A DNA expert will be available to answer your questions!

[UKGuy]

I appreciate your need to explain these things to us like we're idiots, Anti-K. So allow me to extend you the same courtesy: being excluded from being the DNA donor (if such a word can be applied) is a far cry from being excluded from being the killer.

SuperDave,
mmm, now I did think about referring to chromosomes as inherited from both parents and all that stuff.

But hey, we have a degraded evidence sample of touch-dna, but no reference sample to compare against.

So why do we need a lesson in the Product Rule as per probability, and whats the chance the Prosecutor's fallacy has been voiced here?

Are you suggesting someone could be excluded from being the DNA donor, yet still be the killer of JonBenet?


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[Anti-K]

SuperDave,
mmm, now I did think about referring to chromosomes as inherited from both parents and all that stuff.

But hey, we have a degraded evidence sample of touch-dna, but no reference sample to compare against.

So why do we need a lesson in the Product Rule as per probability, and whats the chance the Prosecutor's fallacy has been voiced here?

Are you suggesting someone could be excluded from being the DNA donor, yet still be the killer of JonBenet?


.

No, we do not have a degraded sample of tDNA.

”No reference sample to compare against” is the norm here. Nothing unusual or bizarre or problematic about this.

The Prosecutor’s Fallacy has not been voiced.

Next?
…

AK

 

[observation]

No, we do not have a degraded sample of tDNA.

”No reference sample to compare against” is the norm here. Nothing unusual or bizarre or problematic about this.

The Prosecutor’s Fallacy has not been voiced.

Next?
…

AK

I may have missed it- but AK are you in forensics?


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[UKGuy]

No, we do not have a degraded sample of tDNA.

”No reference sample to compare against” is the norm here. Nothing unusual or bizarre or problematic about this.

The Prosecutor’s Fallacy has not been voiced.

Next?
…

AK

Anti-K,
If you have no reference sample why do we need lessons from you in probability estimation, that is assuming your allele frequencies are accurate?

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[Ellie9]

I have a question for DNA experts. Is it possible that if I was murdered in my home, god forbid, I could have DNA on myself and clothing from people who hung out at our house the past few weeks? Months? Years?

Is it possible and how far back can it go? For this assumption let's say they just hung out, sat on my couch, and chairs, had dinner, for a few hours. At various tones and dates, not people who lived with me for even a short time.


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[Anti-K]

I may have missed it- but AK are you in forensics?


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No. I’m in blue jeans and a Penn’s Sunday School t-shirt. Why? Should I be? Are you?
…

AK

 

[Anti-K]

Anti-K,
If you have no reference sample why do we need lessons from you in probability estimation, that is assuming your allele frequencies are accurate?

.

Your reference to a reference sample is nonsensical. Why don’t you explain to us what a reference sample is, and then tell us why we would need one or why one would even exist.
.

The exact numbers aren’t important to the conversation, as it is the concept that is being explained. However, my frequencies are based on an FBI Caucasian Population Study which concluded an approx 1/13.66 chance of matching at each location for two randomly selected individuals. “Journal of Forensic Science,” Vol 44, number 6

The argument for acceptance of the FBI rate is that the numbers generated are so high that frequency rates specific to racial and/or regional differences are not needed.

Now, about that reference sample you keep referencing: explain, please.
…

AK

 

[Anti-K]

I have a question for DNA experts. Is it possible that if I was murdered in my home, god forbid, I could have DNA on myself and clothing from people who hung out at our house the past few weeks? Months? Years?

Is it possible and how far back can it go? For this assumption let's say they just hung out, sat on my couch, and chairs, had dinner, for a few hours. At various tones and dates, not people who lived with me for even a short time.


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Hi Ellie9,
There are no DNA experts here. I certainly am not one. But, I do have some understanding of the subject.

Before I try to address your questions I want to explain a little bit about tDNA (touch DNA). I don’t know how much you already know so I’m just going to write as if you don’t know anything. Just skip over the parts that you already understand.

