what is the problems of DNA evidence?

Answer 1

It is easy to contaminate samples and generate spurious results.

In forensic science you still have to match against a "suspect" and a DNA database of everyone is not a good thing froma civil liberty point of view.

Answer 2

The problem is that it is believed in and of itself.
In every process there is the possibility of error, DNA fingerprinting is a complicated and involved process.
It is as accurate as they say, false positives exist but only one in a few billion.
And that's the problem, someone messes up the result, and it's you who pays the price. No questions asked.

Answer 3

Twins are the problem. If your identicle twin commits a crime, then you have a problem because your DNA is identicle, so you could get the blame. If you slept with twins and found you were pregnant, you cannot find out who the father is. Also, it is not 100% accurate.

Answer 4

There is no problem.

Each persons DNA is unique, so it's the best evidence there is

Answer 5

lowheight; low weight are the problem off dna

Answer 6

It can send you too prison. i was Innocent i tell you Innocent

Answer 7

I didn't know there was any !!!

Answer 8

This is a example of a mess up of DNA
Sorry this is a true example of a court case. If this offends please do not read. I am trying to show how a apeal can be quashed through inconclusive DNA

[1] Following the reference of his case to this court by the Scottish Criminal Cases Review Commission the appellant has appealed against his conviction on 30 November 1989 on a charge of rape. The grounds of appeal maintain that there was a miscarriage of justice in respect of the existence and significance of additional evidence relating to DNA which was relied upon by the Crown at his trial, and that there is a reasonable explanation why that evidence was not led at his trial.

[2] At the trial the complainer gave evidence that she awoke to find a man kneeling on her bed beside her. He said something to her about jewellery. Although she pointed to her jewellery box, he showed no interest in it. He then asked her for money and she told him that any money which she had was in her purse. He then asked her how many children she had and where her husband was. It appeared that he engaged her in a good deal of conversation. He asked her whether the number of her house was 37, and she told him that it was 59. He seized her by the throat with both hands. He produced a knife which he used to cut the neck of her nightdress, after which he raped her. Almost immediately thereafter he told her to come downstairs with him, and not to put on any of the lights in the house. He made her walk in front of him while he held a knife against her back. At the bottom of the stairs he steered her into the livingroom. She gained the impression that he knew the layout of the house and in particular knew that there was a telephone in the livingroom. He told her to hold the telephone cable in both hands while he cut it with the knife. She recognised the knife as being a carving knife which was normally kept in a cutlery drawer in her house. He then told her to get the key for the back door, and asked her for money again. After she had found her purse he told her to take banknotes out of it and put them in his jacket. As he left he told her that "this was from a friend and if he phones me again, I'll be back". After he left, the complainer went upstairs to check that her children were all right. She then went to the house of a neighbour and informed her that she had been raped and asked her to contact the appellant's wife, who was a close friend.

[3] At the trial the Crown relied on a number of pieces of circumstantial evidence as pointing to the appellant as the attacker. A seminal stain on the complainer's dressing gown was found to have the same blood grouping as the appellant, as well as 39.5% of the population. There was also evidence of a pattern left by a training shoe on a worktop in the kitchen, which was similar to that on the sole of a training shoe belonging to the appellant. The Crown also relied on evidence as to the finding of a knife, which the complainer identified as being the knife which had been taken from her house, on one of a number of possible routes which might have been taken by a person going from her house to that of the appellant. The Crown also maintained that the appellant had had the opportunity to commit the crime during a period between the time when he left a friend and the time when he arrived home.

[4] Finally, and most importantly, the Crown relied on DNA evidence given by Dr. Paul Debenham of Cellmark Diagnostics that, of the 13 single locus probe (SLP) bands detected from the seminal stains found on the complainer's dressing gown, seven matched the profile obtained from a sample of the complainer's blood, while six matched the profile from the appellant's blood sample. Evidence was given that the chances of a match of six such bands in two unrelated persons was less than 1:100,000. It may be noted that a previous DNA analysis by the Home Office Central Research Establishment had proved inconclusive.

