Researching a novel - anyone know anything about DNA

[selliot 10]

Thank you so much everyone for these fantastically helpful replies - you are a clever and knowledgable lot!! It's such a complicated area, and I find I just think I've got it right when something else comes up to throw it all in the air again. Anyway, you've clarified some things for me, and I now have several more avenues to explore. Thank you all you so much! When (if!) the book is eventually published, you helpful forummers will definitely get a mention in the acknowledgements. Watch this space!

[Crayfish 10]

I wonder if anyone can help me with some research for my novel – there’s so much expertise on the Forum that I thought I’d start here! (I’ve already done quite a bit of research, but every re-draft raises more questions!)

 

In my story, a man’s DNA is fed into the national database after he’s arrested for a minor offence. The database throws up a ‘familial link’ to the profile of the unknown perpetrator of crimes dating back to the 70s. The police think the offender must be my character’s father. I know that my character’s DNA would be similar enough to that of his father to throw up a partial match. But how similar would it be to the profiles of, for example:

 

His father’s brother

His father’s son by another woman (not our man’s mum)

His mother’s brother

 

Basically, if the police came saying ‘your DNA profile is very similar to one we have on file, we think it’s your father’. Could my man reasonably argue that it’s as likely to be an uncle or half-brother?

 

Would be very grateful for any help!

 

As I understand it:

 

It should be 50% to the father and about 25% to each of the others... approximately, in theory, over the whole genome. The loci tested for forensic purposes (short tandem repeats) may be slightly more variable.

 

The DNA profile tests carried out for database / 'fingerprinting' purposes are not the same ones that would be carried out for genealogical purposes or paternity tests. Paternity tests generally use a set of Y-chromosomal loci.

 

Alone, the fingerprinting data could suggest relatedness but would not be sufficient evidence without further tests, and would be unlikely to be flagged as similar on the database without prior suspicion, as with this battery of tests, a few people on the database may well be 50% similar without being closely related.

 

If they already suspected his father and did notice this relatedness for some reason, more detailed tests would be required to confirm it (assuming that reasonably intact biological material from the father is still available).

 

However, fiction often takes small scientific liberties and not many people would know the details of forensic DNA testing.

 

Here's wikipedia's take on the matter, which includes links to various sources http://en.wikipedia.org/wiki/Genetic_fingerprinting#cite_note-7

 

Additionally, I don't know the setting of your novel, but sequencing methods improve quickly and it will probably be cost effective to gather much more detailed data in 5-20 years time (I guess this wouldn't affect the original tests on his father, though).

Edited May 5, 2009 by Crayfish

[selliot 10]

As I understand it:

 

It should be 50% to the father and about 25% to each of the others... approximately, in theory, over the whole genome. The loci tested for forensic purposes (short tandem repeats) may be slightly more variable.

 

The DNA profile tests carried out for database / 'fingerprinting' purposes are not the same ones that would be carried out for genealogical purposes or paternity tests. Paternity tests generally use a set of Y-chromosomal loci.

 

Alone, the fingerprinting data could suggest relatedness but would not be sufficient evidence without further tests, and would be unlikely to be flagged as similar on the database without prior suspicion, as with this battery of tests, a few people on the database may well be 50% similar without being closely related.

 

If they already suspected his father and did notice this relatedness for some reason, more detailed tests would be required to confirm it (assuming that reasonably intact biological material from the father is still available).

 

However, fiction often takes small scientific liberties and not many people would know the details of forensic DNA testing.

 

Here's wikipedia's take on the matter, which includes links to various sources http://en.wikipedia.org/wiki/Genetic_fingerprinting#cite_note-7

 

Additionally, I don't know the setting of your novel, but sequencing methods improve quickly and it will probably be cost effective to gather much more detailed data in 5-20 years time (I guess this wouldn't affect the original tests on his father, though).

 

Thanks Crayfish, again, very useful! You're right that not many people would know the details - it's a question of trying to get enough details right to make the 'scientific liberties' plausible. Thanks again.

