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Autosomal Dominance

Discussion in 'MCAT Study Question Q&A' started by plzNOCarribbean, Jul 25, 2011.

  1. plzNOCarribbean

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    Ok, I get that when a gene has a dominant mode of inheritance that you only need 1 copy to express that trait, and the fact that it is autosomal means that it is not sex linked. However, I am confused about how we figure the probability of passing these traits to offspring.

    Say, for instance, that a male is carrying the gene for polycystic kidney disease, which is an autosomal dominant trait. If he has a son, the son has a 50% chance of inheriting the disease? They say its 50%, but this is assuming that the father has only 1 dominant allele, well call it P, for this trait, as opposed to both allele.

    So, if the father were Pp, and the mother carried the normal allele, the offspring would have a 50% chance of having the disease?

    and if the father were PP, then ALL of the offspring would have this disease?

    So basically, for autosomal dominant traits, the chance that offspring carry that trait depends on whether the parent carrying the alleles has 2 copies of the dominant allele or 1 copy of the dominant allele? is this correct?
     
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  3. kkentm

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    google "Punnet squares" and learn the method of how genotypes are predicted. both kaplan and EK books include this method so i assume it must be relatively high yield on the mcat
     
  4. plzNOCarribbean

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    I understand how to predict genotypes; My specific question was regarding SPECIFICALLY predicting the chances of inheriting an AUTOSOMAL DOMINANT trait.

    i know it BB x bb....all offspring are Bb
    if Bb x bb, 50% chance of having an offspring thats bb and 50% thats Bb, with no homozygous dominants. All individuals expressing the B phenotype will be heterozygotes
     
  5. paul411

    paul411 ANES
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    It depends on the genotype of BOTH parents.

    Punnet squares give you the correct answer every time.
     
  6. plzNOCarribbean

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    if both are heterozygous, does the kid have a 50% chance of inheriting the trait?

    since 1/2 +1/2 = 2/4 = 1/2
     
  7. PingPongPro

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    Use the punnet square. It seems like you don't really understand how inheritance works.

    If both parents are heterozygous, the kid has 75% chance of inheriting the trait.
    1/4 of the offspring will be Homozygous dominant
    2/4 of offspring heterozygous
    1/4 homozygous recessive.

    3/4 = 75% will have chance of inheriting the trait.
     
  8. muhali3

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    the above answer is correct.

    I think you might be confusing phenotype and genotype.

    If the dad was homozygous dominant, and the mom was homozygous recessive, then 100% of the kids would have the disease (phenotype).

    However, 50% would be PP and 50% Pp. Different genotypes, same phenotype.
     
  9. plzNOCarribbean

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    Okay, thats what I was wondering and assumed since it is a DOMINANT mode of inheritance and you only need ONE allele to get it. Yea, dont know why I said 50%. Phenotype = physical appearance of the trait; genotype = the genetic make (ie whats on the chromosomes in the nucleotide sequence)
     

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