The investigation into the 2012 murder of Irasema Chavez, which resulted in an arrest this week, highlights the evolution of DNA investigations.

It's one thing to have blood, semen or saliva and a potential suspect identified against whom to compare it. A DNA test, in that case, would simply be used to show whether it's a match or not.

It's a far more complicated investigation when law enforcement finds DNA and has no idea who it belongs to.

That's what happened in the Chavez case, when police found blood at the scene that wasn't hers.

How DNA phenotyping helped revive a 2012 Texas cold case

Four years later, in 2016, investigators sent it to a private company, Parabon Nanolabs, which was able to use phenotyping technology to figure out that the blood likely belonged to a woman with dark hair, dark eyes, and light brown to fair skin. They also created an image of what she might look like.

 Phenotyping is analyzing the parts of the DNA that actually code for the person, and so picking out information about their eye color, their hair color, their ancestry, the overall shape of their face, and we can put that all together into a composite image of that person,  explained Ellen Greytak, Parabon Nanolabs' director of Bioinformatics.

The company's sketch drew renewed attention Monday after police announced the arrest of Mayra Velasquez, whose mugshot bore a striking similarity to the picture created a decade earlier.

Greytak says phenotyping can give police leads.

 In a lot of these cases, there's no witness. There's nothing for investigators to go on to narrow down, you know, among anyone in the world. And we're kind of coming in and saying, 'well, the DNA is there,' and the DNA can sort of serve as a genetic witness to at least give you some description of that person. 

The approach, though, has its limitations.

 We can only know things that are written in the DNA, so things like age, you know, your DNA is the same the day you're born as the day you turn 80, so we can't pick that out,  said Greytak.  Anything that's not genetic, tattoos, facial hair and you think, 'Oh I got a hair

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