-2011 at 12:00 PM.
 The above post was thanked by: Claus_CU (05-31-2011)

#4
05-31-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,684 Threads: 213 Thanked 1,662 Times in 676 Posts Reputation: 1672

Wow great question.

1) What is the probability that III 3 is a carrier of Becker muscular dystrophy?
a) 1/4
b) 2/3
c) 1/2
d) 1

2) What is the probability that III 3 is a carrier of oculocutaneous albinism?
a) 1/4
b) 2/3
c) 1/2
d) 1

3) What is the probability that IV 1 is a carrier of oculocutaneous albinism?

a) 1/4
b) 1/2
c) 2/3
d) 99/100

4) What is the probability that IV 2 will be affected with Becker muscular dystrophy?

a) 1/4
b) 1/2
c) 0
d) 1

5) What is the probability that IV 2 will be affected with oculocutaneous albinism?

a) 1/100
b) 1/50
c) 1/24
d) 1/12

I guessed on the last one! Hope Im right. Im quite weak in genetics though! Looking forward to your answers and explanations.
 The above post was thanked by: Claus_CU (05-31-2011)
#5
05-31-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,176 Threads: 38 Thanked 909 Times in 512 Posts Reputation: 919

Not to offend the OP, but this question really sucks cause the pedigree doesn't distinguish if the mother/father/grandfather/grandmother is a carrier or not.. so you're made to assume.

1) C
2) C
3) A
4) C
5) C
 The above post was thanked by: Claus_CU (05-31-2011)
#6
05-31-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 96 Times in 64 Posts Reputation: 106

haha

Actually, ALL the info is there...this is what you know:
"Subject I 1 and II 2 have Becker muscular dystrophy (X-linked recessive inheritance). Subject III 2 has oculocutaneous albinism (autosomal recessive inheritance), incidence in the general population of 1/10 000.
The disease status for both Becker muscular dystrophy and oculocutaneous albinism is unknown for IV 1 and IV 2."

the rest is just pure probabilities (if the subject is or not a carrier is indeed a probability)

have fun!

Last edited by Claus_CU; 05-31-2011 at 12:45 PM.
#7
05-31-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,372 Times in 906 Posts Reputation: 3382

1) D
2) B
3) D
4) B
5) C?

I think I´m gonna die now, hahaha
 The above post was thanked by: Claus_CU (05-31-2011)
#8
05-31-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 96 Times in 64 Posts Reputation: 106

1) What is the probability that III 3 is a carrier of Becker muscular dystrophy?
a) 1/4
b) 2/3
c) 1/2
d) 1

2) What is the probability that III 3 is a carrier of oculocutaneous albinism?
a) 1/4
b) 2/3
c) 1/2
d) 1

3) What is the probability that IV 1 is a carrier of oculocutaneous albinism?

a) 1/4
b) 1/2
c) 2/3
d) 99/100

4) What is the probability that IV 2 will be affected with Becker muscular dystrophy?

a) 1/4
b) 1/2
c) 0
d) 1

5) What is the probability that IV 2 will be affected with oculocutaneous albinism?

a) 1/100
b) 1/50
c) 1/24
d) 1/12

Thanks everybody!
 The above post was thanked by: sana akbar (05-31-2011)
#9
05-31-2011
 USMLE Forums Newbie Steps History: Not yet Posts: 4 Threads: 0 Thanked 2 Times in 2 Posts Reputation: 12

1. D
2. C
3. D
4. B
5. C
#10
05-31-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 96 Times in 64 Posts Reputation: 106

@awaisnaeem86 check the right answers above
#11
05-31-2011
 USMLE Forums Addict Steps History: 1 + CS Posts: 113 Threads: 8 Thanked 96 Times in 64 Posts Reputation: 106

BTW:

check this link for the explanations
Also you'll find more questions!

http://www.oup.com/uk/orc/bin/978019...ces/risk/quiz/
 The above post was thanked by: bebix (06-01-2011), kiki (05-31-2011)
#12
05-31-2011
 USMLE Forums Scout Steps History: Step 1 Only Posts: 34 Threads: 3 Thanked 37 Times in 14 Posts Reputation: 47

WOW..shoot me now!
 The above post was thanked by: Claus_CU (06-01-2011)
#13
05-31-2011
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 88 Threads: 25 Thanked 19 Times in 11 Posts Reputation: 29

i dint understand the Q.NO 23 n 25 ANSWERS.......
#14
05-31-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,372 Times in 906 Posts Reputation: 3382
Question 23 (aka 3)

Quote:
 Originally Posted by sana akbar i dint understand the Q.NO 23 n 25 ANSWERS.......
Question 23 (aka 3)
probability that IV 1 is a carrier of oculocutaneous albinism?