If you google it, you will find that there are two types of tDNA. The term used to mean one thing and then it came to mean another thing. In the beginning it was in reference to a specific process used to generate results. LCN. Low Copy Number. If you google and find stories on LCN or tDNA being used in reference to LCN – ignore it. We’re not interested in that type of tDNA. At least, not as far as the CODIS or matching tDNA samples are concerned.

Also, remember, the CODIS sample is NOT tDNA. It is probably saliva.

In the current and more common use of the term, the use we are interested in, tDNA simply refers to DNA that is left behind when you touch something. If you leave behind a really, really small sample, then LCN might have to be used. But, in this case, the samples we are talking about were all of sufficient in size as to be processed in the usual way.

I really feel like I should say something about the arguments that are most often raised regarding the reliability of the DNA results, but I think this is going to be long enough already. Remind me if you’re interested. It IS important and always overlooked here.

Okay. I might break this up into two posts, because, before addressing your questions I want to tell you how I (think I) know what I (think I) know about DNA transfer, and, therefore, why I think I can address your questions even though I am not an expert. I read a lot of studies, over about a 3-year period. I spent money. You can find abstracts online, but if you want to read – you gots to pay. I corresponded with a real, live, DNA expert. And, etc…

to be continued.
…

AK

 

[Anti-K]

cont. from above.

DNA can last for a long time. A long, long, long time. how long? Really, really long. However, it is fragile, and we’re talking small, so small. So small that even if it was everywhere, finding it could prove difficult. So, unless we’re talking a sample of significant size or an object that had been stored, any DNA found is probably going to be recently transferred. How recent? It’s variable. What is the object? How recently was it handled? How often is it handled? How was it stored, etc?

Here are a few things that I’ve learned, re: tDNA:
transfer does NOT always occur; type of contact can be relevant; duration of contact can be relevant; emotional state (sweat) can be relevant; recent contact with mouth/nose can be relevant; object touched can be relevant; etc.

transfer does NOT always occur; however:
Primary transfer is always the likeliest method of transfer. Person A transfers Person A’s DNA.

Secondary (and, further) transfer can and does happen. Person A transfers Person B’s DNA. sometimes this involves contact between both persons, but sometimes it involves contact with an object.

With secondary transfer a sample is often a mixed sample: Person A’s DNA + Person B’s DNA. Although, sometimes, Person A can transfer Person B’s DNA without transferring their own (as I said, transfer does NOT always happen).

Anyway, lots to say here before even getting to your couch, but I’m already over-length. sorry. Let’s just skip ahead. Whose DNA is on your couch? Who frequently sits on your couch? Who were the last people to sit on your couch? Did they touch the couch with their skin? Where? Is your couch in anyway comparable to the inside crotch of a brand new pair of panties?

This is a really fascinating area of discussion. sorry that I got carried away and hope I didn’t scare you off.
…

AK

 

[Ellie9]

Thanks for explaining all that. From what I can gather it sounds like DNA can last a long time, but falls apart, so it would be found in small amounts, maybe too small to identify a person with. Also, it sounds like DNA can be transferred from means other than direct contact. I concede that the inside of someone's underpants is a weird place for DNA that isn't their own (or partner if an adult).

However, I do wonder if they recovered one degraded sample, how weird that would be. If a person pulled on another's underclothes, or regular clothes, in a form of sexual assault, I'd imagine quite a lot of their tDNA! Maybe he/she wore gloves? But if say, the person coughed or something, wouldn't there be kind of 'fresh' DNA, the kind that isn't all degraded? You know? And maybe more of it if it was as recent as the night or evening before?

 

[UKGuy]

Your reference to a reference sample is nonsensical. Why don’t you explain to us what a reference sample is, and then tell us why we would need one or why one would even exist.
.

