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Thread: Genetics questions

  1. #1 Genetics questions 
    Moderator Moderator AlexP's Avatar
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    This is not homework. I'm trying to answer questions from an old test for a genetics test I have tomorrow and I'm stuck on a few. Thought I'd see what answers I could get out of you smart people by the end of the night.

    "Prader-Willi syndrome (PWS) is paternally inherited, therefore the gene involved is maternally imprinted. A normal man, Joe, had normal parents and had two normal brothers (normal means no signs of PWS). Each of these brothers had two children, one normal child and one with PWS. This man (Joe) married a woman with no PWS in her family. What is the chance their children would have PWS?"

    I'm going to read over the section in the book on parental imprinting again, but an answer to this would be nice. And I don't understand why maternal imprinting seems to be a consequence of being paternally inherited.

    From the previous question: "From whom did Joe's two brothers inherit the mutation (allele) that can cause PWS? (mother or father)


    "A woman who owned a white purebred poodle (an autosomal recessive phenotype) wanted white puppies, so she took the dog to a breeder, who said he would mate the female with a white stud male, also from a pure stock. When six puppies were born they were all black..."

    ok, so the black is caused by complimentary gene interaction. But I'm not sure about the following question.

    What would be the genotypes of the black puppies? (use whatever appropriate symbols for the genes that would show that you know what is happening in this cross)


    And last...

    "Albinism is a total lack of skin pigment due to a recessive gene. What is the probability of a couple having an albino child if:

    Both are normally pigmented as are their parents, but both have albino siblings."

    Any help would be most appreciated.

    EDIT: One more... I'm given a DNA sequence and asked what the third amino acid is in the open reading frame. The third codon including the start codon is GGT. Since the mention the open reading frame is it safe to say this is the coding strand, and thus when I look up the amino acid on the genetic code chart I don't need to convert to RNA, except for Ts to Us?


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  3. #2 Re: Genetics questions 
    Forum Cosmic Wizard spuriousmonkey's Avatar
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    Quote Originally Posted by Chemboy
    This is not homework. I'm trying to answer questions from an old test for a genetics test I have tomorrow and I'm stuck on a few. Thought I'd see what answers I could get out of you smart people by the end of the night.

    "Prader-Willi syndrome (PWS) is paternally inherited, therefore the gene involved is maternally imprinted. A normal man, Joe, had normal parents and had two normal brothers (normal means no signs of PWS). Each of these brothers had two children, one normal child and one with PWS. This man (Joe) married a woman with no PWS in her family. What is the chance their children would have PWS?"

    I'm going to read over the section in the book on parental imprinting again, but an answer to this would be nice. And I don't understand why maternal imprinting seems to be a consequence of being paternally inherited.

    From the previous question: "From whom did Joe's two brothers inherit the mutation (allele) that can cause PWS? (mother or father)
    The genetic section on the wikipedia entry isn't that bad.

    http://en.wikipedia.org/wiki/Prader-...drome#Genetics

    According to this information the mutation is always paternal in PWS since maternal imprinting silences the maternal allele. The maternal allele can therefore not resque the mutated paternal allele. The error is therefore in the paternal allele.

    So I would say father.


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  4. #3  
    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    Do you have the answer key, I think the answer to the albinism one is 1/16.

    If they both have albino siblings then their parents have to be Aa Aa, and for them to have an albino child they would also have to be Aa Aa, so the chance of an Aa child being born is 1/2, so 1/2 * 1/2 for the probability that the parents would both be Aa, then multiplied by 1/4 for the probability of them having an aa child.

    Aa X Aa Aa X Aa

    1/2 Aa X 1/2 Aa

    1/4 aa
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  5. #4  
    Moderator Moderator AlexP's Avatar
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    Well, the test is over now and it went fairly well. spurious, the answer to that question is actually mother.

    The albinism (same question possibly) question came up on the test... Tell me if my reasoning seems right here. I used for the probability that both parents would be heterozygous because we know they're not albino, and thus aa isn't even an option. They have a dominant allele so it's the chance that they're heterozygous given that they could be AA, Aa, or Aa.
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  6. #5  
    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    Quote Originally Posted by Chemboy
    Well, the test is over now and it went fairly well. spurious, the answer to that question is actually mother.

    The albinism (same question possibly) question came up on the test... Tell me if my reasoning seems right here. I used for the probability that both parents would be heterozygous because we know they're not albino, and thus aa isn't even an option. They have a dominant allele so it's the chance that they're heterozygous given that they could be AA, Aa, or Aa.
    Your reasoning seems right to me, I didn't think about the impossibility of them being aa.

    Edit: Although, I wonder if the probability of each individual parent being Aa is 2/3 rather than the probability of both of them being Aa.

    Edit: It's been way too long since Introductory Stats and Basic Genetics.
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  7. #6  
    Moderator Moderator AlexP's Avatar
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    Well, the mother would have to be Aa, the father would have to be Aa, and the child would have to be aa. So I did .
    "There is a kind of lazy pleasure in useless and out-of-the-way erudition." -Jorge Luis Borges
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  8. #7  
    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    Quote Originally Posted by Chemboy
    Well, the mother would have to be Aa, the father would have to be Aa, and the child would have to be aa. So I did .
    That's what I meant, that would be the right answer I think.

    If not the only other answer I can think of is my first one.
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  9. #8  
    Forum Cosmic Wizard spuriousmonkey's Avatar
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    Quote Originally Posted by Chemboy
    Well, the test is over now and it went fairly well. spurious, the answer to that question is actually mother.
    that shows I have no right to be a scientist.
    "Kill them all and let God sort them out."

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