Father aa
Mother Aa or AA
Probability Mother is a carrier
Incidence = 1/10000 = p^2 => p=1/100
HW eq = 2pq = 1/50
Probability Mother not carrier
1-1/50

Then
If mother is a carrier Aa + Father aa
aa aa Aa Aa = 1/2 * 1/50 "IV 1 is a carrier"
If mother is NOT a carrier AA + Father aa
Aa Aa Aa Aa = 1 * 49/50

Then, probability that IV 1 is a carrier of oculocutaneous albinism
= 1/100 + 49/50 = 99/100
 The above post was thanked by: Ali Khaliq (02-02-2016), AshNiffler (02-20-2015), Charsmd (02-15-2015), Claus_CU (06-01-2011), khushpatel (03-30-2014), nsesereso (01-10-2015)
#15
05-31-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,372 Times in 906 Posts Reputation: 3382
Question 24 (aka 4)

Question 24 (aka 4)
What is the probability that IV 2 will be affected with Becker muscular dystrophy? X-linked recessive

Father X_A Y
Mother X_A X_a (her Father has the disease)

Posible son if Mother X_A X_a:
X_A Y
X_a Y

Then, probability that IV 2 (male) will be affected with Becker muscular
1/2

#16
05-31-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,372 Times in 906 Posts Reputation: 3382

Question 25...I'm still trying to understand the qs...
#17
05-31-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,684 Threads: 213 Thanked 1,662 Times in 676 Posts Reputation: 1672

Wow! Got 3 out 5 .. Not bad!! This was a confidence booster for genetics lol .. but to expect these kinda questions i clearly need wayyy more practice and need to go over everything again!
Thanks for sharing.
#18
05-31-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,372 Times in 906 Posts Reputation: 3382
Question 25 (aka 5)

Question 25 (aka 5)

What is the probability that IV 2 will be affected with oculocutaneous albinism?
We know from qs. 22 (2) that the probability that III 3 is a carrier of oculocutaneous albinism is 2/3.
We also know that II 3 must be a carrier, then the probability that subject II 4 is a carrier is 1/2 (mother & father both carrier OR one of those is carrier and the other is normal)
Now we have to calculate the probability that the subject III 4 is also a carrier.

Using degree of inbreeding (the degree of risk that both alleles of a pair in a person are descended from the same recent common ancestor) the probability that the subject III 4 is also a carrier is 1/2 * 1/2.

If subject III 3 & III 4 are carriers, we'll have:
AA Aa Aa aa

Finally, putting everything together we have:

2/3 (mom) * 1/4 (dad) * 1/4 (aa) = 1/24
 The above post was thanked by: Claus_CU (06-01-2011), nsesereso (01-10-2015)
#19
06-01-2011
 USMLE Forums Scout Steps History: Not yet Posts: 15 Threads: 1 Thanked 4 Times in 4 Posts Reputation: 14

hi bebix ...
thanks for the reasoning ... good one
but i have hard time to understand q 25 line
" We also know that II 3 must be a carrier, then the probability that subject II 4 is a carrier is 1/2 (mother & father both carrier OR one of those is carrier and the other is normal)"
please explain in detail ... thanks
#20
06-01-2011
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,357 Threads: 194 Thanked 3,372 Times in 906 Posts Reputation: 3382

Quote:
 Originally Posted by amritdhillon257 hi bebix ... thanks for the reasoning ... good one but i have hard time to understand q 25 line " We also know that II 3 must be a carrier, then the probability that subject II 4 is a carrier is 1/2 (mother & father both carrier OR one of those is carrier and the other is normal)" please explain in detail ... thanks
Since subject III2 has this autosomal recessive disease (aa), then subjects II 2 (father) & II 3 (mom) must be carriers.

So II 3 must be Aa
Now, her brother (II4) has a 1/2 probability of being a carrier too:
- if mom & dad are are carrier (Aa * Aa = AA Aa Aa aa)
- if one is carrier and the other is not (AA * Aa = AA Aa AA Aa)
#21
06-01-2011
 USMLE Forums Scout Steps History: Not yet Posts: 15 Threads: 1 Thanked 4 Times in 4 Posts Reputation: 14

thanks bebix its working ....
 The above post was thanked by: bebix (06-01-2011)
#22
06-04-2011
 USMLE Forums Scout Steps History: CS Only Posts: 25 Threads: 0 Thanked 21 Times in 16 Posts Reputation: 31

OMG!!! is this for real?
#23
02-20-2015
 USMLE Forums Scout Steps History: 1+CK+CS Posts: 30 Threads: 6 Thanked 3 Times in 3 Posts Reputation: 13
Thanks a lot ...

Quote:
 Originally Posted by Claus_CU Subject I 1 and II 2 have Becker muscular dystrophy (X-linked recessive inheritance) Subject III 2 has oculocutaneous albinism (autosomal recessive inheritance, incidence in the general population of 1/10 000. The disease status for both Becker muscular dystrophy and oculocutaneous albinism is unknown for IV 1 and IV 2. Attachment 1401 click image to enlarge 1) What is the probability that III 3 is a carrier of Becker muscular dystrophy? a) 1/4 b) 2/3 c) 1/2 d) 1 2) What is the probability that III 3 is a carrier of oculocutaneous albinism? a) 1/4 b) 2/3 c) 1/2 d) 1 3) What is the probability that IV 1 is a carrier of oculocutaneous albinism? a) 1/4 b) 1/2 c) 2/3 d) 99/100 4) What is the probability that IV 2 will be affected with Becker muscular dystrophy? a) 1/4 b) 1/2 c) 0 d) 1 5) What is the probability that IV 2 will be affected with oculocutaneous albinism? a) 1/100 b) 1/50 c) 1/24 d) 1/12 Have fun

My genetics professor had fun giving us this to play with a week before midterms...

 Tags Genetics-, Step-1-Questions

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