Differentiating appositive phrases and non-restrictive relative clauses

Solution 1:

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:

Apposition

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. 128)

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 2004 (0410))

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, 1996)

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 & Development 1993)

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.

Solution 2:

I'm for your second sentence. I would, however, reword it slightly. Here's my suggestion:

This allows for the creation of transgenic lines with varying degrees of expression--an important feature, since substrate levels modulate incubation period and disease course.

A bit wordier, but perhaps more memorable, is the following:

This allows for the creation of transgenic lines with varying degrees of expression--an important feature, since substrate levels modulate both the incubation period and the course of the disease.

I wish I knew what the antecedent of this is, but I'll assume the preceding sentence has one.

Solution 3:

Your first example is one of apposition. This expands upon the first clause’s complement. Usually, a comma is sufficient with simple NPs. However, the length and complexity seem to demand more than a comma.

Your second example is a non-defining (descriptive) relative clause – these are separated by a comma. However, they often sound as if they are an “aside” or an afterthought. They tend to be inappropriate in formal writing.

The example would thus be better as two sentences or with the separation of the clauses by a colon.

This allows for the creation of transgenic lines with varying degrees of expression ./: This is an important feature since substrate levels modulate incubation period and disease course.

PS “correct”, in this context, is an ungradeable adjective. It is not possible for something to be “more correct.”