The exact numbers aren’t important to the conversation, as it is the concept that is being explained. However, my frequencies are based on an FBI Caucasian Population Study which concluded an approx 1/13.66 chance of matching at each location for two randomly selected individuals. “Journal of Forensic Science,” Vol 44, number 6

The argument for acceptance of the FBI rate is that the numbers generated are so high that frequency rates specific to racial and/or regional differences are not needed.

Now, about that reference sample you keep referencing: explain, please.
…

AK

Anti-K,

Why don’t you explain to us what a reference sample is, and then tell us why we would need one or why one would even exist.

it would be remiss of me to talk down to you in the manner you adopted towards SuperDave, so I will be brief.

”No reference sample to compare against” is the norm here. Nothing unusual or bizarre or problematic about this.

A Reference Sample is what it says on the tin, this is normally dna sourced from close relatives, or artifacts known to belong to the victim.

So in most investigations you have those two sources of dna, i.e. an evidence sample and a reference sample.

However, my frequencies are based on an FBI Caucasian Population Study which concluded an approx 1/13.66 chance of matching at each location for two randomly selected individuals. “Journal of Forensic Science,” Vol 44, number 6

Thats fine if you are looking for a Caucasian suspect, what if the actual killer is Asian or African-American, where a known hemoglobinopathy is sickle-cell disease, usually rare in Caucasians, lactose intolerance is another genetic mutation specifying a particular continent.

Then there is cross-contamination:

In the case of the Phantom of Heilbronn, police detectives found DNA traces from the same woman on various crime scenes in Austria, Germany, and France—among them murders, burglaries and robberies. Only after the DNA of the "woman" matched the DNA sampled from the burned body of a male asylum seeker in France, detectives began to have serious doubts about the DNA evidence. In that case, DNA traces were already present on the cotton swabs used to collect the samples at the crime scene, and the swabs had all been produced at the same factory in Austria. The company's product specification said that the swabs were guaranteed to be sterile, but not DNA-free.

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[Anti-K]

Thanks for explaining all that. From what I can gather it sounds like DNA can last a long time, but falls apart, so it would be found in small amounts, maybe too small to identify a person with. Also, it sounds like DNA can be transferred from means other than direct contact. I concede that the inside of someone's underpants is a weird place for DNA that isn't their own (or partner if an adult).

However, I do wonder if they recovered one degraded sample, how weird that would be. If a person pulled on another's underclothes, or regular clothes, in a form of sexual assault, I'd imagine quite a lot of their tDNA! Maybe he/she wore gloves? But if say, the person coughed or something, wouldn't there be kind of 'fresh' DNA, the kind that isn't all degraded? You know? And maybe more of it if it was as recent as the night or evening before?

You can imagine quite a lot of tDNA, but that doesn’t mean there would be quite a lot of tDNA. In fact, I don’t think there is ever “quite a lot” of tDNA.
Degraded isn’t an indicator of age. Degradation is usually caused by environmental factors. For example, being wet, inside the crotch, between the legs, mixed sample, saliva + urine - degradation can begin to occur almost instantly.

One of the (many) problems with secondary transfer theories is that one sample is probably saliva (wet) and the other matching samples are probably skin cells (dry). One isn’t going to change into the other. The simplest explanation is primary transfer – killer removes gloves to pull down leggings and uses saliva as lubricant for the sexual assault.

If jbr had innocent DNA on her it should have been the DNA of those she was most often and most recently contact with.
…

AK

 

[Anti-K]

Anti-K,


it would be remiss of me to talk down to you in the manner you adopted towards SuperDave, so I will be brief.



A Reference Sample is what it says on the tin, this is normally dna sourced from close relatives, or artifacts known to belong to the victim.

So in most investigations you have those two sources of dna, i.e. an evidence sample and a reference sample.


Thats fine if you are looking for a Caucasian suspect, what if the actual killer is Asian or African-American, where a known hemoglobinopathy is sickle-cell disease, usually rare in Caucasians, lactose intolerance is another genetic mutation specifying a particular continent.

