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Re: Digits, Digits, Digits!!

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.


"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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