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
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
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
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
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
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....
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