








USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam 

Thread Tools  Search this Thread  Display Modes 
#1




Genetic probability Question
Prevalence of homozygotes in a population is 1%. If a woman is a known carrier marries somebody from that population then what is the probability that they'll produce an affected homozygous child?
A 0.09 B 0.07 C 0.05 D 0.025 E 0.25 F 0.50 G 0.10 
The above post was thanked by:  
asmz34 (10092013), danylo (10142013), drzamzam (07162015), radwan5711 (01302011), Taiwan_Guy (05082011), Usmle16Forall (4 Weeks Ago) 
#2




Anybody knows the answer to that

#3




i think the answer is E

#4




Please tell me how did u arrive to this conclusion!

#5




The woman has three possibilities:
married to affected homozygous 1% married to affected heterozygous (should be calculated by Hardy weinburg) married to homozygous normal person (the remaining possibility) These probabilities should be combined If some one can do the math for us to know the answer 
#6




I know this has been up for a while and has not been answered. I would like to know what you guys think...
I think the answer is C 
#7




C
its C 0.05
disease prevalence 1% = 1/100 = q2 allele frequency = q = 1/10 when disease frequency is less then 1/100 p(normal allele frequency) is considered 1 so....carrier frequency will be 2pq = 2q = 1/5 for AR disease , father must be carrier to transmit disease to child... probability that women will marry carrier man is 1/5 probability that women will transmit her diseased allele to child is 1/2 probability that man will transmit his diseased allele to child is 1/2 now just multiply 1/5*1/2*1/2 = 1/20 = 0.05 
The above post was thanked by:  
abeer (10082013), Androide89 (08292010), asmz34 (10092013), danylo (10142013), dr_kg (10122013), drzamzam (07162015), inder (06072010), nsesereso (10122013), Usmle16Forall (4 Weeks Ago), zen228 (04162014) 
#8




Oh okay sweet.
I did it a bit differently as I included the likelyhood of marrying a homozygous ressessive and having a child (ended up being .005) I also didn't assume p as being 1 (rather .9) so i got .045 instead of .05 The grand total was .05. I guess either way works. 
#9




Quote:
I have a doubt..is the probability of man transmitting the dz is 1/4 coz the question says the woman will marry a guy from homozygous population?? Can anybody explain this question again? 
#10




my solution
may i suggest another solution
since 1% of that population is homozygous normal or diseased if the woman marries a homozygous normal from that 1% she wont get a diseased homozygous baby if she marries a homozygous diseased person from that same 1% she will get a homozygous diseased baby but for our calculation purpose we are going to neglect that homozygous diseased population since it will be negligible as it is an AR disease and it will be a small fraction of that 1% so the probabability of that woman marrying a heterozygous carrier is 99% or 0.9 the probability of her having a homozygous diseased baby if she marries a heterozygous carrier = 1/4 or 0.25 the probability of husband having a homozygous diseased baby if he marries a heterozygous carrier = 1/4 or 0.25 multiply all the probabilities 0.9 x 0.25 x 0.25 = 0.05 
The above post was thanked by:  
danylo (10142013), drHubaishi (11192013) 
#11




answer is c

#12




I think answer is 0.25or e. since woman gonna merrie man from that 1% population of homozygous , it's 1/2 chance that that man is affected homozygous and woman transfers her affected allele is 1/2, so calculating 1/2 times 1/2=0.25

#13




The question is asking you this 
Probability of a homozygous child if someone "from the population" marries a known carrier female. Let's call her Miss X. Now by population, it means 2 possibilities  Situation 1  Another carrier from the population marries Miss X. The probability of this event happening is  2pq = 2x0.9x0.1 = 0.18 (Note: the square of q = 0.01, hence 1=0.1 and p=10.1 = 0.9. We can't ignore p in the equation because the prevalence is high.) Now if this mating occurs, probability of having a homozygous child = 0.25 (draw a punnet square if you please.) Hence combining the probabilities, final probability from situation 1 = 0.18x0.25 =0.45. Situation 2 A homozygous diseased male marries Miss X. Note that the question doesn't suggest that the female will only marry a phenotypically normal guy. The probability of this event happening is 0.01 (frequency of homozygotes is given in the question as 1%). If this mating occurs, probability of having a homozygous child = 0.5. Combined probability from situation 2 = 0.05. Now to calculate the final answer, we must understand that EITHER of the 2 situations would give us our homozygous child. In these scenarios, the probabilities are to be added (not multiplied.) So combining situation 1 and 2, final probability = 0.45 + 0.05 = 0.5 I know this is confusing and we probably won't be tested such complicated applications but once you read it slowly and refer to your genetics book, you would get it. I think a similar questions can be found in the Kaplan qbank. Hope that helped. 
The above post was thanked by:  
Tags 
Genetics, Step1Questions 
Thread Tools  Search this Thread 
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Child Development and Genetic Disorders  geof1  USMLE Step 2 CK Forum  4  04032016 10:03 PM 
Probability Calculation  anoop_1198  USMLE Step 1 Forum  4  06072010 04:19 AM 
Genetic Probabilities?  doctorF  USMLE Step 1 Forum  12  04052010 09:47 AM 
How can i memorize Genetic disorder?  Eric Zhuang  USMLE Step 2 CK Forum  3  03012010 08:56 AM 
Genetic Coding Problem  ngnorah  USMLE Step 1 Forum  1  01212010 08:12 PM 