Then there is cross-contamination:


.

I was hoping that by getting you to explain yourself, you would realize your mistake. Oh well.

The conversation was about the CODIS sample and matching tDNA samples. Reference samples, that is known samples for these to be compared against did in fact exist. This is how they were able to determine that the CODIS sample was commingled with the victim’s – they had her reference sample. This is also ho they were able to eliminate others (200+ so far) – known samples were compared.
IOWs, with a little consideration you should have realized that reference samples were used and that your objections – no reference sample – are wrong.
.

Your question regarding if someone were Asian or African-American was already answered in my post which you quoted.
.

Your comments re: cross-contamination were sort of non sequitur. Regardless, contamination is almost always discoverable and so far has not proven to be of concern here.
…

AK

 

[icedtea4me]

You can imagine quite a lot of tDNA, but that doesn’t mean there would be quite a lot of tDNA. In fact, I don’t think there is ever “quite a lot” of tDNA.
Degraded isn’t an indicator of age. Degradation is usually caused by environmental factors. For example, being wet, inside the crotch, between the legs, mixed sample, saliva + urine - degradation can begin to occur almost instantly.

One of the (many) problems with secondary transfer theories is that one sample is probably saliva (wet) and the other matching samples are probably skin cells (dry). One isn’t going to change into the other. The simplest explanation is primary transfer – killer removes gloves to pull down leggings and uses saliva as lubricant for the sexual assault.

If jbr had innocent DNA on her it should have been the DNA of those she was most often and most recently contact with.
…

AK

Saliva contains skin cells.

 

[BOESP]

Saliva contains skin cells.

:giggle:

:goodpost:

 

[Ellie9]

Saliva contains skin cells.

Oh God there's some of that in my mouth right now! Ahhh!


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[Anti-K]

Saliva contains skin cells.

Yes, I know saliva contains skin cells. Saliva itself has no DNA. the DNA comes from skin cells sloughed off the inside of our mouths.

But, surely you recognize the difference between the two sample types. One is dry, one is wet. One is saliva, one is not; even if saliva contains skin cells, it is still saliva!

The use of these descriptors is ubiquitous and widely accepted. But, I’m curious. How do you think I should refer to them? Do you think there is something misleading about referring to them as I do (probably saliva (wet) and probably skin cells (dry))?
.
BTW, as I remember it, many years ago, it was being said, sources forgotten or unknown, that the original note did have a period after the “C” but it was faint and never showed up in any of the copies. Do you know anything about this?
…

AK

 

[icedtea4me]

Yes, I know saliva contains skin cells. Saliva itself has no DNA. the DNA comes from skin cells sloughed off the inside of our mouths.

But, surely you recognize the difference between the two sample types. One is dry, one is wet. One is saliva, one is not; even if saliva contains skin cells, it is still saliva!

The use of these descriptors is ubiquitous and widely accepted. But, I’m curious. How do you think I should refer to them? Do you think there is something misleading about referring to them as I do (probably saliva (wet) and probably skin cells (dry))?
.
BTW, as I remember it, many years ago, it was being said, sources forgotten or unknown, that the original note did have a period after the “C” but it was faint and never showed up in any of the copies. Do you know anything about this?
…

AK

Was an amylase test done on the skin cells scraped off the longjohns?

And, no, I've never heard anything about this faint period after the big C. The note was written with a Sharpie, so I don't see how it could wind up being so faint as to not show up when copied.

 

[Anti-K]

Was an amylase test done on the skin cells scraped off the longjohns?

And, no, I've never heard anything about this faint period after the big C. The note was written with a Sharpie, so I don't see how it could wind up being so faint as to not show up when copied.

BODE said that the tDNA was probably skin cells.
…

AK

 

[icedtea4me]

BODE said that the tDNA was probably skin cells.
…

AK

Probably skin cells from where?