For most of the 20th century, forensic biology meant serology — analysing blood and body fluids by their group and protein markers. It was genuinely useful, but it had a ceiling that DNA would later smash through.
What serology could do
By determining a stain's ABO blood group and additional inherited markers such as certain enzyme and protein types (for example PGM), an analyst could describe the biological "type" of the person who left it. Because these markers vary in the population, combining several could narrow a stain to a fraction of people — say, a few per cent.
The ceiling
The problem is in that word "fraction". Serology could exclude a suspect decisively — if the types didn't match, the stain wasn't theirs. But it could never individualise: the best it offered was "consistent with this suspect, and with millions of other people who share the same common types." For a common blood group, that pool was enormous.
The case that made the point
This is exactly what stalled the 1980s Narborough murders (the Colin Pitchfork case). Serology narrowed the offender to roughly a tenth of adult men — far too many to act on. It was DNA profiling, newly invented, that turned "one in ten" into a virtual certainty, cleared an innocent man, and identified the killer.
The legacy
DNA didn't just improve on serology; it changed what biological evidence could do — from narrowing a pool to identifying an individual. Serology's core lesson still stands, though: evidence that can only exclude is still powerful, and a good scientist is clear about which of the two — exclusion or identification — a result actually supports.