Re: Digits, Digits, Digits!!

["Matthew Bonnan" <mbonnan@hotmail.com>]

Hi all, long time no speak.

Tim Williams writes:

"I'm not particularly knowledgeable on the topic of the molecular basis of
supernumerary digits (hyperdactyly).  However, are you certain this trait is
heritable?  Will Mr Alfonseca's children have six fingers??"

Potentially yes.  These traits, polydactyly, run in families and are 
genetically inherited by some or all of the offspring, depending, of course, 
on the background of the mother and father.  Polydactyly is considered to be 
what's called an autosomal dominant inheritable trait.  That means it is an 
inheritable allele on a "regular" chromosome (rather than a sex-linked gene 
on the X or Y chromosome).  So, Mr. Alfonseca's children will have a certain 
probability of inherting this trait depending on the genetic makeup of the 
mother and whether Mr. Alfonseca's trait for polydactyly occurs on both 
alleles or just one.

Polydactly has been traced through family lineages (pedigrees) for 
generations and is nothing new.  Many people that are born with six or seven 
fingers or toes never know because their parents approve surgery to remove 
these extra digits.


So:

"In this case, a secondarily tridactyl tyrannosaur would be a "one-off", and
could not become the first of a new lineage of three-fingered tyrannosaurs."

Well, we don't know, but if T. rex had a "polydactly" allele, then its 
offspring could inherit the mutated condition.  In any case, the digit thing 
is interesting because we still don't quite understand what influences what 
-- environmental and genetic influences on the development of digits and the 
skeleton in general are much more complicated than previously appreciated.  
Do you intially develop a "plastic" response system that gives predictable 
results in most cases but has "accidents" once in a while?  In some cases, 
this appears to be so -- certain bony crests, patellae, etc., all develop 
under the influence of environmental stressors that are not specified in the 
genetic code.

Bone and cartilage are living tissues that respond plastically to their 
environment, so there is a debate among embryologists and others about how 
much genes code for and how much expression is controlled by the environment 
-- fascinating.  This has huge implications for phylogeny because most of 
our dinosaur systematics is based on bony characters that are influenced by 
environmental and physical factors during the life of the animal.  This 
means you have to understand development and function if you want to 
understand phylogeny.

And, remember that this whole conversation has been "digital" both literally 
and figuratively. =)

Matt Bonnan
soon to be at:
Dept Biological Sciences
Western Illinois University
Macomb, IL
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