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I am trying to understand how mutations cause genetic variance, and I'm stuck on one issue that I'm going to try my best to explain. (I am specifically talking about mutations that cause a phenotypical change). The question is focused on organisms with a distinct germ-line.

My issue is with trying to understand where mutations need to occur to cause variation. For example, does the mutation need to occur in the DNA of either the specific sperm or the specific egg that gave rise to you?

The body of an organism obviously has a vast, vast number of cells. If a mutation occurred in just one of these cells, then surely it wouldn't have any phenotypical effect. Even as an infant or an embryo, if a mutation occurs in just one of your many cells, that is unlikely to cause any phenotypical changes (phenotypical variance) as your body develops. So, is the only way for a mutation to have a phenotypical effect is for it to occur in the gametes that produced you? That way, all of your DNA would inherit the mutation, since all of your DNA would be replicated from the DNA of the sperm and egg.

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    $\begingroup$ Welcome to Biology.SE. This is not simply an answer site, but instead a site that promotes self-learning with some expert help. Consequently, questions that show little or no prior research effort are off-topic on this site as are "homework" questions unless you have shown your attempt at an answer. Please edit your question and tell us where you've looked for answers, what you do know about the topic, and where exactly you still have questions. Please take the tour and consult the help center starting with How to Ask for details. $\endgroup$
    – tyersome
    Jul 20, 2022 at 4:40
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    $\begingroup$ In addition, I encourage you to think about cancer — are you sure you want to claim that "if a mutation occurs in just one of your many cells, that is unlikely to cause any phenotypical changes (phenotypical variance) as your body develops"? $\endgroup$
    – tyersome
    Jul 20, 2022 at 4:43
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    $\begingroup$ I actually think this a reasonable question because, although the answer may appear obvious (and the poster argues for it on the basis of logic) my own impression is that it seems to be rarely emphasised in texts. So I can understand the poster wondering why. $\endgroup$
    – David
    Jul 20, 2022 at 17:20
  • $\begingroup$ The phrase you should look for to understand this is "germline mutation". Also look up "germ cells", which are handled with kid gloves, so to speak, to avoid mutation as much as possible. $\endgroup$
    – Mike Serfas
    Jul 23, 2022 at 21:17
  • $\begingroup$ @David — I still feel like the question is under-researched (and that the logic in what is now the final paragraph is dubious). However, in light of Bryan's answer and your feeling that it is of value, I have withdrawn my close vote and made edits. If you have a chance I'd appreciate your checking to make sure I've actually improved the question ... $\endgroup$
    – tyersome
    Jul 30, 2022 at 22:14

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For example, does the mutation need to occur in the DNA of either the specific sperm or the specific egg that gave rise to you?

To give rise to heritable genetic variation in the sense that it's a mutation that exists in all of your cells and is transmitted (with some probability) to your offspring, yes, it must come from the gametes, with the caveat that it need not occur specifically in the gamete, but in any of the cells giving rise to that gamete in either parent. I suppose it could also occur in the fertilized egg cell, and mutations shortly thereafter may produce a chimera where a substantial fraction, but not all cells carry that mutation, but as mutations are most common during cell division I think it's okay to simplify a bit and presume the source is the gametes for most heritable mutations.

Estimates of how many of these mutations exist per human fertilized egg (relative to their parent genomes) vary, but you can assume that each human has something on the order of dozens in coding regions that their parents lack, plus others in other areas of the genome. A whole human organism has far, far more mutations, and these cause issues with aging and cancer in their cells, but these are not typically heritable.

The number of mutations per replication depends on genome size and also the stringency of that organism's replication machinery. In general, complex eukaryotes have stronger systems for correcting replication errors, so you can expect bacteria to have higher rates of mutation. Some related reading:

Correlation between genome size and mutation rate?

How much variation in mutation rate in there in the human genome?

Bacterial division and mutation rate

Mutation rate in viruses

Veltman, J. A., & Brunner, H. G. (2012). De novo mutations in human genetic disease. Nature Reviews Genetics, 13(8), 565-575.

Each generation (per individual), approximately 74 de novo single-nucleotide variants (SNVs), three novel indels (small insertions or deletions) and 0.02 larger copy number variants (CNVs) arise in our genome

Ségurel, L., Wyman, M. J., & Przeworski, M. (2014). Determinants of mutation rate variation in the human germline. Annu Rev Genomics Hum Genet, 15(1), 47-70.

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