This is a little bit of reading but I think it’s worth putting here. I took it from Dank.Frank on ICmag so credit for the info goes to him
NYC Chem F2 = (Chem D x Chem 91 BX)
ONYCD = Chem '91 x Tres Dawg
Tres Dawg = Chem D x Double Dawg - Chem D Bx2
Double Dawg = Chem D x (Chem D x Afghani#1) - Chem D Bx1
Chem 91, Chem D, Chem 4, ECSD - are all related. The Chem 91 being the original.
So, without using JJNYC's naming schemes - Chem D x (Chem '91 x Chem D Bx2)
What you have is a very serious chem line breeding project that should give you some fairly consistent type plants, even when F2'ed theoretically.
You'll find 9-10wk plants that will stretch about 2.5 times once flipped in flower. The '91 is a more sativa type plant in behavior. Has elongated internodes and chunky flowers that don't exactly make long colas. However, Chem D is a good yielding plant and produces some fat buds when grown right. Is more compact than the '91. Some of the best resin for extracts there is. True FMCD bubble. Essentially, you're going to find all around hybrid type plants that sit very middle of the ground. Outliers will finish 8wk or 11wk respectively. Keep in mind JJNYC used Afghani for the original chem outcross and NO ONE really knows what the genetics of Chem '91 are. It's fair to assume, it's a sativa leaning hybrid of some type. (think 60/40) If you find longer flowering plants, you know you are seeing something that is more heritage and relatively unknown.
I'd be looking for a garlicy, old gym sock, earthy/musty, damp basement smelling skunk, that has a sharp, bleachy smelling stem rub. The '91 should have added back some more subtle sour and lemony type notes absent in the Chem D.
An F2, in any plant, is going to open up a wide range of phenotypes that have some genetic recombination from every parent that has ever been introduced into the line. That means a WIDE world of possibilities.
F1 is generally more representative of the parents used and you can see what amounts to "this plant leans towards the mom / dad".
So, in the F2, you are able to find what is generically referred to as extreme outliers. Strange combinations and phenotypes that you'd not necessarily expect to find in the line. Many people hope to find plants representative of long lost parents that no longer exist.
For a simple example: If you take a pure sativa line and F2 it and then you sort those and find a super squat plant with massively wide leaves. One of two things is possible, it wasn't a pure sativa OR you've found a really rare and unique expression that exists some where in the gene pool from many many years back. A representation of an ancestor.
These extreme outliers are the types of plants breeders look for when trying to create really unique lines.
In much the same way, if you F2 a seed line and all the plants come out more or less the same, you're working with a true breeding line and that can be very useful to know as well.
As far as the seeds you are looking at, if you want Chem line base genetics that are actually going to be representative of the clones, you need look no further than TopDawg Seeds. JJ NYC has been part of the chem scene since day 1 more or less. He is an authority on that gene pool. MANY people have sponged off his work and used it to create their own seed companies. IF you want the best, stick with the guy who has dedicated his life to that specific line of genetics.