[esme 10]

remember there is a huge difference between the dna profile used in forensic testing and the dna pattern as used in the human genome project

 

a dna profile gets produced by cutting up a sample of DNA with enzymes into different sized chunks and this is passed through an electrophoresis gel with bigger bits not travelling as far as smaller bits in the same time, the pattern you get is used as a profile, it's similar to chromatography which you may have done in chemistry at school with an ink blot on filter paper to separate out the component dyes

 

the dna pattern is a list of all the base pairs in sequence and as such is a completely accurate representation of a persons dna

 

you also need to bear in mind that once in the environment DNA degrades over time, light, chemicals and even bacteria will all tend to compromise a sample over time until eventually it is useless

 

and don't forget the confidence limits on the results, no forensic expert will say "this dna profile matches that person exactly" instead they will give a probability of a match of say 1 in 10,000 or 1 in 50,000 and this needs interpreting correctly, most people hear "1 in 50,000" and think there's no chance of a match with anyone else, what this really means is that for every 50,000 people they would expect 1 match and in a country of 65 million that means there should be around 1300 matches and familial groupings will tend to cluster these in specific areas so it's not 1300 over the entire country it's more like 1300 over the entire country with around 1000 within a 10 to 15 mile radius, though I'm guessing on the radius figures as I've seen no research for it

Edited May 5, 2009 by esme

[selliot 10]

remember there is a huge difference between the dna profile used in forensic testing and the dna pattern as used in the human genome project

 

a dna profile gets produced by cutting up a sample of DNA with enzymes into different sized chunks and this is passed through an electrophoresis gel with bigger bits not travelling as far as smaller bits in the same time, the pattern you get is used as a profile, it's similar to chromatography which you may have done in chemistry at school with an ink blot on filter paper to separate out the component dyes

 

the dna pattern is a list of all the base pairs in sequence and as such is a completely accurate representation of a persons dna

 

you also need to bear in mind that once in the environment DNA degrades over time, light, chemicals and even bacteria will all tend to compromise a sample over time until eventually it is useless

 

and don't forget the confidence limits on the results, no forensic expert will say "this dna profile matches that person exactly" instead they will give a probability of a match of say 1 in 10,000 or 1 in 50,000 and this needs interpreting correctly, most people hear "1 in 50,000" and think there's no chance of a match with anyone else, what this really means is that for every 50,000 people they would expect 1 match and in a country of 65 million that means there should be around 1300 matches and familial groupings will tend to cluster these in specific areas so it's not 1300 over the entire country it's more like 1300 over the entire country with around 1000 within a 10 to 15 mile radius, though I'm guessing on the radius figures as I've seen no research for it

 

Thanks for this, esme. I take your point! If I understand it correctly, the dna similarity would only form one part of the evidence in a case like this? I've spoken to a few police officers about the storyline, and they seem to think it was plausible, but that was before I was being asked for more specific details! Most of the idea for this particular storyline in my novel came from the case of the dearne valley shoe rapist, who was caught after his sister committed a minor offence and her dna showed a partial match. Would it be more believable if the police were interviewing my character as one of, say, 30-40 people with a similar dna profile to the one created from the original crime scene?

[Dave59 11]

Hi Selliott

 

Regarding your question.

 

If the DNA test showed up a familial link, then it would be just that - familial. The police would be able to say that this sample links to a general family group, as in the case of the Dearne Valley Rapist. In some recorded cases however, the police have generated a group of up to 100 persons having a similar profile, this profile was then used not as evidence in a trial, but as a basis for further investigation leading to a conviction.

 

Medusa raises an interesting point regarding the loci. In the early days of DNA testing, when the forensics only tested a low number of loci, there were a number of false positives. Perhaps most interesting was the case of a disabled, man with extremely limited mobility who was suspected of being involved in a burglary that included climbing through a window. Perhapsd this might be an interesting addition to your story?

 

I have included a link with relevant research to the stories I mentioned.

 

With my regards

 

DD

 

http://www.genewatch.org/uploads/f03c6d66a9b354535738483c1c3d49e4/NationalDNADatabase.pdf

[Hopman 46]

If you have a library card you can access information via the Council/library website 24 hour service.

There will be sources linked to the Credo Reference service.

[selliot 10]

Hi Selliott

 

Regarding your question.

 

If the DNA test showed up a familial link, then it would be just that - familial. The police would be able to say that this sample links to a general family group, as in the case of the Dearne Valley Rapist. In some recorded cases however, the police have generated a group of up to 100 persons having a similar profile, this profile was then used not as evidence in a trial, but as a basis for further investigation leading to a conviction.

