1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Biology: Genetics: Inherited disease

  1. Dec 4, 2011 #1
    QUESTION 1:
    1. The problem statement, all variables and given/known data

    9. Your first cousin died of Tay-Sachs disease (autosomal recessive, incidence 0.09% (about 1 in 1100)).
    You are not affected, nor is anyone else in your extended family. What is the approximate chance that
    you are a carrier?
    A. 5.8%
    B. 12.5%
    C. 25%
    D. 50%
    E. 67%

    THE BOOK ASNWER IS C: 25%

    2. Relevant equations

    I don't really understand why the answer is 25%. So, I try to make a logical explanation for that. I'm not sure if my reasoning is correct; thus, please help me see if my reasoning is correct or not. If it's wrong, please tell me where I went wrong. Thank you :)

    3. The attempt at a solution

    What I did whas that I I drew out my pedegre. Starting from my First cousin family, I know for sure that the Uncle is is a carrier (Aa) and he must have married a wife who is also a carrier (Aa) and that is how they got an affected son. Thus, this leads to the fact that either both of my grandparents are carriers (Aa x Aa) or only one of them is a carrier (Aa x AA). As a result, the chances that my mom (who is related to the uncle who has a defected child) is a carrier is either (2/3 or 1/2) depending if my grandparents genotype, respectively.

    Thus, if my mom chances of being a carrier is 1/2 then my chance of being a carrier is 1/4. (given that my dad is not a carrier b/c CF is a rare disease)

    However, if my mom chance of being a carrier is 2/3 then my chance of being a carrier is (2/3)x(1/2)=(1/3) As a result, I could either be 1/4 or 1/3. But since the answer only provide the choice of 1/4 and not 1/3. This leave C to be the correct answer.

    This is my interpretation of it; however, I'm not sure if this logic reasoning is correct.

    Picture of my "assumed" pedegry
    2072rsw.jpg


    -----
    QUESTION 2:
    1. The problem statement, all variables and given/known data

    28. The actual carrier frequency of cystic fibrosis in the Caucasian population is about 50% lower than
    what the Hardy-Weinberg formula would predict based on the number of affected individuals. A
    reasonable explanation for this might be (choose none, one, or more than one):
    (A) inbreeding (B) heterozygote superiority (C) carriers may have a mild disease phenotype that is
    somewhat disadvantageous (D) genetic drift.

    ANSWER IS C


    2. Relevant equations

    I was wondering if you can check if my logic reasoning for this question is correct.

    3. The attempt at a solution

    Well, if you think about it, the carrier are lower in # if their frequency is lower in compare to the Hardy Weinberg prediction.
    Since the defected people are dead, leaving only the carrier behind. So, if the carrier is normal, then their number SHOULD resemble that of the Hardy Weinberg prediction, but since the carrier number is small, this indicate that the carrier is also somewhat hurt by having a recessive alleles of CF. (leading to their low in numbers)
     
    Last edited: Dec 4, 2011
  2. jcsd
  3. Dec 4, 2011 #2

    Ygggdrasil

    User Avatar
    Science Advisor

    As you said, Tay-Sachs is a rare disease, so the case where only one grandparent is a carrier is much more likely than the case where both grandparents are carriers.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Biology: Genetics: Inherited disease
Loading...