Genetics questions

[Dreamer29]

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The question from the Kaplan Q-bank:

How would a spontaneous mutation yield disease in an NF patient?
a. A mutation in neurofibromin during mitosis of the patient
b. A mutation in neurofibromin during meiosis of the patient
c. A mutation in neurofibromin during mitosis of the pt's parents
d. A mtutation in neurofibromin during meiosis of the pt's parents

Some info from the passage:
NF is caused my a mutation in the neurofibromin gene. It's autosomal dominant. About half the patients inherit the disease from an affected parent, half the pts have a new mutation with no family history.

The correct answer is D. I was able to rule out B and C immediately, and was debating between A and D. I understand the reasoning behind D as well. But I'm having difficulty ruling out A.

Wouldn't the case presented in A still manifest in disease, but it would not be passed down to the pt's children?

 

[niblet]

Wouldn't the case presented in A still manifest in disease, but it would not be passed down to the pt's children?

It could, but only if it happened very early in development, so as to have every cell that produces neurofibromin be descended from the early cell that originated the mutation in the gene. In all likelihood, a mutation in mitosis happened at a much later stage, meaning neurofibromin could still be produced by the unaffected cells, which would be the majority. So, while mitosis could theoretically cause the disease, I think this is just one of those cases of going with the BEST answer, which would be a mutation during meiosis in the parents.

 

[Testosterone]

A diseased organism is different from a diseased cell. A diseased cell is a local issue that can be monitored by the immune system or undergo some apoptotic/autophagosomal pathway, but a diseased organism from a genetic defect is a much more universal problem that is difficult to deal with. It's hard for a disease to manifest itself from a mutation in a somatic cell unless it is a mutation in an oncogene and falls out of normal cell cycle controls.

Generally, a disease can be either inherited if the allele for it is already present in the parents, or introduced through a random mutation in the germ line if the allele for it didn't exist in the parents/family line/population/gene pool already.