Body: <p>I want to look at violation of the strict molecular clock and compare dn/ds ratio to test differences in evolutionary rates in a set of genes. Some of these genes are really short (30AA), is it likely to prevent the detection of rate changes ?</p>
  
  <p>Thanks</p>
  
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-12-16T11:12:35.317Z
Comments.Text: If your genes are really short you may want to find a mathematical hack to get a confidence interval (several published), otherwise the jitteriness will be too much.
Comments.UserDisplayName: Matteo Ferla
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-12-16T13:22:29.880Z
Comments.Text: I can't find anything about size correction/confidence interval in this type of analysis. As dn/ds is a ratio, why would it be biaised with shorter genes? I feel this can be an issue but I can't find anything explianing why and how to deal with that.
Comments.UserDisplayName: LauraR
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-12-16T13:34:40.223Z
Comments.Text: Would concatenating several genes make the trick? For example, based on the evolutionary model that fit them the best (concatenate all genes under GTR+I model for example) or all genes related to the same function?
Comments.UserDisplayName: LauraR
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-12-16T13:55:59.797Z
Comments.Text: It is not a bias (no size correction needed), it is just that a ratio of two integers gets 'jittery' with low values (say 1/1 = 1, 1/2 = 0.5!). Concatenating different genes would assume the selection pressures are the same, so that would be not advisable. If I am completely misunderstanding and you were going to average (or similarly process) these anyway, averaging them will get rid of the jitters. In terms of errors the simplest and most rudimentary calculation is a [margin of error](https://en.wikipedia.org/wiki/Margin_of_error), but that is pointless if the values are to be combined.
Comments.UserDisplayName: Matteo Ferla
Comments.ContentLicense: CC BY-SA 4.0
Comments.CreationDate: 2019-12-16T14:37:53.227Z
Comments.Text: Ok thanks. I'm not willing to combined the values if it's avoidable.
Comments.UserDisplayName: LauraR
Tags: <gene><positive-selection>
Title: Gene size and power of detection in sequence divergence analyses
ThreadText: # Gene size and power of detection in sequence divergence analyses
  
  (Asked by: username_0 on 2019-12-16)
  
  I want to look at violation of the strict molecular clock and compare dn/ds ratio to test differences in evolutionary rates in a set of genes. Some of these genes are really short (30AA), is it likely to prevent the detection of rate changes ?
  
  Thanks
  
  **Tags:** gene, positive-selection
  
  > **Comments:**
  > **username_1**: If your genes are really short you may want to find a mathematical hack to get a confidence interval (several published), otherwise the jitteriness will be too much.  
  > **username_0**: I can't find anything about size correction/confidence interval in this type of analysis. As dn/ds is a ratio, why would it be biaised with shorter genes? I feel this can be an issue but I can't find anything explianing why and how to deal with that.  
  > **username_0**: Would concatenating several genes make the trick? For example, based on the evolutionary model that fit them the best (concatenate all genes under GTR+I model for example) or all genes related to the same function?  
  > **username_1**: It is not a bias (no size correction needed), it is just that a ratio of two integers gets 'jittery' with low values (say 1/1 = 1, 1/2 = 0.5!). Concatenating different genes would assume the selection pressures are the same, so that would be not advisable. If I am completely misunderstanding and you were going to average (or similarly process) these anyway, averaging them will get rid of the jitters. In terms of errors the simplest and most rudimentary calculation is a [margin of error](https://en.wikipedia.org/wiki/Margin_of_error), but that is pointless if the values are to be combined.  
  > **username_0**: Ok thanks. I'm not willing to combined the values if it's avoidable.  
  
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  ## 0 answers
  
  