I am working on an A levels questions:

Which of the following statements about gene mutation is incorrect?

A. It can occur in both somatic and sex cells

B. It can cause Down's syndrome in humans

C. It can change a dominant allele into a recessive one.

D. It can be brought about by exposure to ionising radiation

The answer at the back of the book says the correct answer is C. Its explanation is

A gene mutation cannot cause a dominant allele to become recessive, but can disallow it from being expressed phenotypically.

But I think that it should be B. From what i understand, Down's syndrome is caused by an extra chromosome (21). And it is not possible to grow an extra chromosome just by gene mutations?

Is the answer from the back of the book wrong, or did i make a mistake? Thank you for helping me with my A levels.

  • $\begingroup$ Mutation is a very general term that defines „change of genetic material“ (not just sequence) and can include the loss or acquisition of complete chromosomes. So down syndrome can be caused by mutation. However, I don‘t know why mutations shouldn‘t be able to convert dominant alleles into recessive ones (E.g, a mutation that halfens an enzymes activity?). In general, this page is not meant to help you with your A-level without efforts from your side. If you have googled the issue, please show us which sources you already checked. $\endgroup$
    – markur
    Sep 24, 2022 at 12:52
  • $\begingroup$ "And it is not possible to grow an extra chromosome just by gene mutations?" Did you google this question? What did you find? $\endgroup$ Sep 24, 2022 at 16:11
  • $\begingroup$ C is not impossible. A mutation in Gene A can cause alleles x and y of Gene B to swap dominance. E.g. if gene A encodes a enzyme that changes the substrate of gene B so that it is first recognized by allele x instead of y. The word "can" quite surely includes theoretical possibilities. $\endgroup$
    – KaPy3141
    Sep 27, 2022 at 15:25

2 Answers 2


Don't feel bad, that question is horribly worded.

Gene mutation is a confusing choice of words, it is not a specific thing.

First, down syndrome IS caused by a mutation, specifically a duplication. Trisomy itself is a mutation, but not caused by a mutation to a single gene, which could be what "gene mutation" is referring to. If so, it is just bad wording and the book answer is wrong. But trisomy itself can be caused by a mutation to a single gene so even then it is iffy.

The question is a "gotcha question" at best or just straight-up wrong at worst. "Which of the following statements about gene mutation is incorrect?" There is no right answer here: a mutation can cause all of these things, since we don't know what exactly is meant with "gene mutation".

At first is seems like all of the answers could be caused by mutations, but there is twisted logic to C being correct.

A mutation can change alleles, thus, it can change a dominate allele into a recessive and vice versa. What it can't do is change a dominate allele into a recessive one... wait that sounds insane, but it is technically true. It is a confusion of language; lets go a little deeper.

A mutation can turn one allele into another different allele...but, if you referring to the allele as in the defined allele and not the gene then a mutation cannot change it. A mutation can never make the blue eye allele as it is defined, recessive, it can however change the gene from blue eyes to brown eyes, thus making gene go from recessive to dominant and thus changing from a recessive allele to a dominate allele. Think of it this way, I can make steel change color by heating it but I can never change the color blue itself. So C could technically be incorrect but from a slightly different angle it is complexly possible, thus the questions is just bad.

Why textbooks often have sucky questions. You run into this problem with textbooks a lot for two reason. The people who write/edit textbooks rarely understand the subject and even when they kinda do, they often reuse and shuffle questions often changing key terms without understanding how it changes the question. this is done to create a new "edition" which they can sell again when they drop online support for the old edition.

  • $\begingroup$ I think you've identified the most reasonable unreasonable way to consider option C the correct answer here, though it makes their "explanation" even worse than the question itself. $\endgroup$
    – Bryan Krause
    Sep 27, 2022 at 21:15

Honestly, I'm not sure what exactly is meant by "changing a dominant allele to a recessive one". But if you have a mutation which breaks a gene, that mutation certainly could be recessive as compared to the original, functional allele.

Down syndrome is clearly not a mere "gene mutation", and I think high schoolers are meant to know that. I think you book is just wrong; maybe there was a typo claiming the answer was C, and someone made up a justification for that that doesn't make a lot of sense.

  • 1
    $\begingroup$ Duplication is considered a type of mutation. It's a bit weird to describe a whole extra chromosome that way, but it's likely technically correct according to the textbook definitions given in a course. I agree that C doesn't make much sense, though. $\endgroup$
    – Bryan Krause
    Sep 26, 2022 at 18:25
  • $\begingroup$ I would think that part, if not the whole point of the question is "do you know know the general kind of genetic differences that causes Down Syndrome? " $\endgroup$
    – swbarnes2
    Sep 26, 2022 at 20:19
  • $\begingroup$ Perhaps, but if that was the goal they could have just asked about Down Syndrome. Instead, they may be trying to communicate the scope of changes that are included when someone says "mutation" and that "mutation" when used in evolutionary terms should not be confused with "point mutation"/substitution. E.g. the Wikipedia page on Mutation includes a section "large-scale mutations" that covers chromosome-level changes. $\endgroup$
    – Bryan Krause
    Sep 26, 2022 at 20:39
  • $\begingroup$ I think that's an area that is especially confused by anti-evolution arguments that fail to consider the importance of duplications of entire genes and larger and instead make silly arguments about the improbability of some combination of point mutations, and I'd think it's an apt target for exams assessing science literacy. $\endgroup$
    – Bryan Krause
    Sep 26, 2022 at 20:40
  • 1
    $\begingroup$ down syndrome is caused by a mutation, trisomy is by definition, a mutation. it is not is a point mutation but it is a mutation. $\endgroup$
    – John
    Sep 26, 2022 at 21:04

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