[5] At the trial the complainer did not identify the appellant as being her attacker. This was despite her evidence that she knew the appellant well, and that the attacker had spoken to her at some length. She did not suggest that her attacker had been masked or otherwise disguised. She did not appear to be reluctant to identify the appellant as her attacker. Evidence was given by one of the police officers who examined the scene that the bedroom curtains were made of heavy lined material and were closed. When the bedroom door was closed and the room was not lit, it was almost pitch black and impossible to see anything. The complainer stated in evidence that she was unable to describe the face of her attacker. However, his voice was "strange", as it was very deep and "throaty". She did not believe that he was disguising his voice. Prior to the trial the complainer attended two identification parades. The first of them was a "voice only" parade in which the appellant took part. She did not identify his voice. She asked to hear the voice of one of the stand-ins a second time, but concluded that she was not hearing the voice of her attacker. At a second parade, which was attended by another suspect, she indicated that a stand-in sounded quite like her attacker. She later identified a suspect as having the same build as the attacker, but was uncertain whether he was tall enough.

[6] The defence relied on evidence that the appellant had consumed a considerable quantity of alcohol that evening. It was suggested that this would have rendered him unable to climb in through the kitchen window of the complainer's house without arousing the attention of the complainer or her children. It was, moreover, inconsistent with evidence given by the complainer that the attacker had been "clean smelling". Further, a police officer who had gone to the appellant's house less than an hour after the attack found that he was smelling of drink. The defence also founded on the fact that the complainer had not recognised the appellant. She had given evidence that the attacker had asked her a number of questions about her children. Since the appellant was familiar with the complainer and her personal circumstances, he would have already known the answers to these questions. Reliance was also placed on evidence that a pubic hair, which had been found on the pillow of the complainer's bed, was inconsistent with samples taken from the complainer, the appellant and the complainer's father who had slept in that bed. As regards the DNA evidence, it was criticised as being misleading, in the light of the fact that the frequency calculations were based only on data in regard to 200 Caucasians in Cheshire. Prior to the trial the appellant's solicitor had obtained a report from Dr. Patrick Lincoln of the London Hospital Medical College, dated 20 November 1989. Dr. Lincoln stated in the report that he was unable to exclude the appellant as a possible source of the DNA found on the complainer's dressing gown. In his view, even taking account of the deficiency of the Cellmark database, the DNA evidence against the appellant was still "extremely strong". The defence also advanced a special defence of alibi. However, this was of limited importance, since the appellant appeared to be unable to account for his movements during a period of about one hour during which the crime was committed.

[7] In connection with this appeal the court authorised the hearing of additional evidence, which was led by the appellant and by the Crown. In order to assist the court in understanding the factual background to the DNA evidence, the Crown evidence was, by agreement, taken first. The witnesses led by the Crown were (i) Dr. Debenham, who had specialised in DNA research and the carrying out of DNA analysis throughout his working life. He was employed by Cellmark between 1987 and 1993. He is currently the Director of Life Sciences with another company; (ii) Mrs. S.J. Howes, who was employed as a DNA scientist with Cellmark from March 1987. She is now employed as a consultant in DNA diagnostics; and (iii) Mr. Lester Knibb, who has 28 years of experience as a forensic scientist, and is the Biology Manager of the Lothian and Borders Police Forensic Science Laboratory. The appellant led the evidence of (i) Mr. Simon Ford, a scientist who has been involved with DNA technology since the 1970s. He developed an interest in the application of that technology to forensic problems in the middle of the 1980s. He has frequently acted as a consultant and an expert witness in reviewing DNA evidence in court cases in the United States of America, and has been critical of DNA analysis; (ii) Dr. A.M.T. Linacre of the Forensic Science Unit of Strathclyde University. He has given evidence about DNA analysis in many cases in the United Kingdom and abroad; and (iii) Dr. W.H. Goodwin, another specialist in DNA analysis, who was formerly in the Department of Forensic Medicine and Science of Glasgow University, and is now in the Department of Forensic and Investigative Science of the University of Central Lancashire.

