 Semiconservative replication

In semiconservative replication, each daughter duplex retains one strand of its immediate parent. Under complete copying without strand loss or exchange, one starting duplex gives $2^n$ duplexes after $n$ rounds, but still only two original strands. For $n\ge 1$, the fraction of duplexes containing an original strand is $2/2^n=2^{1-n}$: all after one round, half after two, a quarter after three. The fraction of all strands that are original is instead $2^{-n}$. A strand made in round 1 is parental when round 2 begins.