Six-year-old Johanna Gill puts a protective hand on her sister, Eva. The twins both have mild autism, a disorder linked to genetic inheritance.

The 15-year-old sisters want to go to the same university and become opera singers. They both like to draw as well but have a different approach to their art. Marta depicts finely detailed faces, while Emma prefers more expansive images: the sky, the rain, objects in motion.

As infants, Ramon and Eurides looked so much alike that their mother gave them name bracelets so she wouldn’t get confused and feed the same child twice. Today at age 34, the twins are next-door neighbors in Florida, living in identical custom-built houses. A topic of family debate: Who has the fuller face? Ramon says it’s Eurides. Eurides (and the mother) say it’s Ramon. Mom thinks it’s because she mistakenly gave Eurides’s portion to the other twin.

When Loretta was diagnosed with breast cancer three years ago, Lorraine was in the doctor’s office with her. Loretta asked if Lorraine should be checked as well. The doctor discovered that Lorraine also had breast cancer. After receiving treatment, the sisters are both in good health.

The nine-year-olds get along well and also have a psychic shopping bond. Their mom sometimes takes them to the mall on separate occasions. Even when one twin doesn’t know what the other twin has selected, they typically want to buy the same clothes.

In kindergarten Spencer was too shy to pose on picture day. He gave his shirt to his brother, who sat for both portraits. Their mom wasn’t fooled: She spotted the mosquito bite on Skyler’s forehead in both. In high school the brothers were wrestlers; the ref would sometimes tell one, “You can’t come back out—you just wrestled.” Now 19, they attend Ohio’s Lake Erie College, one of the few that offers a scholarship to twins—one pays full tuition, the other is a freebie.

Jessica and Jackie Whited, 20, both go to the University of Akron, share the same friends and jobs, and even teach Sunday school together. When they were younger, nail polish distinguished them—Jessica wore purple and Jackie wore pink. They look so alike that their boss at McDonald’s gets confused, but their personalities aren’t identical. Jessica says Jackie, who will do things like dye her hair impulsively, is “more of a thrill ride.”

Five-year-olds Carly and Lily Ayer of Ohio are so inseparable that their mother, Lisa, occasionally worries: “I wonder if they sometimes think they are the same person.” The girls are in the same class at school and in swimming. When teachers tried to move Carly up to a more advanced swimming class, Lily protested: She didn’t want to be left behind.