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Takoradi missing girls: what is forensic DNA testing? – Interview

In light of the recent report on the Takoradi missing girls in Ghana, there have been several speculations about forensic DNA testing and its reliability in the media. The Acting IGP of the Ghana Police Service (GPS), James Oppong-Boanuh, declared last month that some recovered human remains belonged to the missing girls, confirming their death. The declaration was based on DNA tests carried out on the human remains by the Police.

To educate and inform the Ghanaian public about forensic DNA testing, our monthly spotlight features an interview with Dan Nana Osei Mensah Bonsu, a Forensic Scientist with specialization in DNA analysis.

Dan is a lecturer at the Department of Forensic Sciences, University of Cape Coast, Ghana. He is also carrying out a doctoral research in Forensic DNA at the Australian Centre for Ancient DNA (ACAD), The University of Adelaide, Australia.

Aaron Amankwaa: What is DNA? Could you explain DNA testing in simple terms for our audience?

Dan Bonsu: DNA stands for deoxyribonucleic acid. The DNA contains all the necessary information required to produce/create any living organism. The DNA code uses four (4) “letters” called bases: A (adenine), C (cytosine), G (guanine) and T (thymine), to encode every molecule required for life. The bases are arranged in a unique order with a G always pairing a C, and an A always pairing a T to form a shape called double helix. Think of the DNA double helix like a spiral staircase (or ladder) whose steps are formed by the G-C and A-T base pairs! You can see a simplified illustration of it in this figure (Fig 1). Human cells contain two types of DNA, namely: nuclear (found in the nucleus) and mitochondrial (found in mitochondrion). There are about 3.2 billion nuclear and 16,000 mitochondrial DNA base pairs in a single human cell.

Figure 1: a) A simplified illustration of the DNA ladder showing the base pairing; b) The DNA double helix structure is like a spiral ladder

Aaron Amankwaa: Where does the DNA come from?

Dan Bonsu: Half of our DNA is inherited from our mother, and the other half from our father, hence, every cell in a person’s body has the same DNA as they came from the combination of one egg and one sperm from the parents. There is DNA in the white blood cells, skin cells (such as those present in saliva), hair, teeth and bones. Since all the DNA of a person is essentially the same, it allows comparison of a bone sample to a blood sample. DNA (or genetic) testing (or profiling) thus examines specific segments of a person’s DNA with variations, to determine whether the individual has any biological relationship with another. The DNA profile of an individual is 50% match to their mother and a 50% match to their father. During forensic investigations, segments of the nuclear genome are frequently used because it has extreme amount of variation from one individual to another which increases the probability of obtaining a DNA profile that will be unique and individualizing.

Aaron Amankwaa: In simple terms, what are the steps involved when a forensic scientist conducts a DNA test on human remains, such as bones?

Dan Bonsu: The DNA is just one of the many molecules contained in a cell. It is, therefore, crucial, as an initial step, to extract it using an appropriate technique. The forensic scientist generally: (a) breaks open the cells of evidence sample (in this case, bone) containing DNA of unknown origin, (b) separate the DNA from all other cell components and finally (c) isolate the DNA.

Because DNA has a negative charge, a positive charge helps to pull it away from all the other molecules, enabling the scientist to collect clean DNA for further processing. Isolating DNA from saliva and blood can take at least two hours.

Bones generally contain very little DNA and extracting DNA from such hard tissues is more difficult due to the enormous amount of calcium and other bone material. The isolation of DNA from a sample (e.g., blood) collected from a known individual (e.g. family member), which will be used for comparison is generally done at a later time.

Once the DNA has been isolated, it must be quantified to determine how much, if anything, was found. Specific regions of the DNA that are known to be variable and unique to individuals are targeted (because the over 3 billion base pairs in a human cell are too big to analyse routinely) and a DNA code is recorded. The variations in the test results from these specific regions of the DNA are then determined.

In the next step, test results from the unknown and known samples are compared using statistical analysis, to ascertain the likelihood of a familial relationship. Finally, a case report is generated, with an interpretation of the analysis and communicated to the interested parties, as per the guidelines of the specific jurisdiction.

Aaron Amankwaa: What types of DNA analysis can be carried out, and what is their investigative value?

Dan Bonsu: Choosing the appropriate type of DNA analysis is closely linked with the nature of the investigation, the condition of the DNA sample, whether or not a reference (known) sample can be obtained and the specific question(s) to be addressed by the forensic scientist. Three (3) types of DNA analysis, namely: autosomal DNA (atDNA), mitochondrial DNA (mtDNA) and Y-DNA test can be done. Each test type examines a different part of the DNA as you can see in this figure (Fig 2) and can provide valuable information to the investigator.

Figure 2: A simplified scheme of the human cell. The nucleus in the centre contains autosomes (chromosomes 1 to 22); sex chromosomes (chromosome pair 23). The mitochondrion found outside (in the cytoplasm) the nucleus is inherited only from the mother. Nuclear DNA consists of autosomal DNA from the autosome), X-DNA and Y-DNA from the sex chromosomes. The mitochondrion contains the mitochondrial DNA (mtDNA)

Aaron Amankwaa: Can you explain a little bit more about these tests?

