2 person mixture #3 questioned samples from bomb remains, no references

2 Person Mixture #3

Questioned samples from bomb remains, no references

Scenario

• Pieces of wire were recovered from bomb• Three samples gave DNA profiles• Two samples each gave a different

predominate Major profile– Unknown #1– Unknown #2

• The third sample is a rough 50/50 mixture “consistent with 2 persons”

Swab of Wire Ends (Item 1)

Swab of Wire Twist (Item 2)

Swab of Wire Twist (Item 3)

What’s your take on this data?

1 2 3 4

25% 25%25%25%

CountdownCountdown

30

0 of 30

1. We can’t do anything because all samples are mixtures

2. We can compare the major profiles to the 50/50 mixture

3. There’s no references so we don’t do anything

4. If it weren’t for these interactive questions I’d be asleep

Setting up the interpretation

• Using the “Popout calls” feature, I very quickly generate the major profile for Item 1, and rename it Unknown #1

• Then I do the same thing for Item 2• For D8, I just want the 13 here for the major

Combine the Samples

• “Combine Sources”• Gives a new table that

has the two single source derived profiles and the mixture

• (I could generate a CMF right now for CODIS for the two SS profiles)

The Data Table

• At this point, we’re left with the mixture of Item 3 to consider

Match to Unknown #1

• Not all of Unknown #1 alleles are found in the mixture

• Since I previously described it as “consistent with 2 persons” he is excluded

Match to Unknown #2

• All alleles of Unknown #2 are present• But it’s a 50/50 mixture and you can’t assume

anything• How do you do the stats? (if needed)

Why this mixture, there are no references?

• What better time to do a stat then when you have no reference?

• Investigators really want to know if there are any connections between these three items of evidence

• If three people were involved instead of 1 or 2, maybe that is meaningful to the investigation

More reasons for this mixture

• What can you learn about the contributor that is not Unknown #1 or #2? (Fine tune for a database/investigator? Every little bit helps)

• I needed an example of a 50/50 mixture and a scenario where you can’t assume anyone to show various ways to deal with it

Calculating the Stat

• Start at loci with the most information (it’s my default method) – do the 4 allele loci first

• Review the Egram –– All alleles above our (conservative) 300 rfu

stochastic threshold (Except for FGA)– No obvious peak imbalance issues suggesting

hidden third person– No signs of dropout, clean baseline, no hint of

“uncalled” alleles (need GMID to see this)

CPI?

• Only one allele in the “Danger Zone” below 300 rfu

• You could just do a CPI and be done with it• As long as you drop FGA• SWGDAM would say it’s OK• It takes about 2 clicks to calculate the stat this

way

CPI?

• Just click the “Probability of Inclusion” button

• 1 in 100 million(ish)• We had to drop FGA

CPI?

• But…• We already decided that this isn’t an

“indeterminate” mixture• We decided there are 2 contributors• CPI ignores lots of information in the mixture

Let’s Use RMP Instead

• This is a great example for an RMP that uses several “flavors” of RMP

• 50% PHR eliminates some options for some loci – so rRMP

• Some loci cannot be restricted – all possible genotypes are viable – so uRMP

• One locus needs mRMP• (I’m going to try to convince you that the

“flavor” of RMP doesn’t matter)

How to calculate

• Under “Operations” just choose “Mixture Frequency” and restrict globally

“Restrict Globally”

• I just said that I’ll click a box that does a restricted RMP at every locus

• I also said that some loci will have an unrestricted RMP

• Remember, if all genotypes are viable, it’s an uRMP even if you thought you were doing an rRMP

4 Allele loci

• THO1

• D18

• All combinations of contributors (and therefore all genotypes) are possible – 6 total

4 Allele Loci

• So for THO1 and D18, this stat is actually an “unrestricted” RMP, as all options are possible

• Based on our calculation, you end up seeing all combinations on the stat page

• (I realize you can’t read this but 6 types there)

4 Allele Loci

• This approach gives the same answer as the “CPI” based math

• By that I mean if you “sum ‘em and square ‘em” and then “throw out the homozygotes” you get the exact same number for the probability at these two loci as if you calculate all genotypes individually and add them up

3 Allele Loci

• We have three “categories” of 3 Allele Loci• Unrestricted RMP– TPOX– vWA

• Restricted RMP– D21– D5

• Modified RMP– FGA (<300 allele)– We’ll come back to this one at the end

Unrestricted 3 Allele Loci

• vWA

• TPOX


• TPOX will be the example• At 50% PHR, only 4 of 6 possible combinations

are viable• But all possible genotypes are represented


• If we change the PHR to 30%, all combinations show up, but there are no new genotypes

