Biology Reference
In-Depth Information
the same haplogroup: mtDNA can be assigned to a haplogroup based on containing
characteristic mutations; some of these are within the hypervariable region, whereas
others are dispersed around the genome; the evolutionary relationship of different
haplogroups can be shown as a phylogenetic tree [45, 46]. mtDNA haplogroups
show geographical clustering, so, for example haplogroup H occurs at a frequency
of approximately 40% in western Europeans, whereas it is virtually absent from
Africa and Southeast Asia. Based on the relative frequency of the haplogroups in
different geographic populations, the geographical origin of a particular sample can
therefore be inferred [47 - 49]. In a forensic investigation care should be taken not
to overinterpret the 'ethnicity' prediction of a sample left at a crime scene as it
only shows the maternal lineage, which may have become admixed with another
geographic population several generations ago.
Evaluation of mtDNA profiles
Declaring a match is straightforward, but exclusions can be more problematic. When
a questioned sample and a reference sample differ at only one position the likelihood
of that one base difference occurring though a mutation has to be assessed. In such
an instance the results are usually classified as inconclusive - when there are two or
more differences between a questioned and known sample it is normally classified
as an exclusion [21].
If a match is declared, the statistical significance has to be assessed. The mtDNA
genome is inherited as a single locus and this limits the evidential value in forensic
cases. Haplotype frequencies have to be measured directly by counting the occurrence
of a particular haplotype in a database and reporting the size of the database [50].
When databases are relatively small, for example 100, many of the less common
haplotypes that are within a population will not be represented. There are mecha-
nisms that compensate for the limitations of reference databases, such as minimum
haplotype frequencies, employing standard error calculations and correction factors
to allow for subpopulations [51, 52].
If a mtDNA haplotype occurs at a relatively high frequency in the population of
interest then additional information can be gained by typing some of the polymor-
phisms that occur outside the hypervariable regions, thereby increasing the evidential
value of the mtDNA [53 - 55].
When reporting the results of mtDNA analysis, the caveats associated with mtDNA
have to be clearly explained so that there is no confusion with 'standard' (autosomal
STR typing) analysis. In particular that 'it is inherited only from one's mother, and
therefore all individuals who are related by a maternal link will have the same mtDNA
profile', and that 'it varies less between individuals, and therefore more individuals
chosen at random from the population will have the same mtDNA profile' should be
made very clear.
