Electrophoresis & PCR: Applications

This is where the techniques earn their keep. A pattern of bands on a gel can identify a body, settle a paternity case or clear an innocent suspect. Reading those patterns is a skill, and once you know the rule it is surprisingly quick.

What is actually being compared

Almost all of your DNA is identical to almost everyone else’s, so comparing whole genomes would be useless. Profiling targets the parts that vary most.

VNTRs stands for variable number tandem repeats. These are short stretches, roughly 20 to 50 bases long, that are repeated over and over. Non-coding regions like these are free to vary without harming the organism, so the number of repeats differs enormously between individuals.

More repeats means a longer fragment, which travels less far on a gel. So the pattern of band positions is really a read-out of how many repeats a person has at each VNTR site. Apart from identical twins, everyone’s combination is different.

The procedure, start to finish

Reading a profile: the one rule

Everything about paternity questions comes down to a single statement.

So work through the child’s bands one at a time:

• If the mother has that band, the child could have inherited it from her. It tells you nothing about the father, so move on.
• If the mother does not have it, it must have come from the father. Any man lacking that band is eliminated.

Keep going until only one candidate survives, then look for one more band as a confirmation.

Ancestry and paternity testing

Because VNTRs are inherited, roughly half of a child’s bands come from the mother and half from the father. A parent should therefore share about 50 % of the offspring’s bands.

The same logic extends outwards. More distant relatives share fewer bands, so profiles can be used to check whether two people are related and roughly how closely.

Forensic investigations

DNA profiling is used to identify suspects and victims.

• Cells or fluids — blood, saliva, hair roots, skin cells — are collected from the scene or the victim.
• The DNA is extracted, amplified and profiled.
• The profile is compared with samples from suspects, from the victim, and with control samples from people with no connection to the crime.
• The suspect whose profile shares the most bands with the crime-scene sample is the likely source.

Just as importantly, profiling can eliminate people. Someone whose DNA happens to be at the scene for innocent reasons can be cleared quickly, and past convictions have been overturned this way.

Contamination is the constant danger. A single stray skin cell from an investigator can add bands that were never in the original sample, so samples are handled with strict controls.

Nature of science: reliability

No test is ever completely certain, and that matters more than usual when evidence is going before a court.

In profiling, reliability is improved by increasing the number of VNTR markers compared. Two unrelated people might share the same pattern at one or two sites by chance, but the probability of matching at ten or more sites becomes vanishingly small.

This is the general principle in a specific setting: repeating a measurement, or taking more measurements, increases the reliability of the conclusion drawn from it.