Little pieces of you

 

At a crime scene, the most common DNA evidence comes in the form of bodily fluids like blood or semen, or from hair. The average person sheds about one and a half pounds of skin a year, microscopic flakes that disappear into the carpet. Unless the criminal in question has eczema, differentiating victim from perp from passerby using only skin flakes could be the work of a lifetime. The best bet for acquiring a suspect’s DNA is by pulling hair or other material from the scene, skin or sweat in discarded clothing, blood from wounds inflicted by a victim, or other bodily fluid evidence. Finding any of this can be difficult, painstaking work, and it only sometimes pays a dividend.

Your house is a different story. Someone trying to build a genetic profile of you could snatch all they needed from your garbage can. It’s certain that your saliva or hair is in your kitchen and bathroom garbage, and even badly degraded samples can be reconstructed into genetic profiles. Indoors, your toothbrush, your comb, your razor, and all those pounds of skin you’ve dumped on the floor over the years are available.

When the police build a DNA profile, they do not sequence the entire genome. Instead, they use a system called Short Tandem Repeat Analysis. This method recognizes that most of the human genome is basically identical from person to person. The variation from person to person is only about half a percent, which means you only need to look at a very tiny bit of DNA to find what is unique about it. It also means that very little detail about a person is revealed. DNA profiles do not include eye color, predispositions for diseases, or even ethnicity. They can be used to find familial relationships by comparison with other profiles, but other than that they contain about as much information as a fingerprint, just with a much higher degree of accuracy.

Having established the uniqueness of a sample, the investigator then needs to match that sample with a suspect. If they already think they know who committed a crime, that person can be compelled by a search warrant to submit to a cheek swab. If the DNA matches, they have someone to accuse.

If someone is attempting to profile you for reasons other than a criminal investigation, they may want to sequence your entire genome. For about $400, they can. Within a very short time, your profiler will have a complete genetic map of you—total access to your ancestry, potential health issues you might have, drug resistances, allergies, and more. All of that information comes from only a small fraction of the three billion base pairs in human DNA. As our understanding of genetic markers deepens, more and more information about you will become available, and much of that information will not be the sort of thing you want in the hands of strangers.