3 person mixture #4 (or is it 2?) the hardest mixture i know

3 Person Mixture #4 (Or is it 2?)

The Hardest Mixture I Know

Bed Sheet Stain 1

Bed Sheet Stain 2

Suspect Underwear

Data Table

• Is Victim included?


• We have to deal with family sexual assault cases

• These by definition include biologically related people

• Sometimes step-parent situation• Sometimes biological parent situation• But they all live together so no “random man”


• Evidence commonly shows all alleles of both Mom and Dad (or Mom and Step-Dad)

• Some advantages:– Not “unrelated” people (well, step-dad)– Not “random” people

• Therefore, no statistics apply– “The probability of a random, unrelated person

contributing to the mixture is…”

• If someone knows a relevant stat, let us know


• OK - so we don’t worry about a stat• What do we need to worry about?– Hp vs. Hd– (But I don’t know a relevant LR to use)

• But we are asked to answer a question:– Is the victim contributing to that semen stain

found on the parents bedding and Dad’s boxers or not?


• Is she “there” because it’s mixture of her biological parents

• OR• Is she “there” because she has been sexually

assaulted but her biological father• These types of cases are the most difficult

mixtures I’ve ever seen (obvious reasons)


• So it’s what I call a “real time” LR• It has nothing to do with any LR stat that I

know• Instead I assume each hypothesis is true and I

test them• Then I decide if one hypothesis is supported

better than the other


• How do I do that?• Start with Defense hypothesis:– The evidence is consistent with a 2 person mixture

of Mom and Dad only– Use the 2 person calculator with only Mom and

Dad applied

• Does it “graph” well?• Is there a “valid” combination at each locus?– In other words, does our validated PHR and P

calculator show that the data “fits”


• Then the Prosecution hypothesis:– The evidence is consistent with a three person

mixture of Mom, Dad, AND Victim (bio daughter)– Use the 3 person calculator with all three

reference profiles applied

• Does it “graph” well• Is there a “valid” combination at each locus?– In other words, does our validated PHR and P

calculator show that the data “fits”


• A note about testing the three person hypothesis

• Bio Mom, Bio Dad, and Bio Child is worst case scenario for “not excluded” options– The “double” or “circular” sharing types– We will not get much in terms of P and PHR

• The “graph” will be pretty lousy• BUT we can still see if there is a “valid”

contributor option in our 3 person calculator


• The three person graph for this scenario at the time this handout was prepared is basically worthless

• At all the loci where a “not excluded” combination is viable, we see empty space in the graph – all we get is the name of the locus on the X-axis

• At all loci where two contributors share a type, we don’t even get an X-axis label


• So I just will give a “count” for the number of loci that show a valid three person option

• Our programmers are working feverishly to get this working better by the workshop– (Buy stock in Mt. Dew)

• Show P for the one person that doesn’t share with a designation that the other 2 share a type

• Show a designation for “not excluded” loci• Ultimately solve the “not excluded” dilemma(?)


• We made a bunch of validation samples from three volunteers I know very well– I had to bribe my daughter – She felt she deserved a new Barbie in exchange

for her “Buckle DNA”

• We made it “worst case” by keeping Mom and Dad equal contributors

• When either Mom or Dad is a minor, it makes it easier to determine if Daughter is “really there”


• Because there are so many “not excluded” options, we have to do a further test than just “viable” at our normal validated PHR of 50%

• We will instead test at 80% PHR• In real life, you would do this in iterative steps

and watch what happens for each hypothesis maybe all the way from 50-90%

• For this hand out, it’ll just be 80% and 90%


• First we test to see if all three people have relevant alleles present to check if Mom or Dad is low level or dropping out but daughter still all there

• All concerns about <300 etc. are thrown out• This is testing what fits, not worrying about a

potential missing allele from a random, unrelated person – these are all related people living together

Sheet Stain 1

• Mom and Dad at 80% PHR• Support at 11 loci (not counting Amel)But Amel can be useful

Suspect varies 43-69%

43-59% if you ignore D16(16% window)

Sheet Stain 1

• Mom and Dad and Daughter at 80% PHR• Support at 9 loci

Lots of “not excludeds” and “2 people sharing types” loci – won’t graph

Sheet Stain 1 Answer

• Remember this is a mixture I made in the lab• Hp – 11 of 15 loci show support (not bad)• Hd – 9 of 15 loci show support (still OK)• I’m going “inconclusive” as to V’s status• Not much difference

• The mixture is really only Mom and Dad

Sheet Stain 2

• Mom and Dad at 80% PHR• Support at 9 lociProportions not very consistent (Amel flips)

Suspect varies51-71%(20% window)

Sheet Stain 2


Sheet Stain 2

• Mom and Dad at 90% PHR• Support at 4 loci (plus Amel flips Major/minor)

Sheet Stain 2

• Mom and Dad and Daughter at 90% PHR• Still support at 10 loci

Sheet Stain 2 Answer

• Hp – – 11 of 15 loci show support at 80% PHR– 10 of 15 loci show support at 90% PHR

• Hd – – 9 of 15 loci show support at 80% PHR– 4 of 15 loci show support at 90% PHR

• I’m going with the Daughter “really there”• The mixture is Mom and Dad and Daughter

with Daughter present at 10ish% (10:10:2)

Suspect Underwear

• Mom and Dad at 80% PHR• Support at 6 loci

Major/minor flips all over the place!!

Suspect varies40-68%(28% window)

Suspect Underwear


Do we even need to bother checking at 90%??

Suspect Underwear

• Mom and Dad at 90% PHR• Support at 4 loci

Look at that cool flip-flop!

But, it’s not in the “correct for degradation” order

Suspect Underwear


Suspect Underwear Answer

• Hp – – 14 of 15 loci show support at 80% PHR– 12 of 15 loci show support at 90% PHR

• Hd – – 6 of 15 loci show support at 80% PHR– 4 of 15 loci show support at 90% PHR

• Is there really a question?• The mixture is Mom and Dad and Daughter

with Daughter present at 33ish% (10:10:10)

3 person mixture #4 (or is it 2?) the hardest mixture i know

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