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New type of twin?

King’s College researchers study theory that some siblings may be half-identical

Elizabeth Williams, 3, peers at twin sister Alexa as her father, Marty, holds her while Joe Margotta swabs her cheek Wednesday afternoon as part of the King’s College twin study.

Times Leader staff photo/S. John Wilkin

Chris Falcone, right, and his twin brother, James, from Boston, swabbed the inside of their cheeks for DNA samples as part of King’s College’s twin study Wednesday afternoon.

Times Leader staff photo/S. John Wilkin

Driven by curiosity, Diana and Martin Williams subjected their twin 3-year-old girls to a DNA experiment.

Dressed in little pink coats, with white ribbons tied neatly in their curly hair, Alexa and Elizabeth stared cautiously at the young biology research student in a white lab coat. He took a swab from inside each of the children’s cheeks and placed the samples in a small container for testing.

Nearly 25 sets of twins took part in the second phase of a King’s College DNA study that researchers hope will lend definitive proof to the existence of a third type of twin.

Biologist Ann Yezerski believes that in addition to fraternal and identical twins, a third type, the half-identical twins, explain why some suspected fraternal twins look almost exactly alike. She began her research last year when 25 sets of twins volunteered their DNA for the genetic research.

Identical twins, also called monozygotic twins, are formed when a single fertilized egg splits in half. Dizygotic twins, commonly known as fraternal twins, form when two eggs are fertilized by separate sperm cells.

The concept of the half-identical twins says the twins are formed when a single unfertilized egg splits in half and each half is fertilized by one sperm. A piece that splits from the egg, known as the polar body, usually dies, but it’s believed that sometimes when the egg is old, the polar body receives nourishment and survives.

Told by her doctor that her daughters were fraternal twins, Diana Williams said she always gets comments about the similarities in Alexa and Elizabeth’s physical features.

“I think they are fraternal,” she said. “But everyone always says they think they are identical twins, so I got curious about this.”

Williams hopes the test will give her an exact answer.

Among some of the findings from the first round of genetic research, Yezerski said she was surprised by the amount of differences in the DNA of identical twins. Yezerski said that since 90 percent of a human’s DNA has no outward physical or internal effect on the body, there’s a lot of room for differences that, while slight, would technically mean identical twins aren’t 100 percent the same.

“At first I thought my results were a mistake,” she said. “But after I tried again, we did find that there were differences in identical twins.”

Yezerski said she and her students will develop a practical DNA test that can determine the percentage of similarities in twin genetic material.

“This is really just the beginning,” she said.

The research could go a long way in showing how one identical twin develops a disease while another doesn’t, which would explain why Nancy Harworth’s identical twin sister developed multiple sclerosis, while she did not. Her sister, Carol Brague, couldn’t attend Wednesday’s event because of her illness, but Harworth said she would take the sample from her sister and return it to Yezerski’s lab.

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