Ancestry wants your spit, your DNA and your trust. Should you give them all three?

It markets its DNA kits with promises that tug at the heartstrings: Discover ancestors. Strengthen family ties. Understand your life.

Aided by venture capital and a flood of savvy marketing, Ancestry LLC has grown to become the world’s largest DNA testing conglomerate. Since 2012, it has lured more than 5 million people to spit into tubes and add their genetic code to the world’s largest private database of DNA. It has also banked away the world’s largest collection of human spittle, numbering in the hundreds of gallons.

In the age of Facebook and Google, consumers seem comfortable surrendering their personal information to corporations that aggregate it and monetize it. But Ancestry and other DNA testing companies have added an audacious tweak: Consumers are now paying to hand over their genetic code, their most sensitive individual identifier, to companies that could monetize it far into the future.

Ancestry officials say they have state-of-the-art systems to prevent hacking and security breaches. So far the company has sidestepped privacy scandals that tripped up companies like Facebook, which allowed a political data firm, Cambridge...