All About Alleles

Day Two in my new life as a blogger. I have been involved in a criminal case for the past two and a half years. It is a cold-case murder, and I have been volunteering my legal services to the accused. I am not the primary defense attorney in this case. The case is being tried in Georgia, and I am not licensed there. (Remember My Cousin Vinny?) However, I have been donating time and mental energy to the research and strategy involved.

As it turns out, I have been able to provide a fair bit of help on the dna evidence the prosecution has introduced. I always loved math and science in my earlier academic years, and I still do today. So, I did quite a bit of work analyzing and discussing the data with our forensic expert.

We are still in the prosecution's case-in-chief, so I certainly have to be careful about saying too much about the strategy we are using, and/or what I cannot say. However, I can say at this point that there is a lot about dna evidence that most people probably never realized. When you think of a dna match, you tend to think about that 1 in a billion, 1 in a trillion sort of odds that essentially means that statistically there is no way anyone on Earth committed this crime except for you, your identical twin, or your clone that was sent back to the present from the future.

Actually, that is inaccurate. I will try and put this in simple terms, as I am no expert myself, and I want to be careful not to stray beyond what I think I know. Human beings share a great deal of their genetic code with each other--over 99%. However, there are a number of different sites (loci) on the dna molecule where you can find the differences that make us unique. Much like a fingerprint, we all have these sites, but they may have different values represented there. These values are the "alleles" that are the traits represented at that site. (Eg: hair color, skin color, etc). However, a similarity or a match at one site is hardly conclusive, as we share so many of the same alleles. Imagine, there are many people with black hair, but that does not mean they all also have the same eye color, or skin color.

So, if my dna matches evidence at one particular loci, it does not mean I will match at other loci. The more matches I have with that evidence, the higher the statistical probability that my dna actually matches the evidence in question. Now, two important things about that: 1) just because I can be a contributor of that allele does not mean I AM the contributor of that allele--this is particularly important in mixed samples; and 2) it is possible to exclude a person as a contributor to a sample if they do not share the same alleles.

Of course, there is a lot in between, which is why someone might be considered a probable match, a likely match, a possible match, or not a match at all. Knowing how many of the sites match is very important in knowing how strong of a case you do (or don't) have if it is predicated partially or wholly on this dna evidence. So when handling a case like this, get a good expert. Make sure you truly understand what they're telling you, so you properly can handle the direct or cross examination. It is likely many/most of the jury won't know this any better than you do, and you need to be able to get them to understand your point. This can really make or break a case!

Jay