• Fraternal twins can run in families.

Between the lines: There's no single "twin gene."

• Instead, when fraternal twins keep showing up in a family without fertility treatment, the reason may be an inherited tendency to hyperovulate, or release more than one egg in a cycle.
• Scientists have linked a few common gene variants to that tendency, including some that involve a hormone that helps eggs mature.

Yes, but: Identical twins appear to turn up largely at "random," scientists have found.

• And researchers are still trying to understand what causes a fertilized egg to split.

The intrigue: Sylia Wilson, associate professor at the Minnesota Center for Twin and Family Research, recently encountered one family tree with three sets of identical twins.

• A woman enrolling in Minnesota's infant twin registry had just given birth to identical twins. Her sister also has identical twins, and their mother is an identical twin, Wilson says.
• Wilson cautions that the story is "anecdotal," but it raises questions about whether there may be a genetic component scientists don't yet understand.