The clear implication made in the books is that when Jon Arryn, Hand of the King, said "The seed is strong" he was referring to the indications - based on documented bloodlines and inherited genetic characteristics - that Robert Baratheon did not father Joffrey, Tommen, or Myrcella on Cersei Lannister.
(Of course, in Westeros, no one would think of them as "genetic characteristics". But the inherited traits which we know to be genetics have long been recognized as having a bloodline relationship - more so, frankly, in systems where nobility intermarries and documents their births and marriages better than is the average. And seed is slang for sperm, which leads to births, or so I'm led to believe...)