[8] The method of forensic DNA analysis by SLP, as performed by Cellmark in 1988, was, in outline, as follows. The laboratory received samples such as a semen stain from the scene of a crime, and blood samples from the complainer and a suspect, to which were given identification numbers. The DNA from each sample was extracted and cut into fragments by means of an enzyme. Each sample was mixed with a blue dye and loaded into a well which had been formed in an agarose gel. In accordance with the practice which was followed at that time, samples from the complainer and the suspect, such as the appellant, were loaded into a well on either side of the well into which the crime scene sample was loaded. As a result of being exposed to an electrical field the DNA fragments moved within the gel, so as to form a pattern of bands. That pattern was transferred to a nylon membrane by means of a blotting process. The membrane was then exposed to a number of radioactive DNA probes in solution, which resulted in the DNA in each probe binding with complementary sequences on the membrane in a process called hybridisation. Between the use of each probe the membrane was washed in order to remove any remaining excess probe. The radioactivity enabled the pattern to be detected on x-ray film, which was developed in order to make the pattern of DNA bands visible.

[9] In the present case the carrying out of a probe known as G3 demonstrated two bands in the lane relating to the DNA extracted from the semen stain on the complainer's dressing gown (17540). One of these bands corresponded to a band in the lane relating to the reference sample from the complainer (17683). The other band appeared to correspond to a band in the lane relating to the reference sample from the appellant (17685), but was of a lower intensity than it. The evidence of Dr. Debenham at the trial was based on the correspondence between the band in the lane relating to the crime scene sample and that in the lane relating to the appellant. In the appeal it is maintained that there was a miscarriage of justice in respect that there is evidence, which was not led at the trial, that there was a risk that this apparent correspondence arose from the DNA in 17540 being contaminated by DNA from 17685.

[10] Evidence was given to this court by Dr. Debenham and Mrs. Howes as to the qualification and experience of the scientists who were allocated to the task of performing the DNA analysis. The procedures followed standard operating procedures. While one scientist loaded the DNA into the wells, another scientist was present in order to see that the samples were loaded into the correct well. In the case with which we are concerned, Mrs. Howes and Dr. Debenham carried out these respective roles. Mrs. Howes explained that the DNA samples were dyed blue in order to enable them to be seen against the clear gel and the buffer solution in which it was submerged. The volume of the sample was set automatically by the pipette, which had a maximum volume of 20 microlitres. The well had a volume of 30 microlitres. The pipette was taken directly to the relevant well and not over other wells. The tip of the pipette was smaller than the well and was easily accommodated. The pipette was emptied completely before being removed from the well. The DNA sample in solution with the dye was heavier than the buffer solution and would sink through it. It also had a surface tension which, on its entering the buffer solution, held it in a defined shape. Dr. Debenham explained that at the time it was considered to be important to have the reference samples adjacent to the crime scene sample in order to ensure accurate measurement. There were concerns that DNA fragments could migrate more rapidly through different parts of the gel as a result of properties in the electrical field. Thus if DNA fragments of equivalent mass were separated, there was a risk that such DNA would travel different distances and could not then be accurately compared.

[11] We turn then to evidence as to the risk of cross-contamination. Dr. Ford and Dr. Linacre gave evidence that it was possible for material to move from one well to another as a result of slight imperfections in the gel which were caused at the time when the comb which was used to form the wells was removed. Dr. Linacre also stated that sometimes there were small deposits of salt in the wells which caused a problem when DNA was being loaded. Further, small air bubbles in the pipette could cause fluid to be carried to the surface in the well. In regard to the loading process, Dr. Ford stated that it was possible for a little piece of biological material to fall, or be spilt, into a different well from the intended one.

