At 17 months, Triniti and Ghabriael are chubby-cheeked twins, born 11 weeks early at three pounds each and now healthy and a joy to their parents.

But when their mother, Khristi Cunningham of Akron, Ohio, takes them in public, the babies get a lot of second looks and questions.

Triniti has ebony-colored skin and all the classic dark features of an African American, but "Gabe," as his parents call him, is ivory-white with steely blue eyes and blond hair. He's now 10 pounds heavier than his sister, but it's their racial identity that gets people scratching their heads.

"People ask, 'How did it happen?' Are you sure they are twins?" said Cunningham, 29. "We get a lot of stares, and I am sure people make comments behind my back."

Their mother is white and their father, Charles Cunningham, is black.

"I don't know how it happened," said Cunningham. "They are fraternal twins, so they aren't any closer than if they had been born years apart. Ours just happen to have the same birthday."

Geneticists say racial differences involve many genes and are more complex in determining looks than those for eye color, but the startling difference between the twins raises an interesting question about how mixed-race families are viewed in a country that is increasing biracial.

Even the Cunningham's pediatrician was baffled by the black and white babies.

"She asked if they were identical twins," said Cunningham. "That was the last time we went to see her."

As for the Cunningham's pediatrician, Bachman says only a careful study of the placenta by the obstetrician or a DNA test can definitively determine if twins are monozygotic (identical) or dizygotic (fraternal).

Identical twins develop from one zygote that splits and forms two embryos. Fraternal twins are two eggs that have been fertilized by separate sperm.

In fact, identical twins are not exactly alike, genetically, according to Bachman. "There are scant variations between the two, who grow up in different places in the uterus and as they grow in time have various somatic mutations [that are not passed on]. There are gene changes within them."

Because the United States is such a diverse country racially, couples can carry an assortment of genes from multiple racial backgrounds. Skin color, according to Bachman, is determined by "multiple genes, not a single gene."

"An assortment of genes go into the egg and sperm to get skin color," he said. "This family is no different. The twins are just like siblings in biracial families."

The Cunninghams met in 2006, while working in a juvenile prison in Columbus three hours away from their families. They later moved, got new jobs in an auto plant and married.

They knew carrying the twins would be challenging. She lost a son the year prior to conceiving the twins because of an incompetent cervix.

The goal was to get to 24 weeks when the babies would be viable outside the womb, but they held on for an additional month and were delivered in September 2009.

At first their developmental milestones were a bit delayed, but now they are "well adjusted and caught up," according to their mother.

Although the Cunninghams pay little attention to the skin color of their babies, the world is not color blind.