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Re: Rconstructing DNA (was Re: Dino-fuzz found in amber?)



I think the underlying assumption is that parsimony dictates that chicken DNA has not changed since dinosaur DNA. The post below is only a long way to say that.

Dora

----- Original Message ----- From: "Roberto Takata" <rmtakata@gmail.com>
To: <dinosaur@usc.edu>
Sent: Saturday, September 17, 2011 2:19 AM
Subject: Re: Rconstructing DNA (was Re: Dino-fuzz found in amber?)


On Sat, Sep 17, 2011 at 2:38 AM, Tim Williams <tijawi@gmail.com> wrote:
It would take too long to explain why.

Well, it seems that it would take longer to explain you why my
assertion is a reasonable one..

Why do you assume that the choice of base in the chicken is ancestral?

Excuse me. Where did I have assumed such a thing? I'm not assuming
chicken sequence as ancestral. What I'm assuming is that chicken and
T-rex sequences are homologous, are derived from a common ancestral.

Lets use parsimony for simplification.

T-rex second codon must be: TTA, TTG, CTT, CTC, CTA or CTG for leucine.

Since we have CTT for chicken, by parsimony, we infer that the T-rex
and chicken common ancestral codon would have a C in the first
position. So no additional evolutionary step is required. If ancestral
was supposed to have T in the first position, an additional step of
change from T->C in chicken will be required.

Of course, comparing this just two sequences, using only parsimony -
since the peptide sequences is identifical, we will endup assuming
that the common ancestral will have the chicken sequence. We could do
it, and assign a margin of error due to mutation fixation rate.

Lets say that the mutation rate is constant over the time and along
the sequence of, say, 1 SNPs per 100 mya. In this case, about 4 SNPs
is expected.

The "_T. rex_" peptide sequence just happens to show 100% match to the chicken
peptide sequence....

Yep.

[]s,

Roberto Takata