so we’ve all conjectured that sherlock’s commentary on the crap telly (“course he’s not the boy’s father, look at the turn-ups on his jeans”) is foreshadowing that the baby isn’t john’s, and in particular i’ve seen people pointing out mary wears her jeans cuffed, but if it IS intended as foreshadowing then i think it’s a little more figurative than that.
what a weird, arbitrary piece of evidence, right? sherlock’s level of deductive skill is absolutely the stuff of fantasy, but they DO generally try to provide us with coherent, logical explanations for his train of thought. this particular piece of evidence is utterly opaque, decontextualized, and ludicrous, however. how could the way a man cuffs his jeans possibly ever tell us anything about a child’s paternity? i think that’s supposed to draw our attention a little more closely to this line, and when you look a little closer you realize that the obvious reinterpretation that brings the evidence (“the turn-ups on his jeans”) into the paternity context is swapping in “genes” for “jeans” (which is pretty hard to argue is a coincidence, given the apparent randomness of the sartorial evidence sherlock is ostensibly citing and the fact that we’ve basically been told coincidences don’t really exist in this narrative).
but what about “turn-ups”? obviously we aren’t talking about cuffs on dna. well, as it happens, john himself provides us with the key to that mystery just a few minutes later:
course john’s not the father. look at the totally unexpected content of the baby’s genes.