What you have in the first example is not apposition, but rather a verbless clause as supplement with everything before the comma as an anchor.
Apposition is defined by the Oxford Dictionary of English Grammar p31 as:
apposition A relationship of two (or more) units, especially noun phrases, such that the two units are normally grammatically parallel,
and have the same referent, e.g.
Our longest reigning monarch, Queen Victoria, reigned from 1837 to 1901
The third edition of OUP’s biggest dictionary, the Oxford English Dictionary, is being published online
And again by the Cambridge Grammar of the English Language p1357:
The construction with a specifying NP as supplement is known as
apposition. More particularly, this is the supplementary type of
apposition, corresponding to the integrated apposition of the opera
‘Carmen' or my husband George [...]. Thus the appositive NP can be
substituted for the whole supplementation yielding an entailment of
the original:[The first contestant, Lulu, was ushered on stage]
entails Lulu was ushered onstage
Further examples of supplementary apposition are given in:
The murderer, the man with the scar, will be arrested soon.
A university lecturer, Dr Brown, was arrested for the crime.
A surprise present, a bouquet of roses, was delivered to my door.
An entire genre, the comedy thriller, has been made obsolete by the
invention of the mobile phone.
A Sevfert galaxy - a galaxy with a brilliant nucleus - usually has a massive red-shift.
The two elements of your sentence:
(1)This allows for the creation of transgenic lines with varying degrees of expression,
a main clause
(2)an important feature since substrate levels modulate incubation period and disease course.
a verbless clause including:
- an ascriptive NP an important feature
- an adjunct of reason since substrate levels modulate incubation period and disease course
(2) is not in apposition to (1) as it cannot take its place (and so is not grammatically parallel).
Rather, this is a case of a verbless supplementary clause. These are allowed quite freely whenever the anchor is the entire preceding clause, or a dependent clause within it.
Some examples with a similar structure (supplemental verbless clause made up of an ascriptive NP + an adjunct):
At the 3-year point, however, there was a dramatic reduction in
dieting and frequency of weighing-in among this group , an important
finding because decreasing dieting and weighing-in were primary
objectives of Eat For L.I.F.E. (Journal of Psychology, Sep94, Vol.
The write-up is also being financed by White, through the Shelby
White-Leon Levy Program in Archaeological Publications , an ironic
twist because it was Levy and White's insistence on constant
excavation that prevented publication to begin with. (Publish or Be
Punished; Atwood, Roger; Archaeology, Mar/Apr 2007)
American military and Pentagon officials continue to hold that as
Iraqi security forces increase in numbers and effectiveness, they will
be able to gather even more detailed and timely information, an
important consideration if the insurgency is to be stifled.
(ESTIMATES BY U.S. SEE MORE REBELS WITH MORE FUNDS, New York Times
To forestall Congress from passing protectionist legislation in 1985,
the administration organized concerted exchange rate intervention
against the dollar , an unnecessary step if the dollar was beginning
to decline for other reasons. (Reaganomics: Myth and reality,
Roberts, Paul Craig; Perspectives on Political Science, Spring '90)
The Pistons' starters sat the entire fourth quarter , an important
break since the Pistons face the New York Knicks on the road Tuesday. (Detroit Free Press, 2019)
It turns out that Atrus's father, Gehn, is also trapped there , an
interesting development since Gehn himself created that world by
writing' it in one of his magic books. (The Next Myst, Newsweek,
Anterior resultant shear angles were directed more distally in the
model than in clinical data , an expected result since there was no
relief for the tibial crest in the model. (Normal and shear stresses
on a residual limb in a prosthetic socket during ambulation...,
Sanders, Joan E. Daly, Colin H., Journal of Rehabilitation Research &
All of the above could also be paraphrased by a supplemental relative clause which is/was..., but there is no requirement to do so. The choice is a stylistic one, not a matter of grammatical rules.