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I came across a passage on enhancers that sounds out-of-whack to me:

...an enhancer sits on just one of DNA's two strands (usually the same strand as the protein-coding DNA gene itself). This is unlike a protein-coding DNA gene, which might need to be on both DNA strands——in a so-called homozygous state, in order to surface as a phenotype——like the classic case of blue eyes.

This is what makes no sense to me, since, AFAICT, whether something is in one or two strands has nothing to do with zygosity.

The authors continue:

And this is [an] evolutionary advantage: an organism doesn't have to wait for a change on both DNA strands. The bottom line is that evolutionary tinkering is in principle much easier with enhancers...

Is there a grain of truth in what the authors' write, despite their muddled explanation? IOW: does the fact that enhancers "sit on just one of DNA's two strands" somehow facilitate "evolutionary tinkering"?

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  • $\begingroup$ Firstly, the author is deviating from traditional jargon when he tries to represent different strands as paternally of maternally derived DNA. Finally does the author add some sort of example for his statement? More so, I would be very interested how you decide an enhancer sits on either the maternal or paternal DNA. $\endgroup$ – FoldedChromatin Oct 2 '16 at 11:46
  • $\begingroup$ The two authors are a computer scientist and a linguist (from the Amazon description), which explains everything - they are not biologists, and don't know what they're talking about. $\endgroup$ – MattDMo Oct 2 '16 at 15:40
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...an enhancer sits on just one of DNA's two strands (usually the same strand as the protein-coding DNA gene itself).

Yes and no. Enhancers are in fact sometimes palindromic, so they're mirrored on both strands of the DNA.

This is unlike a protein-coding DNA gene, which might need to be on both DNA strands

No. Protein-coding DNA is always on one strand only because transcription doesn't suddenly switch strands.

in a so-called homozygous state, in order to surface as a phenotype - like the classic case of blue eyes.

No. Homozygous means on both chromsomes and has nothing to do with strands or the need for an allele to be homozygous to produce its phenotype because it's recessive.

And this is [an] evolutionary advantage: an organism doesn't have to wait for a change on both DNA strands. The bottom line is that evolutionary tinkering is in principle much easier with enhancers...

This is completely all over the place and makes no sense to me. Either "strands" is used in the correct way, in which the previous statement is wrong, or "strand" is used as "chromosome", in which case there is no reason why an enhancer should only be on one chromosome. Just like the gene, it can be hetero- or homozygous. The statement about "having to wait for a change on both DNA strands" is more relevant to whether something is dominant or negative. Regardless, I don't think it's possible to make any statements here about enhancers allowing for evolution to progress faster.

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