Genetic diversity of modern humans

7 Apr

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 level 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 the same DNA. However, individuals, even ones that appear very dif- ferent, 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 share a common ancestor around 6 million years ago, we find that our genomes have di- verged by only around 5%; the DNA sequence has diverged by only 1.2% [8] and insertions and deletions in both human and chimpanzee genomes account for another 3.5% divergence [8, 9].

This means that we share 95% of our DNA with chimps! Modern humans have a much more recent common history, which has been dated us- ing genetic and fossil data to around 150000 years ago [10, 11]. In this limited time, nucleotide substitutions have led to an average of one difference every 1000-2000 bases between every human chromosome, averaging one difference every 1250 bp [4, 12] – which means that we share around 99.9% of our genetic code with each other. Some additional variation is caused by insertions, deletions and length polymorphisms, and segmental duplications of the genome.

There have been attempts to define pop- ulations genetically based on their racial identity or geographical location, and while it has been possible to classify individuals genetically into broad racial/geographic groupings, it has been shown that most genetic variation, around 85%, can be at- tributed to differences between individuals within a population [13, 14]. Differences between regions tend to be geographic gradients (clines), with gradual changes in allele frequencies [15, 16]. From a forensic point there is very little point in analysing the 99.9% of human DNA that is common between individuals. Fortunately, there are well characterized regions within the genome that are variable between individuals and these have become the focus of forensic genetics.

The genome and forensic genetics

With advances in molecular biology techniques it is now possible to analyse any region within the 3.2 billion bases that make up the genome. DNA loci that are to be used for forensic genetics should have some key properties, they should ideally:

-be highly polymorphic (varying widely between individuals);
-be easy and cheap to characterize;
-give profiles that are simple to interpret and easy to compare between laboratories;
-not be under any selective pressure;
-have a low mutation rate.

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