A line, for example the V.P.I. axanthic is named for the first person to breed it out.
There isn't really a way to make a line. It happens. Well unless you have a bigger knowledge of genetics than anyone I know personally and are charting genomes on ball pythons.
You can, decide to breed specimens of a morph that look especially different than the other morphs like it. More crisp, brighter, darker for example. But it has to be a difference that sticks around (is genetic)
That's as close as I can come to giving you an answer
singingtothesnakes 
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