Researchers have discovered a pair of twins who are identical through their mother's side, but share only half their genes on their father's side.
The 'semi-identical' twins are the result of two sperm cells fusing with a single egg — a previously unreported way for twins to come about, say the team that made the finding. The twins are chimaeras, meaning that their cells are not genetically uniform. Each sperm has contributed genes to each child.
Apparently this was discovered because of a genital anomaly in one of the twins:
Souter and her colleagues investigated the twins' genetic makeup because one was born with ambiguous genitalia. This twin turned out to be a 'true hermaphrodite', with both ovarian and testicular tissue. The other twin is anatomically male.
Genetic tests revealed that each twin contained some 'female' cells with two X chromosomes, and some 'male' cells with an X and a Y. The proportion of each type varies from tissue to tissue in each twin, the researchers report in Human Genetics1.
The babies, now toddlers, were conceived and born normally, and each twin's growth and mental abilities appear normal.
Mother Nature has quite a sense of humor.