UC San Diego biologist Ethan Bier says it was the coolest thing he's ever seen come out of his lab: "I was speechless. It's hard to articulate the degree of surprise."

The jaw-dropping result? Nothing more than a bunch of yellow fruit flies. Bier says that the thing is, fruit flies only get that color from a rare gene pairing.

"You have a yellow gene, which, if you have two mutant copies of it, you have yellow flies. But if you have only one mutant copy of it, and one normal copy of it, you have the dark pigmentation."  

That's why Bier was shocked when his grad student discovered a way to ensure every fly born in his lab came out yellow. They inserted a modified yellow gene into a fly's DNA, and they got that gene to copy itself onto both chromosomes, which means they were guaranteed to get a yellow fly.

They got it to copy in offspring too. That set off a chain reaction: all the babies from this engineered fly also turned yellow and so did all their babies. By modifying one single fly, Bier could hatch a lab full of yellow flies in just a few weeks.

"It determines the genetic characteristics of all of their descendants. They are all going to be the same," Bier explains.

If you're scratching your head thinking "So what, they turned a bunch of flies yellow, big deal," you should hear from UC Irvine's Anthony James. He’s an expert on malaria. "It's still the most serious mosquito-borne disease," he says.

James has discovered ways to genetically engineer malaria out of the mosquitoes in his lab.The problem is he doesn't have a way of spreading his anti-malaria genes into wild mosquitoes.

James explains, "What we need then is something that would move these genes into field insects.”

He needs something people in his field call gene-drive. And James found it in Ethan Bier's study:

"I looked at the data and I wrote back and I said that's gene drive. That's what we've been looking for and working on."

Malaria kills six hundred thousand people per year. James says if this new technique can spread genes through mosquitoes as efficiently as it does in fruit flies, it could be a game-changer in countries devastated by the disease.

And mosquitoes modified with different genes are already putting a dent in other tropical diseases.  Health officials in Panama say they reduced dengue fever in one small town by releasing modified mosquitoes. But many scientists say until further research is done, modified bugs should remain confined to highly secure labs like this one.

To prevent a takeover of yellow flies in San Diego, Bier had to keep his flies confined in carefully sealed boxes behind a series of heavy doors.

A lot of scientists think genetically modified insects should stay locked down for now, or they could loose the public's trust. Harvard geneticist George Church says, think about how people have reacted to genetically modified crops. Many already distrust GMOs in their food, even after extensive regulation. Church ventures,

"Imagine the reaction of an unscheduled, unapproved release of an aggressively spreading GMO that flies, that doesn't sit still like a plant."

Church's preferred gene-drive method includes safeguards to prevent modified DNA from surviving outside the lab. He says the UC San Diego researchers didn't take that precaution. Bier agrees that safety is a huge concern. But he doesn't want scientists to shy away from using genetic modification against deadly diseases:

"I do believe that the positive applications of this technology are numerous, and will have real impact on human welfare. And I think that those benefits greatly outweigh the risks."

Bier says the domino effect this technique could have against malaria and other infections would be profound. Scientists just have to set up their dominoes carefully before tipping one over.

A version of this story originally ran on KPBS in San Diego.