TPR bio question 1 chapter 6

[echoyjeff222]

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question 1 in the freestanding asks about "a woman is phenotypically normal but had a brother who had an autosomal recessive disorder that resulted in death during infancy. what is the probability that this woman is a carrier for thee disorder that affected her brother"

the answer key claims it's 2/3 and simply looked at the 25% from the hetereo and 50 from the homozygous dominant and totally ignored the 25% from the homozygous recessive ... I thought it was going to be 1/2 -- is it convention to only regard the parts that matter ... or only for this question, since you're technically not going to be alive for a long time if you have the homozygous recessive??

 

[JMMTB]

If her brother was homozygous recessive that then means that both of her parents are carriers. If we do a heterozygous cross we would see genotypes AA, Aa, Aa, aa but because we know that she is phenotypically normal we can ignore the homozygous recessive genotype. Thus 2/3.

This is another look at it - lethal alleles.