The probability that two individuals will have identical markers at each of 13 different STR loci within their DNA
exceeds one out of a billion. If a DNA match occurs between
an accused individual and a crime scene stain, the
correct courtroom expression would be that the probability
of a match if the crime-scene sample came from someone
other than the suspect (considering the random, not
closely-related man) is at most one in a billion [14]. The
uniqueness of each person’s DNA (with the exception of
monozygotic twins) and its simple numerical codification
led to the establishment of government-controlled criminal
investigation DNA databases in the developed nations
around the world, the first in 1995 in the UK [23]. When a
match is made from such a DNA database to link a crime
scene sample to an offender who has provided a DNA sample
to a database that link is often referred to as a cold hit.
A cold hit is of value as an investigative lead for the police
agency to a specific suspect. China (approximately 16 million
profiles, the United States (approximately 10 million
profiles), and the UK (approximately 6 million profiles)
maintain the largest DNA database in the world. The percentage
of databased persons is on the increase in all countries
with a national DNA database, but the proportions are
not the same by the far: whereas in the UK about 10% of
the population is in the national DNA database, the percentage
in Germany and the Netherlands is only about
0.9% and 0.8%, respectively [24].
Lineage markers in forensic analysis
Lineage markers have special applications in forensic
genetics. Y chromosome analysis is very helpful in cases
where there is an excess of DNA from a female victim
and only a low proportion from a male perpetrator. Typical
examples include sexual assault without ejaculation,
sexual assault by a vasectomized male, male DNA under
the fingernails of a victim, male ‘touch’ DNA on the skin,
and the clothing or belongings of a female victim.
Mitochondrial DNA (mtDNA) is of importance for the
analyses of low level nuclear DNA samples, namely from
unidentified (typically skeletonized) remains, hair shafts
without roots, or very old specimens where only heavily
degraded DNA is available [25]. The unusual nonrecombinant
mode of inheritance of Y and mtDNA
weakens the statistical weight of a match between individual
samples but makes the method efficient for the
reconstruction of the paternal or maternal relationship,
for example in mass disaster investigations [26] or in
historical reconstructions. A classic case is the identification
of two missing children of the Romanov family, the
last Russian monarchy. MtDNA analysis combined with
additional DNA testing of material from the mass grave
near Yekaterinburg gave virtually irrefutable evidence
that the two individuals recovered from a second grave
nearby are the two missing children of the Romanov
family: the Tsarevich Alexei and one of his sisters [27].
Interestingly, a point heteroplasmy, that is, the presence
of two slightly different mtDNA haplotypes within an
individual, was found in the mtDNA of the Tsar and his
relatives, which was in 1991 a contentious finding
(Figure 3). In the early 1990s when the bones were first
analyzed, a point heteroplasmy was believed to be an extremely
rare phenomenon and was not readily explainable.
Today, the existence of heteroplasmy is understood
to be relatively common and large population databases
can be searched for its frequency at certain positions.
The mtDNA evidence in the Romanov case was underpinned
by Y-STR analysis where a 17-locus haplotype
from the remains of Tsar Nicholas II matched exactly to
the femur of the putative Tsarevich and also to a living
Romanov relative. Other studies demonstrated that very
distant family branches can be traced back to common
ancestors who lived hundreds of years ago [28].
Currently forensic Y chromosome typing has gained
wide acceptance with the introduction of highly sensitive
panels of up to 27 STRs including rapidly mutating
markers [29]. Figure 4 demonstrates the impressive
gain of the discriminative power with increasing numbers
of Y-STRs. The determination of the match probability
between Y-STR or mtDNA profiles via the
mostly applied counting method [30] requires large,
representative, and quality-assessed databases of haplotypes
sampled in appropriate reference populations,
because the multiplication of individual allele frequencies
is not valid as for independently inherited autosomal
STRs [31]. Other estimators for the haplotype
match probability than the count estimator have been
proposed and evaluated using empirical data [32],
however, the biostatistical interpretation remains complicated
and controversial and research continues. The
largest forensic Y chromosome haplotype database is the
YHRD (www.yhrd.org) hosted at the Institute of Legal
Medicine and Forensic Sciences in Berlin, Germany, with
about 115,000 haplotypes sampled in 850 populations
[33]. The largest forensic mtDNA database is EMPOP
(www.empop.org) hosted at the Institute of Legal Medicine
in Innsbruck, Austria, with about 33,000 haplotypes
sampled in 63 countries [34]. More than 235 institutes
have actually submitted data to the YHRD and 105 to
EMPOP, a compelling demonstration of the level of networking
activities between forensic science institutes
around the world. That additional intelligence information
is potentially derivable from such large datasets becomes
obvious when a target DNA profile is searched against a
collection of geographically annotated Y chromosomal or
mtDNA profiles. Because linearly inherited markers have