What are the Different Ways DNA Can Help Identify People?

In the last few decades, DNA technology has opened up several new methods for identifying humans (whether adoptees, people of unknown or uncertain parentage, human remains, or tissue or fluid samples left behind by suspected criminals). These methods have proven increasingly useful for both family tree building and the investigation of crimes. The following contains a succinct overview of the different techniques. Based on your own research needs, you may find one or more of these methods useful:

(Photographer: George Prentzas, public domain)

1. Direct Matching

Direct matching of DNA samples represents the simplest and most straightforward method. Say someone takes a DNA test, logs their results in a database, and then leaves behind a sample of their DNA at a crime scene. When investigators test the crime scene sample (whether some hair, skin, bodily fluid, etc.), they can then upload the sample DNA profile to that database and see a 100% match with the culprit’s previous results. Many felons in the United States are required to submit a DNA sample to the government’s CODIS (Combined DNA Index System) database. This allows one-to-one matching of DNA results. Other, non-criminal databases a person might upload their DNA profile to include AncestryDNA, 23andMe, FamilyTreeDNA, MyHeritage, or GEDmatch. Investigators could even use this method of DNA matching to help rediscover the identities of amnesiacs or human remains, assuming the person had previously done a DNA test.

Direct matching of DNA samples usually represents the last step of all genetic investigations, whenever possible. When investigators use genetic genealogy to help identify a violent criminal who left genetic material behind at a crime scene, this–at most–can represent a very high quality lead. Police investigators can then use this lead to test the individual directly, either by getting a DNA sample directly from the suspect, or by retrieving a sample unwittingly generated by the suspect (like picking up a cigarette or cup that the suspect has placed their lips on and then thrown away, for instance). Adoptees searching for a biological parent might also use genetic genealogy to zero in on a high-quality lead, and then ask the suspected parent to take a direct DNA test to see if a parent-child match exists. This helps genetic genealogists to rule out rare but still-possible situations that could fool a researcher, such as when the suspect had one, more, hitherto-unknown sibling, given up for adoption perhaps.

(Creator: National Cancer Institute, public domain)

2. Genetic Genealogy

Genetic genealogy involves a bit more inference than direct DNA matching. It consists of triangulating the identity of an unknown individual or someone with uncertain parentage, by comparing their DNA profile against that of numerous other test-takers in a database. By finding clusters of cousins, often distant cousins, the genetic genealogist can infer likely candidates for the sample-contributor’s ancestors. By then tracing all the descendants of these suspected ancestors, the researcher looks for spots where the family tree of one cluster of cousins touches the family tree of another cluster of cousins. The family trees might touch at a point where one person from one family tree marries or dates one person from the other family tree, or at a point where one person from one family tree lived in the right geographic location at the right time to impregnate a person from the other family tree. This method uses “autosomal DNA,” or DNA from the 22 non-sex chromosomes (we call these 22 chromosomes “autosomes”). Researchers find autosomal DNA especially useful for doing genetic genealogy because humans can pass down these genes to any descendants, regardless of whether they or their descendants have male or female sex chromosomes.

DNA from the 23rd chromosome, the so-called “sex chromosome,” which all humans have, can also prove useful. For researching people with a male sex chromosome (those who have a Y chromosome along with their X chromosome), Y-DNA–as it is called–provides strong links between paternal ancestors and descendants, between brothers who share a father, and between paternal cousins. Y-DNA gets passed down from fathers to sons with almost perfect replication. Every so often, a small mutation might happen. Researchers pay close attention to the handful of locations on the Y chromosome where these mutations have occurred. Mutations might take one of two forms:

One: A mutation might cause a segment of the DNA to repeat several times (for instance, “GATC GATC GATC GATC GATC”). Geneticists call these locations “short tandem repeats” (or “STRs” for short). If two males test their Y-DNA and find that, on all of the tested STR locations, they have the same repeat values (for instance, if both of their DYS393 STR locations have a repeat value of “14,” both of their DYS390 STR locations have a repeat value of “24,” etc.), then they can rest assured that they have a very recent common ancestor in their paternal lines. The more different their STR values, the less recent their common ancestor on their paternal lines.

Two: A mutation might cause a substitution of one molecule (or, more precisely in this case, a “nucleotide”) for another inside someone’s Y chromosome. Each time such a point mutation happens, we call it a “single nucleotide polymorphism” (or, for short, a “SNP,” pronounced “snip”). Each new SNP creates a new tribe (or “haplogroup”) of male descendants. Again, if two males have the same molecule in the same location at that SNP site, the overwhelming odds say that they both descend from the same, common, paternal ancestor who generated that SNP. The more SNPs two males have in common, the more closely related they are on their paternal lines.

Researchers can also use DNA that only mothers pass down to their children. While the 23 chromosomes (sex plus autosomal DNA) float around in the nucleus of every cell, the mitochondria of every cell have their own pieces of DNA, separate from the 23 chromosomes of nuclear DNA. This mitochondrial DNA exists in the shape of a circular chromosome, and children only receive it from their mothers, not from their fathers. Like Y-DNA testing, mitochondrial DNA (or “mDNA”) testing helps researchers pinpoint the relationships of maternal ancestors and descendants, siblings who share a mother, and cousins who have a common ancestor in their maternal lines.

A portrait of a woman created artificially through phenotype profiling (Creator: Daniel Ahlberg, Source: Wikimedia Commons)

3. Phenotyping

Phenotyping remains an evolving frontier, as much an art as a science. As genetic science gets a better and better grasp on which genes have responsibility for which physical appearance traits in human beings, phenotyping can become increasingly accurate in predicting what a person’s natural body should look like, given the patterns observed in their DNA. This accounts for things like natural hair color, eye color, skin tone, the presence or absence of freckles, etc. Although just one tool in genetic detectives’ toolbelt, investigators can combine a phenotype with other pieces of evidence to help generate tips from the public and to narrow in on a suspect when investigators have several suspects to rule out. If the phenotype of a suspect shows that they have red hair, for instance, this can help rule out non-redheads from the list of suspects. We have to take phenotyping with a grain of salt, of course, not only because science is still figuring out the complex and tricky relationships between the thousands of genes a human has, but also because people can alter their appearance through wigs, dyes, contact lenses, makeup, cosmetic surgery, body modification, injuries, disease, and aging. Phenotyping has also not quite made it onto the consumer market of easily affordable and accessible testing, like autosomal, Y-DNA, and mDNA testing have. At this point in history, investigators must directly contact labs to have phenotype profiles produced.

Are you a family history researcher, adoptee, or law enforcement who needs more information about what kind of DNA testing would prove most helpful for your project? Contact Josiah Schmidt today to start with a free consultation!

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