Biology Reference
In-Depth Information
The largest portion of the genome - over 50% - is composed of repetitive DNA;
45% of the repetitive DNA is interspersed, with the repeat elements dispersed
throughout the genome. The four most common types of interspersed repetitive
element - short interspersed elements (SINEs), long interspersed elements (LINEs),
long terminal repeats (LTRs) and DNA transposons - account for 45% of the genome
[4, 13]. These repeat sequences are all derived through transposition. The most com-
mon interspersed repeat element is the
Alu
SINE; with over 1 million copies, the
repeat is approximately 300 bp long and makes up around 10% of the genome.
There is a similar number of LINE elements within the genome; the most common
is LINE1, which is between 6 kb and 8 kb long, and is represented in the genome
around 900 000 times; LINEs make up around 21% of the genome [4, 13]. The other
class of repetitive element is tandemly repeated DNA. This can be separated into
three different types: satellite DNA, minisatellites and microsatellites.
Genetic diversity of modern humans
The aim of using genetic analysis for forensic casework is to produce a DNA profile
that is highly discriminating; the ideal would be to generate a DNA profile that is
unique to each individual. This allows biological evidence from the scene of a crime
to be matched to an individual with a high degree of confidence and can be very
powerful forensic evidence.
The ability to produce highly discriminating profiles is dependent on individuals
being different at the genetic level and, with the exception of identical twins, no
two individuals have been found to have the same DNA. However, individuals,
even ones who appear very different, are actually very similar at the genetic level.
Indeed, if we compare the human genome to that of our closest animal cousin, the
chimpanzee, with whom we shared a common ancestor around 6 million years ago,
we find that our genomes have diverged by only around 5%; the DNA sequence
has diverged by only 1.2% [14] and insertions and deletions in both human and
chimpanzee genomes account for another 3.5% divergence [14, 15]. This means
that we share 95% of our DNA with chimps! Modern humans have a much more
recent common history, which has been dated using genetic and fossil data to around
150 000 years ago [16, 17]. In this limited time, nucleotide substitutions have led to
an average of one difference every 1000 bases between every human chromosome,
averaging one difference every 1250 bp [5, 18] - which means that we share around
99.9% of our genetic code with each other. Some additional variation is caused
by insertions, deletions, length polymorphisms and segmental duplications of the
genome [6 - 10, 19].
There have been attempts to define populations genetically based on their racial
identity or geographical location, and while it has been possible to classify individ-
uals genetically into broad racial/geographic groupings, it has been shown that most
genetic variation, around 85%, can be attributed to differences between individuals
within a population [20, 21]. Differences between regions tend to be geographic
gradients (clines), with gradual changes in allele frequencies [22 - 27].
