My sentence in full length is “A case in point is Japan, they have experienced a drop of TFR from 3.44 in 1950 to only 1.55 in 2015.”
Divide the sentence by comma into two parts, I would like to know these two parts can be linked together with the first part as dependent and the second part as independent.
A case in point is Japan.
This is an independent clause that can function as a complete sentence. It has a subject ("a case in point"), a verb ("is"), and a complement ("Japan"). This is an S-V-C sentence. As Jeff Blair explains ("SVC Sentence Pattern"), the complement describes the subject. This independent clause should be treated as such with appropriate punctuation, like a period (separating the clauses) or a colon (connecting the clauses that are directly related – see the UNC Writing Center website for more).
If you are joining two independent clauses, you need a coordinating conjunction like "for." Alternatively, you could make one of the clauses subordinate or dependent with a subordinate conjunction. (For more details see Robin L. Simmons, "The Coordinating Conjunction" and "The Subordinate Conjunction," Chomp Chomp). The result would look like this:
A case in point is Japan. They have experienced … (separate sentences)
A case in point is Japan: they have experienced … (colon joins two independent clauses)
A case in point is Japan, for they have experienced … (two independent clauses joined by a coordinating conjunction)
A case in point is Japan, since they have experienced … (an independent clause joined to a now-subordinate clause by a subordinate conjunction)