Dan Bonsu: Y-DNA analysis: Only males inherit the Y-chromosome from their father. Hence, a Y-DNA analysis is performed if we want to find out whether samples from one male matches with samples from other males by comparing their Y-chromosome markers. If the male test subject matches descended from a family bloodline just as any of the others tested, then their Y-DNA will be the same or very similar. This type of DNA analysis is only relevant to direct paternal line (son via father, grandfather, great grandfather, great-great-grandfather, etc.)

mtDNA analysis: Every human (male or female) has mtDNA inherited from their mother, unlike the Y-chromosome which only passes from father to son. Fathers cannot pass on their mitochondrial DNA to their children, though the mtDNA of males can be tested. An mtDNA analysis compares the mitochondrial DNA of a person with that of others, and a match indicates descendants from the same female ancestor. The sequence of mtDNA inherited through the maternal lineage remains unchanged and can allow comparison of a close maternal relative to that of an unidentified person. mtDNA profiles can provide circumstantial identification and may be used to estimate a person’s age at death [1], especially from bone and teeth samples.

atDNA analysis: Autosomal DNA is a random mix of DNA segments inherited directly from both parents and passed down from several generations of grandparents. Therefore, unlike Y-DNA and mtDNA which relate specifically to a male or female, any person of any gender and age can be tested, with the most reliable and accurate matches often recorded for recent generations and immediate family. atDNA tests may include X-DNA analysis if inheritance patterns that apply specifically to the X-chromosome are of significant investigative value.

Aaron Amankwaa: How would the scientist tell if the DNA comes from a particular person and no other living or deceased person? What is the meaning of a match?

Dan Bonsu: As noted earlier, test results from an unknown sample (e.g. skeletal material) are compared to a reference panel: a known sample (e.g. a probable family member) or a DNA database. Based on the specific variable regions of the unknown and known samples processed, confidence scores are generated that indicate the level of similarities and differences between both samples. Similarly, the unknown sample can be compared to all DNA profiles held in a database, to provide a list of people (living or dead) whose profiles have similarities (or matches) with the unknown sample. The match is thus a score (amount/percentage) that indicate the estimated relationship, based on shared DNA, between the unknown and known sample or profile(s) in a database.

Aaron Amankwaa: Based on the similarity score, can we conclusively say that a particular DNA profile comes from a specific person without any other corroborative evidence?

Dan Bonsu: The scores generated, based on the comparison, informs the scientist of how much DNA evidence there is for the unknown sample to be actually related to another DNA profile match. It is, therefore, possible to decide that a profile from a specific person shares an adequate amount of DNA with a match to the exclusion of others and that the relationship is recent enough to be conclusive. DNA testing can be solely used and is accepted in several jurisdictions to conclusively determine identity, without any other corroborative evidence due to the robustness of the technique. However, there should be caution in such interpretations since there could be coincidental matches because only parts of the entire DNA is analysed. For example, the DNA of identical twins may be the same. Siblings and other relatives have a higher level of similarities in their DNA. As stated earlier, Y-DNA and mtDNA profiles may be similar for several relatives (known or unknown). Ideally, there should be other corroborative evidence to support any conclusions from the DNA match. Further, there should be transparency in what types of analyses and comparisons were carried out and the probability score generated.

Aaron Amankwaa: What is the margin of error in the conclusions of the scientist? or could there be any errors?

Dan Bonsu: There is no specific margin of error as this is test and case dependent. Each statistical analysis of a match takes into account the odds of the DNA profile belonging to a specific person determined other than a randomly-selected, unrelated person in the population who was not tested. For a full matching DNA profile made up of 20 markers or areas from autosomal DNA, the chance of a random match is very low. The chance of a random match increases with partial profiles or matching profiles with low markers. This is why corroborative evidence may be required to support the DNA match evidence. Apart from these interpretation issues, errors may arise if there is any contamination of samples. For this reason, there should always be a clear chain of custody in the analysis of DNA and techniques to minimize contamination.

Aaron Amankwaa: Thank you very much for your time and its been lovely talking with you Dan. Before we conclude, Do you have any thoughts on forensic science practice in Ghana?

Dan Bonsu: Perhaps, it is time for the government and stakeholders to take a critical look at all aspects of forensic science training and practice in Ghana. In a recent publication in Forensic Science International: Synergy [2], we espoused relevant policy issues encompassing legislation, governance framework, service provision, quality assurance issues, education, research and development, and impact of forensic science on the criminal justice system in the country.

References

1. Zapico S, Ubelaker D. Relationship between mitochondrial DNA mutations and aging. Estimation of age-at-death. J Gerontol A Biol Sci Med Sci. 2016; 71:445–450

2. Amankwaa AO, Nsiah Amoako E, Mensah Bonsu DO, Banyeh M. Forensic science in Ghana: A review. Forensic Science International: Synergy. 2019; 1;1:151–60.

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3 replies »

  1. Very informative .
    I am student Statistician and I am really enlightened on the confidence interval and sampling aspects.

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