Re: Digit Identity WAS (Re: Yay! Cladobabble! :-))

[Nick Pharris <npharris@umich.edu>]

Quoting Matthew Bonnan <mbonnan@hotmail.com>:

> Apparently, then, digit identity is gene encoded PRIOR to the Sonic hedgehog
> signal.

Well, yes, but it also appears that any digit has the genetic potential to 
assume any identity, until the Sonic hedgehog signal determines which genes 
will be activated.  (Then again, the cells in your finger have the *genetic* 
potential to grow a spleen, or a brain, or a bone, or any other tissue, don't 
they?)

So say that you have an animal with four fingers, homologous to original 
amniote digits I-IV.  Digit IV forms first, digit I last.  Suppose also that 
the genes for "thumb"-hood are activated by a high concentration of a protein 
that diffuses from the inner edge of the manus.

Now suppose a mutant arises that produces only three digits (II-IV).  If 
nothing else changes, the concentration of the "thumb-determining" protein 
will now be highest in ancestral digit II (since I isn't there at all), and so 
the digit homologous to ancestral digit II (being the third digit formed) will 
develop just like ancestral digit I.  Voila`:  frame shift.  Am I missing 
something?  Why would something like this not be *expected* to happen?

Nick Pharris
Department of Linguistics
University of Michigan