[12] It is clear that Dr. Debenham and Mrs. Howes were very experienced scientists who had worked with the formation and loading of gel trays for some time. In 1988 Cellmark was the leading exponent of DNA testing in the United Kingdom. Dr. Debenham did not dispute that it was possible for DNA material to reach the wrong well. Apart from the examples given above, he accepted that the effect of forcibly ejecting a sample out of the pipette could cause part of it to enter an adjoining well. However, he maintained that any contamination of this sort would have been avoided by adherence to proper procedures or by detection in the course of the work. He emphasised that both the operator and the witness were very well experienced in gel loading and would have taken extreme caution to ensure that no transference could occur. Since it was known that cross-contamination could be an issue, they worked stringently to avoid it. Mrs. Howes said that in her 10 years of carrying out such work, she had never experienced difficulty with a gel tray. Gel trays were "pretty robust". Gel was left to set for longer than was actually necessary. The comb which was used to form the wells was removed carefully, and the wells checked visually in order to ensure their integrity. The operator had to make sure that there were no air bubbles and that the buffer liquid had completely filled each well. As regards the loading technique, the operator would look for the sample coming out of the tip until the pipette was empty. It was quite easy to see that this was the case. The operator did not move the pipette across other wells before emptying it. After it was emptied the pipette would be raised vertically so that anything which was left in its tip would fall straight down into the well. The witness would be looking at the operation on a slightly higher level than the level of the tray. She had seen a gel tray which had been spoiled due to the way in which a sample had been loaded, but "it is quite difficult to do". She had never personally spoilt a tray (transcript at 267). If DNA had travelled after it had entered the well the operator would see traces of this. The developed film in the present case showed that tracks appeared to be all within the same width as the cells. As far as she was concerned there was no possibility that the gel might have been damaged.

[13] It is, however, necessary for us to bear in mind the evidence given by Dr. Linacre. He said that it was very difficult to see small damage which had occurred between wells when the comb was removed. If there was a flow from one lane to another into which nothing had been loaded, it would be possible to see a small leakage of blue dye. One of the main purposes of the dye was that it would be seen. However, this would not necessarily be seen if the latter lane was one into which a sample had already been loaded. It may be noted that the order in which the samples had been loaded, in accordance with normal practice, was that the crime scene sample or samples were loaded after the reference sample from the complainer, and before the reference sample from the suspect. Dr. Linacre observed "Obviously one attempts for this not to happen, but blue into blue is difficult to spot" (537).

He added that in the case of the loading of polycrylamide gels in the forensic science laboratory leakage in the gels occurred quite frequently.

[14] Mr. Knibb expressed the opinion that under all but the most unusual circumstances the results which had been obtained in the present case indicated the presence of DNA in the crime scene sample which matched that of the appellant. It was unlikely in the extreme to have been the result of error during analysis. He had reviewed records for a number of years and had found only one clear example of leakage from one well to another. He added, however, that he would only find what had been detected. Spillage did happen from time to time, but the chances of it going undetected were, in his view, and in his experience, "virtually nil or very, very rare" (348).

[15] Dr. Linacre said, in regard to the suggestion that there was a risk of leakage from one well into an adjacent well,

"It is not simply a risk. It is something that having loaded countless, hundreds of gels in my time, it is something that does happen". (534).

He found it difficult to comment on whether it would be seen by the operator and the witness. The risk of their not seeing the spillage was not negligible, although it would be reduced because they were skilled (557). He could not exclude cross-contamination (543).

[16] Dr. Goodwin said that there was an element of risk of cross-contamination, adding "I would move it towards the low risk. Not necessarily towards the end of the low risk" (574). He also said

"I think individual tests have to be taken on their own merits. If, for example, there would be bands within this autorad of similar intensity, I would say that it was very unlikely that it was caused by leakage almost because you would have had to have had half the sample leak over. Where in this case you get the crime scene sample at a lower intensity so less DNA than in the reference sample, it makes any leakage more possible". (574-575).

