Twins are normally either identical—the result of a single sperm fertilizing a single egg, which then splits and duplicates itself—or fraternal, developing from two separate eggs fertilized by two separate sperm. But last March, doctors reported the first known case of semi-identical twins.
“When we did some detailed genetic analysis, we discovered that there was a single maternal contribution to the twins but they had a mixture of two paternal contributions,” says Melissa Parisi, a doctor on the gender-assessment team at Children’s Hospital and Regional Medical Center in Seattle. “So that suggests that there were two sperm involved but maybe only one egg.”
With these twins, whose identities were not revealed, each paternal contribution contained a different sex chromosome. This resulted in the conception of a boy with normal genitalia and a girl who was developing both ovarian and testicular tissue. It was the sexually ambiguous genitalia that brought her to doctors’ attention. After several years of studying genetic material from the twins and their parents, the team came up with two possible scenarios. In the first, a single egg divided on its own and was then fertilized by two separate sperm from the dad. The second theory proposes that the egg was fertilized by two sperm and then divided.