What does it mean to be a DNA match?
Detectives ran the DNA profile of the Golden State Killer, from a sample collected at a crime scene decades earlier, through a database of 960,000 DNA profiles amassed by GEDmatch, a publicly available genealogy website. Users can upload their genetic profile to the site’s database after obtaining it from a number of testing companies. Each profile is connected to a name, which the person submits when they upload their data.
The analysis found 10 to 20 distant relatives of the killer, believed to be the equivalent of his third cousins, according to the Washington Post. Investigators used that to trace back to a common ancestor, then began filling in the blanks of the sprawling family tree using other tools, including Ancestry.com, a gravesite locator, old news clippings and census data, the Post reported.
They then narrowed down the pool of suspects to men around the killer’s age who had connections to Sacramento and other California cities where the crimes were committed.
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Humans have 6 billion pairs of nucleotides — A, C, G and T molecules that bind together to form DNA. All people, whether they’re related or not, share 99.9 percent of the same DNA. Only a tiny percentage of the 6 billion pairs, between roughly 500,000 and 1 million pairs, actually matter when it comes to tracing one’s lineage and connecting a person with a blood relative. These 500,000 to 1 million locations on the human genome are called markers.
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The more closely related you are to someone, the more markers you share. Identical twins, for instance, share virtually all markers. Siblings share half the same markers. A grandparent and grandchild share one-fourth. Cousins share one-eighth.
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Investigators reportedly built out the family tree to the great-greatgreat-grandparent of the Golden State Killer suspect. A person would share 1 out of 32 markers with that ancestor.