He agreed that crime scene samples often showed a low intensity because they had less DNA in them but added:

"I would say that this is a flaw in the methodology used in this case, that the crime scene sample was next to the reference sample". (575).

He could not exclude the possibility that spillage had occurred (576).

[17] Dr. Ford gave evidence in regard to the publication by the National Research Council in the United States on DNA Technology in Forensic Science in 1992. Under the heading "Anomalous Bands" it stated at page 59:

"Another possible cause of extra bands is leakage between adjacent sample lanes or misloading of two samples in a single lane. Such an occurrence can be exceedingly difficult to detect and could result in an incorrect conclusion. It is therefore important to leave a blank lane between a suspect sample and an evidence sample, so that leakage can be detected and will not lead to false - positive results".

It is evident that in about 1990 the method of DNA testing changed generally and in the Cellmark laboratory. Under the new procedure crime scene and reference samples were not placed in adjacent wells. Dr. Ford stated that the change was made because laboratories in the United States became aware that there was a danger of cross-contamination if an empty well was not left in between, so that the operator could see if there had been a spillage of any kind. There were also references in the literature, and the point was being taken in legal proceedings. As a result it rapidly became part of normal practice for a well to be left empty. Dr. Debenham said that, as he recalled, the change came about as a result of a combination of two things. Due to the development of improved marker systems which enabled the size of DNA fragments in different tracks to be compared, it ceased to be necessary to do "side by side comparisons for the analysis" (46). Later he referred to

"a new, far more detailed molecular probe ladder which came along with probes that are no longer used with radioactivity ... that combined I presume with reacting to issues possibly raised in the US. I wasn't involved with the transition at that time but that must have been the combination of events ... " (146-147).

The evidence of Mr. Knibb (at pages 393-394) was to the same effect.

[18] As can be seen from the evidence to which we have already referred, there was no suggestion that any of the results of the DNA tests in the present case positively suggested that there had been cross-contamination. Dr. Debenham maintained that certain of the results pointed away from that being the case. He referred to a photograph which, he said, showed substantial florescence in the well of the reference sample for the appellant. In his report dated 6 June 2002 to the Crown Office he stated:

"This is presumably due to some proteinated DNA within the reference sample that would not enter the gel matrix. If this reference sample had overflowed into the crime sample well, even at trace level, then some of this same DNA would be expected to be seen fluorescing in the crime scene sample loading well. The photograph shows no evidence of fluorescent in the crime scene sample well at all".

He went on to state:

"The G3 probe autoradiograph of the gel reveals a band of high molecular weight DNA within the Kelly reference sample. This DNA 'band' is an electrophorsis artefact associated with high molecular weight DNA within the sample, probably uncut by the restriction enzyme step, which enters the gel but is severely retarded in its migration in the gel ... If any of the reference sample had transferred into the crime sample then this constituent of the reference sample would have equally transferred to create such a band in the crime scene sample. It is quite clear that there is no trace of this DNA band present in the G3 probe result from the crime scene sample".

In his evidence he stated that he could not think of any mechanism by which there could not have been a transfer from the reference sample to the crime scene sample (88). Referring to the photograph he said:

"One would have expected some degree, even a trace, of brightness reflecting the same elements of the DNA sample having transferred into that well and I can't see it in the original or in the photocopy." (81).

Mr. Knibb in his report stated that the overwhelming evidence for the integrity of the Cellmark procedures lay in the autoradiograph from the G3 probe analysis where a clear and strong band present in the reference sample lane from Mr. Kelly was absent from the adjacent dressing gown sample lane. "If there had been leakage or other contamination the band should have been present in both samples" (349).

[19] Dr. Goodwin stated:

"The fact that this high molecular weight band is present, the undigested DNA is present in the reference sample and not in the crime scene sample in my opinion makes it less likely that spillage has occurred because for that to be present in one and not the other there is a window of spillage that could be permissible before that could start to show up". (577).

On the other hand, Dr. Ford regarded this point as not very compelling.

