DNA Testing and the Results from Zimmerman’s Gun

I. Introduction

I was going to publish an article on a Motion for a Change of Venue today, but decided to postpone it and do an article on DNA instead.

II. Forensic DNA Testing

All crime labs use the same PCR test that the Florida lab used. The test was developed by the FBI Crime Laboratory. It uses the PCR process developed by Dr. Kary Mullis, a biochemist who won the 1993 Nobel Prize in Chemistry for inventing it. He is a true genius and wild man and his story is worth checking out.

Basically, PCR imitates the cell division/ replication process to create millions of copies of targeted sequences of DNA.

The FBI selected 13 polymorphic (i.e., variable) human genes that are independent of each other.

Let’s say, for example, that each of the thirteen genes might have 10 known variations or genotypes in the human population.

Databases, each consisting of Caucasians, Blacks, Hispanics, Native Americans, and Asians, have been genetically studied to identify the frequency of each known genotype within each group.

Let’s say the distribution of the 10 genotypes with the first gene, which I will call Nikko, because that is my parrot’s name, is as follows:

Gene = Nikko

Database: Human Population

Genotype A: 1%
Genotype B: 5%
Genotype C: 10%
Genotype D: 15%
Genotype E: 19%
Genotype F: 23%
Genotype G: 17%
Genotype H: 6%
Genotype I: 3%
Genotype J: 1%

Total: 100%

The distribution of the 10 Nikko genotypes in each of the racial groups will differ somewhat from their distribution in the human population and from their distribution in each of the other racial groups.

This part is key, so pay attention. The 10 genotypes of each of the 13 genes are known to occur independently of each other. For example, your Nikko genotype (let’s say Type H) does not make it any more or less likely that you will have a particular genotype at any of the other 12 genes.

To illustrate this point, let’s consider the genes responsible for eye and hair color. We know they are not independent because those genetic traits are obviously linked. Blonde hair and blue eyes are an example and you can probably think of others.

Forensic DNA testing selects genes that are known to be inherited independently from each other.

Why is that important?

Because you can use the Product Rule to calculate the odds that a person will have a particular set of genotypes or genetic profile for the 13 genes.

For example, as before let’s say your Nikko genotype is Type H, which you share with 6% of the human population.

Let’s call the next genetic site Brutus. Let’s say your Brutus genotype is Type D, which you share with 17% of the human population.

What percentage of the population would be expected match your genetic identity at Nikko and Brutus?

The answer is 0.0102 (0.17 X 0.06 = 0.0102) or about 1% of the human population.

To calculate the distribution or frequenct of your genotype in your racial category, you would use the Product Rule to multiply the percentages for the distribution or frequency of your genotype for the Nikko and Brutus genes in your racial category or database.

You might find, for example, that the frequency of those two genotypes occurring together might vary by up to as much as 10% from one racial category to another, but usually they do not vary by that much.

III. DNA Test Results in the Zimmerman Case

A. The grip on Zimmerman’s gun

Let’s take the swab collected from the grip.

First, the analyst tested for the presence of blood by swabbing a discrete portion of the grip and the test reacted positively, indicating the presence of blood.

Note that no blood was detected on the other locations (trigger, slide and holster).

BTW, red blood cells do not have a nucleus. White blood cells do and DNA tests are designed to extract and purify the DNA in the nucleus of white blood cells.

The absence of blood does not mean that no DNA will be present as that can happen if saliva or living skin cells are present. The cells in saliva and skin cells have a nucleus.

The analyst ran the PCR test on the blood that was present on the grip and obtained a complete DNA profile of the major contributor (i.e., a result identifying all 13 genotypes of the questioned sample, plus the sexual identifier).

This profile matched the known profile of George Zimmerman that was obtained from a buccal swab.

Using the Product Rule that I explained, the frequency of this profile is:

1 in 11 quadrillion Caucasians:
1 in 1.5 quintillion African Americans; and
! in 57 quadrillion Southeast Hispanics

Obviously, those numbers exceed the known population of the world. They were generated by multiplying the 13 genotype frequencies of his complete profile in each of the three listed databases.

The bottom line is George Zimmerman is the major contributor of the blood on the grip of his semiautomatic.

The analyst further determined that there was at least one other human contributor to the DNA sample obtained from the grip, but excluded Trayvon Martin as a possible source of that DNA.

How is that possible?

B. Alleles and Genotypes

All genotypes are composed of two alleles. One allele is inherited from the father and the other is inherited from the mother.

Let’s assume that the mother’s alleles are 1 and 2 while the father’s alleles are 3 and 4. What are the possible allele combinations their children might have?

1, 3
1, 4
2, 3
2, 4

What if they shared an allele? For example, the mother is 1, 2 and the father is 2, 3? You still have four possibilities. The possibilities are:

1, 2
2, 2
1, 3
2, 3

Why are alleles important to forensic DNA testing?

The answer is they are useful in determining if a person can be excluded from a mixed sample.

C. Exclusion of Martin as a minor contributor of sample obtained from the grip

If the minor sample contained alleles that are not present in Martin’s known DNA sample obtained at autopsy, and his DNA sample contains alleles that are not present in the sample from the grip, he can be conclusively excluded as a contributor to the mixed sample.

D. Swab collected from the trigger

These results were not interpretable “due to the limited nature of the results.” This means not enough DNA was present to get an interpretable result.

E. Swab collected from the slide

Due to the limited DNA results obtained, this data is insufficient for conclusion purposes.

Results are consistent with the presence of at least one male individual. No determination can be made regarding the possible contribution of Zimmerman or Martin because not enough DNA is present.

This means they found some alleles common to both of them, but not enough to draw any conclusions.

F. Swab from the holster

The analyst found a mixed profile of at least three individuals. The major profile matches Zimmerman.

No profile for the minor contributor could be developed.

No determination could be made regarding whether Martin could have contributed to the mixed sample.

This means a few alleles matching alleles in his sample were present but not enough to draw any conclusions.

In other words those matches were not uncommon and could have been due to chance.

IV. Conclusion

Martin’s blood is not on the gun and his DNA cannot be confirmed as present on any of the four areas tested.