New options with phr<50%A

AB

BA

C

C


• TPOX and vWA have all genotypes in the stat• Homozygotes are allowed• In a CPI stat, you could just “sum ‘em and

square ‘em” and be OK, because the homozygotes don’t change the assumption of the number of contributors

• But I won’t get the same answer as “CPI” for this locus


• I won’t get the same answer here because we calculate the homozygote types the “long” way:

p2 + p(1-p)Θ

• But if I set Θ = 0, I would get the same answer as a plain old CPI calculation at these loci


• Again, I know you can’t read this• The point is just to show that vWA and TPOX

have 6 distinct genotypes in their stat, the same as THO1 and D18 (the 4 allele loci)

Restricted 3 Allele Loci

• D21

• D5


• For D5, we can’t restrict by much…• Only the 11, 11 homozygote is not present in

the stat, as that would require a 9, 13 heterozygote for the other contributor

• That person would then have a PHR of 37%


• For D21, we can limit the genotypes in the stat a bit more

• Only combinations:– 31.2, 32.2 and 30, 31.2– 31.2 (homozygote) and 30, 32.2

• Two homozygotes are not possible– 30, 30 (would require 31.2, 32.2 – 41% phr)– 32.2, 32.2 (would require 30, 31.2 – 37% phr)– But 31.2, 32.2 and 30, 31.2 are OK with another

heterozygote


• So even though you can’t read this, the calculation for D21 and D5 is more informative than a CPI calculation at these loci

2 Allele Loci

• It’s pretty hard to limit any genotypes here• There are 3 “families” of combinations that

exist:– AA and AB (homozygote and heterozygote that

share one allele)– AA and BB (2 homozygotes and no sharing)– AB and AB (2 heterozygotes that share the same

type)

2 Allele Loci

• Even if a “family” of combinations is missing, there are only three genotypes possible, and they’ll pretty much show up no matter what

• Bottom line, it’s pretty hard to “restrict” anything with only 2 alleles

2 Allele Loci

• D19 is the only locus with all three “families” present and all three genotypes are allowed

2 Allele Loci

• The rest of the 2 allele loci all have only 2 combinations viable, but all three genotypes are possible

• So it’s an uRMP – we couldn’t restrict it• CSF is the example

1 Allele Locus

• D8 has a single allele• Should we “inc” the locus?– Drop it from the calculation?– Make it 1?

• That wouldn’t make any sense to me• We are confident we are seeing a contribution

from both contributors at every other locus in the profile

1 Allele Locus

• D8 is a very small molecular weight locus• If I’m detecting a contribution from the minor

contributor at CSF and D2 and FGA… why would that person not contribute here?

• Did the enzyme get lazy?

1 Allele Locus

• 2 Choices:– 13, 13– 13, Any

• (We decided not to use the 3rd choice – we’re not going to “inc” the locus – make it 1.0)

1 Allele Locus

• D8 got entered into the stat page as a 13, 13

• I can change it by double clicking on that combination, and changing it to 13, Any

Back to FGA

• FGA has an allele <300 rfu (Danger Zone)• So we need to account for a 26, Any type• We treat this locus as a “modified” RMP– We define this as Allele, Any– Sum ‘em and square ‘em, then do all the other

types that could go with the 26 allele

Back to FGA

• We sent the locus to the stat page as a “restricted” RMP

• To change that, double click each combination to remove it

• Then choose “modified RMP” option

Back to FGA• Here is what you see in the stat box

• (p + q + r)2 + [2(r)(1-(p + q + r))] or• (20 + 23 + 26)2 + [2(26)(1-(20 + 23 + 26))] or• CPI(kind of) + 2(r)(what you didn’t detect) • r means 3rd allele (the 26) 261 means <300

Final Stat

• 13 loci we tried to do rRMP, but lots ended up being uRMP as no type(s) dropped out

• D8 is the 13, Any and FGA is the modified RMP

• 1 in 823 Million (Moral = an RMP is an RMP)• CPI was 1 in 100 Million(ish)

Or, Use LR

• Hp = Unknown #2 and someone else

• Hd = We have no clue who either one of those two guys are

• Just hit the LR button and pick the mixture

LR Stat

• 67 Billion times more likely

2 person mixture #3 questioned samples from bomb remains, no references

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