Seeing double - twins gather in England

Twins relax in the grounds of St. Thomas' Hospital at an event celebrating genetic research of twins in London, England, June 8, 2013.

The Department of Twin Research at King's College London, based at St. Thomas' Hospital, has carried out important genetic research for the past 21 years.

By studying volunteer twins, from the TwinsUK registry of 12,000 twins, scientists have identified over 400 genes associated with over 30 diseases.

Twins Jenny Carrick (L) and Jeanette Carrick (R) pose for a photograph in the grounds of St. Thomas' Hospital at an event celebrating genetic research of twins in London, England, June 8, 2013.

Twins relax in the grounds of St. Thomas' Hospital at an event celebrating genetic research of twins in London, June 8, 2013.

Twins Janese Samuels (L) and Janet Morgan (R) pose at an event celebrating genetic research of twins, June 8, 2013.

Twins Gail Green (L) and Tonia Green (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Helen Watson (L) and Janet Barron (R) pose for a photograph with Janet's twins Maisy (2nd-L) and Isla (2nd-R) at an event celebrating genetic research of twins, June 8, 2013.

Twins Davina Bond (L) and Sabena Bond (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Wendy Rees (L) and Christine Binding (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Joan Robbens (L) and Joyce Shuter (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Margaret Simpson (L) and Sylvia Lindsay (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Camilla Robbins (L) and Victoria Robbins (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Lynda Plaskett (L) and Susan Downward (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Karina Hayklan (L) and Sonia Taylor (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Sarah Hope (L) and Victoria Bacon (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Lisa Dykes (L) and Alison Croasdale (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Diana Bartholomew (L) and Sandra Horner (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Jacqueline Mason (L) and Gillian Thornton (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Twins Mary Norris (L) and Angela Ambrose (R) pose for a photograph at an event celebrating genetic research of twins, June 8, 2013.

Hundreds of pairs of twins gather at an event celebrating genetic research of twins, June 8, 2013.