More like nonexistent.
With few exceptions, many, if not most, convicts have exercised the ability to procreate before incarceration or will exercise that ability upon release.
Jails could only possibly have a genetic impact if criminals were caught before they ever had sex and were held there until after they could no longer effectively produce viable offspring. Also, there’s the assumption there is a genetic component to criminality, which is more than a little a stretch.
Of course, this all assumes natural selection is at play. The Lamarckian result would, of course, be quite different.