Biology Reference
In-Depth Information
C, or T), it is not feasible to use one or a handful of known SNPs to discriminate among
individuals in any given population. However, technological advances have progressed to
the point that it is now possible to type several hundreds of thousands of SNPs in any one
individual so that we can use this technology to distinguish one individual from any other
(because the probability that any other individual will have exactly the same combination
of SNP variants is at or close to zero).
As of this writing, anthropological researchers interested in questions of evolutionary
significance tend to use SNPs regularly, while at present, forensic geneticists do not (though
this might change in the next five years or so, due to the possibility of using large numbers of
SNPs for individuating purposes).
SHORT TANDEM REPEAT (STR)
A
S
hort
Ta
ndem
Re
peat, or STR (pronounced s-t-r), is a genetic variant consisting of
a linear repetition of a characteristic sequence motif. For example, at the same genomic
region, one individual's genome may read AGTCAGTCAGTCAGTC, while another's
genome may read AGTCAGTCAGTCAGTCAGTC. Here the repeat motif is “AGTC,” with
the former individual exhibiting four repeats of the motif
d
denoted as (AGTC)
4
d
and the
latter, five [(AGTC)
5
]. An STR typically contains a repeated unit of between two and six bases,
and can form series of up to 100 bases in length.
A difference in the number of repeats between parents and offspring is fairly
common
d
much more common than for SNPs (as above) or transposable elements (as
below). It is not really known why this is the case, but it probably has something to do
with a “slippage” of strands occurring during the replication of DNA prior to cell division.
Unlike the estimated low mutation rate for SNPs, STRs have relatively high estimated
mutation rates, ranging from 10
-6
to 10
-2
bases per generation (
Fan and Chu, 2007
).
STRs are only found in the nuclear genome; the mitochondrial genome is devoid of them.
ALU
ELEMENTS
An
Alu
element (or simply, “
Alu
”) is a transposable element, also known as a “jumping
gene.” Transposable elements are rare sequences of DNA that can move (or transpose) them-
selves to new positions within the genome of a single cell.
Alu
elements are about 300 bases
long and are found throughout the human genome. Their sheer size combined with their
unusual “copy and paste” mechanism means that an
Alu
element insertion in any particular
genomic region is unusual and therefore very unlikely to happen more than once. Therefore,
we can assume when any two individuals share a specific
Alu
element insertion, it is likely
due to common ancestry.
As with STRs,
Alu
elements are only found in the nuclear genome.
Obtaining DNA Data
A relatively straightforward way to obtain DNA data is to use data already generated by
other researchers. Sometimes researchers publish journal articles or book chapters with
summary results and interpretations based on DNA data that they generated themselves,
but the data themselves are not immediately accessible. In that case, writing to the corre-
sponding author of the publication with a request for the data is in order. However, it is
becoming more common for researchers to make data available as downloadable digital files,
