28.1 DNA Forensics in Criminal Investigations

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expression) they have a high degree of polymorphism among individuals. Degrada-

tion of the DNA samples, which may have been exposed to adverse environmental

influence before collection, limits the use of the longer VNTRs. The smaller STRs

require PCR amplification in order to ensure that enough material is available for

detection after chromatographic or (more usually) electrophoretic separation.

The pioneer of the use of DNA forensics, Alec Jeffreys, used the so-called

“mini-satellites”, repeatedtilde 10∼10–100-mers, present intilde 1000∼1000-mer fragments gener-

ated from the sample DNA using restriction enzymes, radiolabeled with phosphorus,

and size-separated using gel electrophoresis, then overlaid with a photographic plate

in order to record the size-dependent positions. This electrophoretogram constituted

a unique set of features associated with an individual, which could be compared with

a similar electrophoretogram generated from the fresh DNA taken from a suspect. ³

Subsequently it was found that STR typing provides a comparably unique set of

features. STRs are 3–5 bases long (e.g., GATA) and are found in fragments, called

“markers”, less then 500 bases long. The fragments can be readily generated in

numbers sufficient for subsequent analysis using appropriate primers in PCR. To

avoid linkage bias, markers from different chromosomes are usually selected and,

since 2017, there is now international agreement on a standard set of 20 markers for

human STR typing. Each human being has two alleles, maternal and paternal, for

each marker, each of which may contain 1 tilde 201 ∼20 STRs. ⁴ The number of repeats in

each marker is readily determined using capillary electrophoresis.

A significant potential pitfall is the possible uncertainty in repeat number determi-

nation (plus or minus 1±1—due to so-called PCR “stutter”), alongside the almost inevitable pres-

ence of contaminating DNA. Stutter peaks are much weaker than the main peaks,

but may be confounded by the main peak of a minor contaminant. One way of

overcoming this problem would be to take several samples from slightly different

environments at the crime scene. Another way is to use probabilistic genotyping,

which proposes different possible genotypes and finds the mixture giving the best fit

to the experimental data.

The usefulness of STR typing has been enormously increased by the presence of

national and international databases. Hence, the technique is no longer restricted to

comparing a sample from the crime scene to a suspect’s DNA; the sample can be

compared with the database. Initially the database was made up from samples taken

from convicted felons, but often samples taken from suspects were retained and, with

the enormous rise in DNA typing for ascertaining genealogies, much of which data

is uploaded into public repositories, in effect this also forms part of DNA data that

can be consulted. ⁵ In the USA alone, there are at least 16 million DNA profiles held

in CODIS. Given the prevalence of recidivism, it is a useful resource for detectives.

Collaterally with the rise of suspect databases, there has also been the establishment

3 Jeffreys et al. (1985); Jeffreys (2013).

4 Derived from the US combined DNA index system (CODIS), which originally specified

13 markers.

5 Indeed, there have been cases in which criminals were found due to assiduously following links

with matches to distant relatives.