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free120/Block 2/Question#31 (reveal difficulty score)
A 10-year-old girl is brought to the office ...
It is a polymorphism 🔍 / 📺 / 🌳 / 📖

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 +23  upvote downvote
submitted by sugaplum(487)

Incase anyone else was thinking the same:
I was stuck between this and variant of unknown significance. However, variant of unknown significance is a sequence not a single nucleotide

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nbme4unme  Thank you for explaining, I selected the unknown significance answer as well! +5
dhkahat  lol fuck this test +19
chaosawaits  Because knowing this will definitely make me a better doctor /s +1

 +4  upvote downvote
submitted by bwdc(697)

A genetic variation in a particular nucleotide is by definition a polymorphism. Note that the question specifically states that the protein remains unchanged.

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 +3  upvote downvote
submitted by melchior(79)

A single-nucleotide polymorphism is a single nucleotide substitution that is present in at least 1% of the population (in this question, it is 7.5%). This fits the definition.

I'm not sure how to rule out variant of unknown significance. I guess since we known that the protein is unchanged, we can say that it is not significant (and therefore the significance is not unknown).

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 +2  upvote downvote
submitted by drdoom(1203)

Mom and her daughter have the same sequence of FBN1. What is special is that compared to a “normal” (wild-type) FBN1, mom and daughter have one little tiny mutation in their sequence: a single nucleotide (A,T,G,C) that is different than most other folks.

(Interestingly, this nucleotide difference does not alter the amino acid encoded by this part of the gene; that makes Choice D (change the folding of the protein) and Choice E (truncated protein) very unlikely.)

Back to our question. Even though mom and daughter have the same mutation, mom shows no signs of Marfan syndrome; this eliminates Choice A (disease-causing mutation in the patient and her mother).

You might think this is an example of variable expressivity (not an option but a good guess), and it might be.

In the stem they say:

The same nucleotide change is found in 15 of 200 individuals without Marfan syndrome.

Interesting! This means that in a “normal” sample of people (who have no signs of Marfan), 15 out of 200 still have this mutation.

(They don't say anything about how many Marfan people have the mutation but we don't need that information to answer the question.)

When a gene can be found in multiple “versions”, we call the gene polymorphic (literally, many-shapes or many-forms).

So the best response is Choice B (polymorphism).

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 +1  upvote downvote
submitted by meryen13(48)

genetic polymorphism means that there are different alleles (alternative form of a gene that arises by mutation) of the same gene in the population. that is exactly what this question was explaining. now sequence variant means: A “sequence variant” is a surrogate term covering any unintentional amino acid substitutions, omissions, or insertions during protein biosynthesis.

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