"In part my problem with that is this thing is an artefact. It is something that we don't know why it is there. It is a problem with the test ... It seems flawed reasoning to use something we don't know anything about, something that is intrinsically a problem in the test, to explain something else". (471).

He would not expect to see anything in an adjacent well unless there had been an enormous amount of leakage (475). There was, in addition, the point that the band referred to was not seen on other probes which were carried out by Cellmark at the time.

[20] We require to be satisfied in the first place that the additional evidence is not merely relevant but also that it is of such significance that it is reasonable to conclude that the verdict of the jury, which was reached in ignorance of its existence, must be regarded as a miscarriage of justice. To that end we require to be satisfied that it is important evidence and of such a kind and quality that it was likely that a reasonable jury properly directed would have found it to be of material assistance in its consideration of a critical issue at the trial. It requires to be capable of being regarded as credible and reliable by a reasonable jury, and likely to have had a material bearing on, or a material part to play in, the determination by such a jury of a critical issue at the trial (Megrahi v. H.M. Advocate 2002 J.C. 99 at paragraph 219).

[21] As we have already noted, it was not suggested that there is evidence positively indicating that cross-contamination did, or may have, occurred. On the basis of the evidence tendered by the appellant, it is maintained, on the other hand, that there was a risk of cross-contamination arising from the practice at that time of using adjoining wells for DNA samples from the crime scene and the suspect, and of such cross-contamination being undetected. It was not in controversy that it was possible for there to be leakage between adjoining wells or for DNA material to fall accidentally into a well next to the one for which it was intended. Up to a point the evidence of Dr. Debenham and Mrs. Howes as to the procedures which were followed, and the special care which was taken, countered the risk that such a mishap would in practice occur or be undetected. However, such evidence does not in our view provide a complete answer. In particular there was, on the evidence, a risk that the leakage of DNA from the well for the suspect's reference sample to the adjoining well which already held the crime scene sample would not be detected. It was, of course, a low risk, but it was of sufficient importance to be recognised by experts, including those who contributed to the 1992 report of the National Research Council to which we have referred.

[22] We have considered the evidence which Dr. Debenham and Mr. Knibb gave as to the significance of a band which was interpreted as representing partially digested DNA. However, that evidence is not so overwhelming that no reasonable jury could have regarded it as not excluding cross-contamination. As we have already indicated, not only the value, but also the validity of that evidence was questioned. In our opinion there is evidence which is capable of being regarded as credible and reliable as to the existence of a risk of cross-contamination occurring without it being detected. The risk was a low risk. It may be that in other circumstances the fact that the jury did not hear such evidence would not lead to the conclusion that there had been a miscarriage of justice. However, in the present case it is otherwise since the DNA evidence was plainly of critical importance for the conviction of the appellant. If the jury had rejected that evidence there would, in our view, have been insufficient evidence to convict the appellant. Accordingly, while the evidence related to a low risk of cross-contamination, the magnitude of the implications for the case against the appellant were substantial. For these reasons we have come to the conclusion that the appellant has established the existence of evidence which is of such significance that the fact that it was not heard by the jury constituted a miscarriage of justice.

[23] The remaining issue is whether there is a reasonable explanation why that evidence was not heard at the trial. At one time it appeared that this would be the subject of strong contention between the parties. In the end of the day, however, little was said about it. We have two reasons for considering that the appellant has shown that this test is met in the circumstances of the present case. First, it is clear that it was only in the late 1980s that concerns were being expressed that undetected cross-contamination could result in the incorrect conclusion in the comparison between samples. This appears to have been raised initially in the United States of America. Secondly, the advice which was obtained by the defence from Dr. Lincoln did nothing to alert them to the possibility that there could be anything amiss in the procedures which led to the conclusions reached by Dr. Debenham in comparing the DNA sample from the scene with that obtained from the appellant.

[24] In these circumstances the appeal will be allowed and the appellant's conviction quashed.