DNA profiling has become todays modern fingerprint in the sense that it provides unique identification to an individual. DNA profiling has been able to provide reliable decisive proof of an individual's profile. This is done by analysing biological material to uncover the genetic information held within it. DNA (Deoxyribonucleic acid) basically provides fundamental genetic inheritance by developing a vast sequence of codes which are made up of four base units called nucleotides which in turn form the DNA strand. In the four nucleotides there are two corresponding base pairs which will form to one another. Therefore each nucleotide includes the single stranded DNA which in turn will also bind to the corresponding base to form what is known as a double stranded arrangement.
In a forensic context there are only certain parts of the DNA that would be analysed by the Forensic scientist and these hold no genetic function and are discarded in that sense. This is merely because these parts of the DNA seemingly are just repetitive DNA codes often referred to as VNTR's (variable number of tandem repeats). The analysis of VNTR's from a range of loci can determine a profile of an individual which can be used to differentiate whether an individual has a high compatibility rate.
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In a eukaryotic genome it is normally comprised of repetitive DNA sequences. They come in a variety of sizes and nominated by the length of the core repeat unit and the quantity of adjacent repeat units or on the whole length of the repeat regions. In these long repeats there could be hundreds or even several thousands bases in the core repeat. These regions could be found nearby the chromosomal centre therefore establishing the name satellite DNA. In a medium-length repeat the core repeat unit is often referred to as a minisatellite or VNTR which tend to be 10-100 bases in length. On the other hand DNA regions which contain 2-6 bp are referred to as microsatellites, simple sequence repeats (SSRs) or short tandem repeats (STR's).
The advantages and the popularity of STR's as DNA repeat markers are that they are effortlessly improved by PCR (polymerase chain reaction) without the problems of discrepancy in the amplifications. This could be down to the fact that both alleles from a heterozygous individual are comparable in size simply because they are replicas of size. The amount of repeats in STR markers can be highly changeable between an individual which makes this process of determining human identification very successful.
PCR primers objective is to pursue the invariant along the sequence regions.
In human identification determination a main requirement is to have DNA markers which demonstrate the highest possible variation or a decrease in numbers of polymorphic makers to be combined which in turn will obtain the capability to categorize between samples.
Single Nucleotide Polymorphism (SNP)
Single Nucleotide Polymorphism are basically DNA sequence variations which occur when a single nucleotide (A, T, C or G) in the genome sequence are changed. Single nucleotide polymorphisms make up 90% of human genetic variation which occur every 100-300 bases along the three billion base human genome. Generally every two out of three Single Nucleotide polymorphisms imply the replacement of Cytosine (C) with Thymine (T). Single nucleotide polymorphism occur in the coding and non coding regions of the genome.
When looking at variations in DNA sequence in a biomedical point of view it is useful in displaying how humans act in response to environment, disease, bacterial, toxins etc. It is also thought that by studying Single nucleotide polymorphism it can help by assessing the multiple genes involved in complicated illness like cancer and hopefully being able to find a cure to these untreatable illnesses.
Single Nucleotide Polymorphism (SNP) can be population specific due to their low mutation rate so in a forensic context it could be possible in the future to predict a perpetrators ethnic origin by analysing a small sample of population specific loci.
When looking at short tandem repeats (STR's) in a forensic context they are very reliable and effective DNA markers because the can be easily improved by polymerise chain reaction (PCR). PCR products for STR's are usually similar amounts which makes the analysis a simple procedure. An individual will inherit one STR from each parent but the repeat size may differ. The amount of STR repeats markers in an individual can be variable. This makes this procedure of human identification very successful.
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When using this procedure for human identification it is very important to obtain DNA markers which have the highest number of variation this allows for discrimination between the samples. The disadvantage in a sense is that it can be hard to obtain PCR amplifications products from forensic samples due to the fact that the DNA present in the samples can often be tainted or mixed this can be seen in cases involving sexual assaults.
Therefore it is more beneficial to have smaller STR alleles which make STR markers better contenders for forensic purposes where DNA is often tainted. In PCR intensification of DNA samples which have been tainted can be corrected by small target sample sizes.
Due to their small size STR alleles can be separated from other chromosomal sites easily so that small loci are not selected. Close loci do not produce random distribution in population. This causes statistical analysis difficulties. On a good point STR alleles have lower mutation rates thus making the data more stable and predictable.
The advantage of STR's in human identification purposes are that they have a significant level of distinction so they prove very valuable in cases of analysing perpetrator identity or in missing person cases.
In a forensic context there is a requirement of at least 13 STR loci required to make the evidence acceptable and to be upheld in a court of law for example. Worldwide variations of the allele frequencies at these loci do not compromise the power of the identification of individuals.
Data collection of these loci for forensic purposes has gone beyond just human identification the origin and past migration history of modern humans can now be reconstructed from worldwide variations at these loci.
As well as this complicated unsolved forensic cases can now be investigated with the help of validated STR loci for addressing these issues by using multilocus genotype data of individuals belonging to seven different populations: US European-American, US African-American, Jamaican, Italian, Swiss, Chinese and Apache Native American.
Genomic research is discovering new classes of polymorphic loci e.g. single nucleotide polymorphism (SNPs) and lineage markers e.g. Mitochondria DNA and Y-chromosome makers. The aim of the research undertaken by Budowle et al, 1999 was to determine how many SNP loci are required to address most problems associated with human identification including decoding DNA mixtures; it was found that 13 loci was a suitable number. If a suitable number of SNPs are used that would match the power of the STR loci they alone cannot resolve complex cases unless they are complemented by the validated STR loci. (Budowle et al, 1999)
Mitochondria DNA (mtDNA)
Mitochondria DNA (mtDNA) are inherited at the fertilization process. It is located in the tail end of the spermatozoa it is therefore never passed to the egg therefore deleting the male mtDNA information from the offspring genome.
The mitochondrial genomes are highly polymorphic which is very good in human identification. There are two changeable regions HV1 and HV2 these are improved and sequenced to associate the evidence with the sample.
MtDNA genes are found in high amounts because of this it is an ideal candidate to use when analysing samples which have been tainted or mixed up or lack nuclear DNA. In a forensic context this analysis is often used on evidence such as hair it can be used on hair which has no root available to sample as well on evidence like teeth and bones. These types of evidence can contain samples of DNA which have been tainted. Again the reason why these prove so successful is that a profile can still be made because the mitochondria appears in high enough amounts to be able to fit a profile together for that individual.
Paternity testing involves the use of genetic data following two steps.
The genotype of the mother and child is given. It must be taken into consideration how many randomly accused males can be excluded as potential fathers. This is called random man not excluded. Evaluation is made by level of average degree of polymorphism at the loci (average over all the possible combinations genotypes of both mother and child). This step of exclusion probabilities can be undertaken without having the assumed father's sample present.
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The second step is when the assumed father is not excluded (based on the genotype of the assumed father in combination with those of mother and child) a possibility ratio i.e. paternity index can be computed. This basically explains the odds of finding the specific mother child assumed father genotype combination under the assumption that the assumed father is the true biological.