Note on Jack F-1 Phenotypes, Bud compactness, and Effect: I popped two Jack F-1 seeds and got two phenotypes. Both reached 8 feet tall, but the leaves on the one were fatter and shorter, and slender on the other. The sativa-looking plant was more vigorous and christmas-tree shaped with side branches creating a wider girth. The indica-looking plant had side-branches that were fewer in number, and her leaf morphology reminds one more of Spliff's White Widow （a "mostly indica" strain） than a sativa. See my photos comparing these two Jack F-1 phenos alongside Spliff's WW after about one month of veg, in early May.
I was puzzled at first because Spliff's states -- see Basic Info page above -- that "Jack F1 has an Indica appearance, but is definitely a Sativa." For me, that was confusing because the more vigorous plant, the one which got absolutely huge and produced the larger quantity of bud, was clearly a sativa-pheno that Spliff's did not mention. So at first I was not sure which phenotype was the pheno Spliff's had selected and back-crossed for. Now it is clear to me that Spliff's selected for what I am calling the indica-looker, designated as pheno #1 here on seedfinder.
"Jack F-1" growers will note that these phenos exist. By definition, an F-1 hybrid is a cross of two IBLs and is 50% stable. It can show variation from one individual to the next. Jock Horror is IBL, but numerous posts by Spliff's say the breeder used a Skunk F1 to produce "Jack F-1", so not all plants will have the indica look. By back-crossing with Jock Horror, the chances of getting the indica-looking pheno may have been locked in to a good degree, maybe 75% or 87.5%, depending on the number of BX.
My sativa-looker （pheno #2 here on seedfinder） produced an abundance of slightly fluffy buds （I've got 9 quart jars curing）. Her buds have some compactness -- there is a "hard" interior if squeezed -- but are slightly fluffy on the outside. A large amount of rough leaf material was also prominent on pheno #2, making manicuring messier. The indica-looker （pheno #1） actually has very tight and compact -- "hard" -- buds, which were much easier to harvest, manicure, and cure. See photo of finished bud.
There is no difference in effect as between the two phenos. Jack F-1's effect compares very well with that of the original Jack Herer, being a nicely balanced blend of stimulating and clear haze head high with a subtle and pleasant indica body buzz. When I grow Jack F-1 again, I will try to select for the indica-looking pheno in hopes of getting these highly compact buds. Compactness makes harvesting, curing, and smoking easier.