A question regarding one of the questions in Exam 3 of introduction course.
I need your input please read!
Question:
A couple has a child affected with neurofibromatosis type 1 (AD inheritance). The risk that their next child will be affected is?
The correct answer is designated as 50%
I am thinking it can also be 75%
Reasoning:
It is 50% if only one parent is heterozygous. Aa * aa
It is 75% if both parents are heterozygous. Aa * Aa
now from the "basic genetic handout" page 4 we learn that
1. Affected persons are "usually" heterozygotes.
2. Affecteds are "usually" derived from the mating of a normal parent with an affected
heterozygote.
I believe the fact is that the question is missing the necessary information needed to calculate the precise answer. "The parents genotype".
who agrees with me on this and who doesn't with reasoning.
Dr. Dixon strongly believes that the original 50% answer is the only correct answer b/c:
In the handout, p. 4-- 50% risk for any child of an affected parent is stated clearly as a characteristic of AD inheritance. The only thing tested here was that in AD inheritance-- if a child has it, 1 and only 1 parent is affected-- and then we have Aa x AA and that gives a 50% risk for future children.
Also the standard AD pedigree shows that affected children have 1 (just 1) affected parent. You were not to consider the very rare matings that "might" be theoretically possible.
I still disagree, the question did not give us the parents genotype and we had to consider all the possibilities.
Beside the handout says "USUALLY" so that leave room for other things and the question didn't exclude any assumptions.
Did any one complain about this question?
I will post this for other medical schools too to see what percentage agree and disagree.
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