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HYB: Pigments 101 -- "Final Exam"

The depth of the plicata/luminata/glaciata discussion tells me a lot people 
here do understand the difference between a simple allelic series and an 
epistatic one -- and I suspect that even more grasp the distinction at an 
intuitive level.  For those of you who aren't sure, however, I think it's 
time to provide a brief answer to the "Final Exam" questions I posted several 
weeks ago:

A.  In hybridizing tangerine pinks, the t-factor is a widely-accepted, 
widely-used model. It certainly has pragmatic value, and we've covered it in 
some detail.

B.  In analyzing tangerine pinks, the epistatic model is almost inevitably 
invoked.   It has great theoretical value, and we've used it for background 
when discussing the relationship between alpha-carotene and lycopene. 

Essay Question #1:  Compare the two models, find the point they have in 
common, explain why they are complementary rather than contradictory, then 
discuss why hybridizers prefer one and scientists the other.  [Many of you 
can already do this.  Others may find that the attempt itself raises more 
questions.  If so, please post them until "The Boss" says we've taken this 
too far!]

The key point here is that the t-factor is an allele-based model.  A set of 
alleles occupies a specific location on a particular chromosome and affects a 
specific trait.  

An epistatic series, however, affects a complex process.  It is a logical 
model that may involve a number physically separate sets of allelles.

Thus, an accurate allele-based model fits seamlessly into an accurate 
epistatic model.  Perhaps more importantly, though -- if the attempt to meld 
them doesn't work the contradictions revealed during that attempt suggest 
experiments that could resolve the conflict and thus refine both models.

So "the point they have in common" is that the t-factor model is actually a 
PART OF the epistatic one.

In practice, most hybridizers have already selected for a specific trait and 
are working with stock for which earlier sets in the epistatic series have 
been stabilized.  Allele-based models are therefore not only appropriate but 
much easier to use.

Scientists, however, seek to understand how various allelic sets interact -- 
so they need to invoke the more complex epistatic model.

Essay Question #2:  Describe scenario(s) in which the epistatic model would 
be of use to hybridizers.  What additional question(s) might it answer?  
Describe the experiment(s) that could provide the answers. [This is very 
open-ended and I certainly don't expect everyone to tackle it.]  

We've already jumped headlong into an excellent example.  

Plicatas, Luminatas and Glaciatas are clearly related.  But are they members 
of the same allelic set?  Or do they belong to different sets within the same 
epistatic series?  And how does the plicata ring fit in?  

This is a bit more complicated than the tangerine pinks we started 
discussing, because it involves patterns rather than a simple blend of 
pigments.  There, the allele-based t-factor model explains the conditions 
under which we get tangerine pink while the epistatic model it fits into 
explains the rich continuum of colors in the family.  Here, we are looking 
for the simultaneous appearance of two patterns, rather than a simple blend 
of pigments.

Now for the kicker:  Remember that it's quite possible to have two separate 
sets of alleles that control different chemical pathways with similar effects 
in appearance.  We went into this in some detail when we were discussing the 
dominant inhibitors found in TBs.

Now looking forward to more Plicata/Luminata/Glaciata theorizing....

Merry Christmas!
Sharon McAllister

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