Genetics AAMC question

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deleted783484

Hi! Would appreciate any help

I picked A but the answer is B and it doesn't make sense to me..

Why can't both parents be heterozygous for the disease and so it's a recessive disease and if the child gets both recessive copies then he's affected?

Their explanation is:

"Both the father and mother have cholesterol levels of 3.0 mg/mL, which is in the
moderately affected range. This suggests they have one copy of the HC gene and one
copy of the normal (wild-type) gene and that the two versions of the gene (alleles) are codominant.
When a gene is recessive, its effect (or lack of effect) is masked by a
corresponding dominant gene. The child has a cholesterol level of 7.0 mg/mL. This is in
the severely affected range according to the passage. This child appears to have acquired
one HC gene from each parent for a total complement of two HC genes and no normal
genes. This is consistent with the idea that the normal and HC genes are co-dominant.
Therefore, the best answer is answer choice B."

@aldol16

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Why can't both parents be heterozygous for the disease and so it's a recessive disease and if the child gets both recessive copies then he's affected?

Both parents are heterozygous for the disease. The problem is that if it's a recessive disease, then heterozygous people are not affected at all. Apparently, according to the passage, the parents are affected but not as severely as their child. So that would suggest that both alleles play a role, or are co-dominant. In other words, if you have two normal alleles, you're fine; if you have one normal and one bad allele, you're affected but not that much; if you have two bad alleles, you're seriously affected.
 
Both parents are heterozygous for the disease. The problem is that if it's a recessive disease, then heterozygous people are not affected at all. Apparently, according to the passage, the parents are affected but not as severely as their child. So that would suggest that both alleles play a role, or are co-dominant. In other words, if you have two normal alleles, you're fine; if you have one normal and one bad allele, you're affected but not that much; if you have two bad alleles, you're seriously affected.

I read the passage quickly so I thought that 3.0 was the normal level.. Thanks a lot though !!
 
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