 

Medusa raises an interesting point regarding the loci. In the early days of DNA testing, when the forensics only tested a low number of loci, there were a number of false positives. Perhaps most interesting was the case of a disabled, man with extremely limited mobility who was suspected of being involved in a burglary that included climbing through a window. Perhapsd this might be an interesting addition to your story?

 

I have included a link with relevant research to the stories I mentioned.

 

With my regards

 

DD

 

http://www.genewatch.org/uploads/f03c6d66a9b354535738483c1c3d49e4/NationalDNADatabase.pdf

 

Thanks Dave59 - I've pm'd you

[selliot 10]

If you have a library card you can access information via the Council/library website 24 hour service.

There will be sources linked to the Credo Reference service.

 

Thanks Hopman - I didn't know about this!

[esme 10]

Thanks for this, esme. I take your point! If I understand it correctly, the dna similarity would only form one part of the evidence in a case like this? ...

yes it's only part of the evidence although there are officers who still try to tell you that "we've got your DNA we don't need anything else" to try and elicit a confession

Would it be more believable if the police were interviewing my character as one of, say, 30-40 people with a similar dna profile to the one created from the original crime scene?

probably but here's a question, how are the 30-40 people with similar dna going to be identified in the first place, the police dna records currently only hold records on people who have been arrested for something even if they have subsequently been released without charge and the court of human rights has told the government in no uncertain terms to remove and destroy the dna of those not convicted of a crime, which somewhat surprisingly they seem to be doing http://uk.news.yahoo.com/16/20090506/ttc-government-revises-dna-database-plan-6315470.html, so you'd be left with known family members, which omits the possibility that grandad put it about a bit when he was younger and your protagonist has relations living in the area they know nothing about who can match the sample at least as well as they can

 

so the ideas getting there but there's a few things you need to think about

 

...and I'll settle for a percentage of the gross when you are rich and famous

[selliot 10]

yes it's only part of the evidence although there are officers who still try to tell you that "we've got your DNA we don't need anything else" to try and elicit a confessionprobably but here's a question, how are the 30-40 people with similar dna going to be identified in the first place, the police dna records currently only hold records on people who have been arrested for something even if they have subsequently been released without charge and the court of human rights has told the government in no uncertain terms to remove and destroy the dna of those not convicted of a crime, which somewhat surprisingly they seem to be doing http://uk.news.yahoo.com/16/20090506/ttc-government-revises-dna-database-plan-6315470.html, so you'd be left with known family members, which omits the possibility that grandad put it about a bit when he was younger and your protagonist has relations living in the area they know nothing about who can match the sample at least as well as they can

 

so the ideas getting there but there's a few things you need to think about

 

...and I'll settle for a percentage of the gross when you are rich and famous

 

Thanks esme, great stuff! I've pm'd you with another question - hope you don't mind. As for when I'm rich and famous, there's a bit of a queue forming... I think I'm going to end up owing my soul to the forum anyway!

[Bago 10]

Thanks for this, esme. I take your point! If I understand it correctly, the dna similarity would only form one part of the evidence in a case like this? I've spoken to a few police officers about the storyline, and they seem to think it was plausible, but that was before I was being asked for more specific details! Most of the idea for this particular storyline in my novel came from the case of the dearne valley shoe rapist, who was caught after his sister committed a minor offence and her dna showed a partial match. Would it be more believable if the police were interviewing my character as one of, say, 30-40 people with a similar dna profile to the one created from the original crime scene?

 

You also have to remember that frontline officers may not necessarily know of the elimination factors within the forensics department, and the detail of the forensic aspect unless they are of that department.

 

I do not think that the judges would use one single evidence to conclude acase. Most people would throw it out by using one single source alone. A lot of cases are using various evidences to support and deduce the culprit.

 

What makes a gripping story is how the culprit was deduced in the end, and what gave the game away.

 

You should indeed pick up a book on forensics, as it is quite fascinating. I remember doing a module in this at uni. I now find it funny that CSI is so popular! That program covers that of what you get in a basic university module, or A-level standard.

 

I think we've raised your game a bit here.

 

Make that 2% for me please.