Junk DNA

Dr. James Mon é jmone at marauder.millersv.edu
Mon Apr 17 12:30:30 EST 2000


Ian,
You wrote:
You didn't read what I said. You double the total amount of DNA and you 
double the total number of errors. But if the coding amount is still 1X, 
that region still gets 1X the number of errors. 
To use your analogy, draw your 1 cm circle on your windshield; that's the 
coding region. Will there be fewer drops of rain hitting it if you make 
the windshield ten times larger? 

That's not the question.
If you have a small circle inside a larger one, fewer random drops will hit
the small circle than the larger one, which in my mind is analogous to the
junk DNA vs. non-junk DNA scenario.
BTW, the error rate for DNA pol is approx 10(-8) per base.  Therefore, the
smaller the region, the fewer mutations.

You wrote:

Only if the selective advantage was high enough to counteract random 
increases. By your argument, there would never be any inherited 
disorders, because the selective disadvantage of having hemophilia would 
instantly get rid of it. There's a balance between random disadvantage 
and selective advantage. 
Mutations, even lethal ones, can remain in the population if the mutation
doesn't kill you before you reach reproductive age.  This is the case with
diseases such as Huntington's chorea, which is autosomal dominant disease,
but doesn't manifest itself until the fourth decade of life, well after you
have passed the mutation on.
Sickle cell anemia, a recessive disorder, only manifests in homozygotes,
whereas heterozygotes are not diseased, and actually have a selective
advantage in certain environments.

You wrote:

That's no to say that any of those arguments applies in this case, but you 
always have to consider the null hypothesis. You can't simply declare 
that because something is there it must have a selective advantage, or 
that becase it hasn't been removed there's no advantage to doing so. 

I agree, but in natural situations (devoid of the technology to allow
carriers of genetic defects to survive to reproductive age), deleterious
mutations, or mutations which don't confer selective advantage, are usually
lost over time.  An example can be seen in melanin production.  High levels
of melanin production are retained in populations in the equatorial regions
of the earth, but were lost in populations living nearer the poles.

Jay Mone'
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